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Engine Operation and Systems
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Contents:
…The Engine; ...Getting Old.;...Engine Systems; …In an hour; …Use of aluminum; ..Engine History; ...Pilot training; ...Preflight Neglect; …Fuel problems; …Fueling; …Engine TBO; ...Engine Operation; …Constant Speed Propellers; …Primer; …Engine maintenance; …Trend monitoring; …Engine savers; …Engine operations to avoid; …Ways of losing power; …Engine Warnings; …Top Overhaul; ...Factors reducing engine power; ...Valves; ...Breaking in an Engine; ...Exhaust System; …Drag and cooling; ...Shock Cooling; ...Engine Heat; ...Magnetos; ...P-Lead; ...Distributor; ...Spark Plugs; ...Ignition Problems ; ...Carburetors; …Carburetor Heat; ...Air Intake; ...Vacuum Pump; ...Engine Monitor; ...Oil; …Oil analysis; ...Oil Pressure Gauge; ...Oil Temperature Gauge; ...Cylinder Head Temp Gauge;. ...Carburetor Temperature Gauge; ... Tachometer; …RPM; ...Engine Isolators; …Lycoming Shut Down; …Fuel Efficiency; …Tire History; ...Basics of Tire Construction;...Tire Classification; ..Tire Care; …Flying with Pressure; …Altimeter Readings; …Rusting Out; …
The four-stroke/cycle engine was invented by N.A. Otto in 1876.
His engine operated by having a piston sliding in a
cylinder. The piston has a connecting rod fastened inside the
piston and extending to a crankshaft which in an airplane has
the propeller on one end. The reciprocating motion of the piston
is changed into the rotation of the propeller. The aircraft
engine develops full power for 90% of 2000 hours during which
the comparable life of an automobile engine is developing
20% power for slightly over 100,000 miles. Most propellers have
from 50 to 87% effective thrust.
The four cycles of the piston are timed to the opening and closing of valves with the spark to a sparkplug. The first stroke draws fuel through the open intake valve, the second stroke closes the valve and compresses the fuel to make it volatile. The explosion of the fuel on the piston head gives the engine its power. The fourth stroke gets rid of the residue of the explosion and is called the exhaust stroke.
All the letters and numbers of an aircraft engine tell the
significant things about the engine. An "I" in its title
shows that the engine has a fuel injection instead of a carburetor
fuel system. O says that the engine is a horizontal opposed with
the cylinders flat and in pairs opposite to each other. The number
following the 0 is the total piston displacement volume in cubic
inches. Different aircraft engine manufacturers have different
lettering and numbering systems.
Single-engine planes have two to five degrees of downward tilt
to the engine from the horizontal. This is the reason the rudder
is the first positive control during takeoff. This also keeps
the propwash away from the horizontal-tail and reduces the pitch
changes that occur from power changes. This also serves to reduce
the noise of singles when compared to twins.
There are several ways to learn about how engines operate. There are videos, reading, and actual taking apart a small engine. Visit a maintenance show where the engine is opened up so you can see the parts and get an idea of what the insides look like. Los Banos has such a shop.
Lycoming Model Codes for Reciprocating Engines
Each engine designation is made up of a prefix of a series of
letters, a three-digit number and a suffix, which combines letters,
and numbers. Some examples:
TO 360 C1A6D
IO 540 AA1A5
IO 360 A3B6D
PREFIX DISPLACEMENT SUFFIX
L - Left-hand rotation Cubic-Inches A - Power Section & Rating
crankshaft or
AA
T - Turbocharged
(exhaust gas driven) 3 - Nose Section
I - Fuel Injected B - Accessory Section
G - Geared (reduction gear) 6 - Counterweight
Application
S - Supercharged (mechanical)
D - Dual Magneto
V - Vertical Helicopter
A - Aerobatic
AE - Aerobatic Engine
O - Opposed Cylinders
Engines deteriorate slowly and deceptively. The trouble will
be structural due to excessive temperature or operational in one
of the four operational systems; ignition, lubrication, carburetor,
or cooling. 22% of aircraft accidents are the result of engine
failure. Only 10% are due to mechanical failures. Of these 4%
is only a partial loss of power. 12% are the result of running
out of fuel to the engine and carburetor ice.
--Know what normal is.
--Is something not right?
--How is control response?
--Can the malfunction be corrected from the cockpit?
Ignition
Carburation
Fuel
Lubrication
At 2400 rpm
piston up/down 40 times second
valves open/close 20 time second
sparkplug fires 20 times second
In
an hour
Crankshaft 144,000 revolutions
Pistons reverse direction 288,000 times
In 1800 hours (engine life) propeller will turn 259,200,000 times
Use
of aluminum:
Lighter than steel
Changes with heat more than steel
Good ability to transfer heat
Parts of uneven thickness heat and cool unevenly causing stress.
This stress will weaken and eventually crack given enough time.
This stress can be reduced by slowly heating and cooling engine
(AC No: 20-105B 6/15/98)
The history of engine operation reveals that there has been little
change in the causes of small single and multi-engine aircraft
engine failures over the past forty years. 51% of all engine failures
are directly related to pilot error related to preflight, inspection,
or use of controls. Training programs could have prevented 70%
of all engine failure accidents for pilots, mechanics, and trend
monitoring.
The J-3 Cub, and an Aeronca Champ, had the same Continental engine.,. This engine was later modified from 65HP to 100 HP and became the O-200 found in Cessna 150's. the O-200 is a carbureted engine with the carburetor below the engine in front of the oil tank. This keeps the induction system cool and allows a little more power because of the cooler air in the cylinders. It also makes carburetor icing more likely. Flying behind a small Continental requires the application of full carburetor heat before throttle reduction.
In the sixties, Piper came out with a trainer modified from the Tri-Pacer series. This "Colt" had a Lycoming four cylinder engine. Lycoming engines had the camshaft above the crankshaft instead of beneath it like the Continental. This allowed a large oil pan on the bottom of the engine. By running the induction system right through the middle of the hot oil in the pan it let the engine to run a little cooler. An additional benefit was that it heated the air/fuel mixture. With Lycoming engines, you don't need use carburetor heat overtime.. The POHs said to add carb heat only if there was some sign of icing. The Lycomings are in all later Piper models. Piper pilots were occasionally surprised by icing when they failed to add carb heat on downwind.
All modern training aircraft use the Lycoming engine. The Cessna 150 was changed to the Cessna 152 to note the engine change. The early C-172's used the Continental O-300 which is a spin off of the O-200 in the C-150 with two more cylinders. It also has the ice problem of the O-200. The Lycoming C-172's do not have ice problems.
Pilot
training
--Use of operating manuals with emphasis on fuel management,
power settings, carburetor heat and systems design, location and
controls.
--Adherence to operational instructions, placards and limitations
--Use of checklists during normal and emergency situations.
---Recurrent training related to replacement parts, airworthiness
directives and technical publications.
--The correct use of the primer must be a part of all checkouts
and instruction. The primer pumps fuel to one or more
cylinders in unvaporized liquid. Failure to set and lock the primer
is a frequent cause of a rough engine and engine
damage. A cracked primer O-ring can have the same effect even
with the primer locked.
--The correct care of tires and oleo struts must be a part of
all checkouts and instruction. The oleo strut is a combined
cylinder of oil and air. The air is compressible while the oil
is not. Both of these are sealed at the top by an air valve
and filler cap and from below by a rubber O-ring. If the exposed
part of the oleo strut is not cleaned prior to every
flight the accumulation of dirt and grit will be ground into the
O-ring. In a relative short time the O-ring will be unable
to seal either the air or the oil and the strut will deliver any
landing or taxiing shock directly to the aircraft structure. No
maintenance program can overcome the destructive expenses of a
poor checkout or pilot training programs. Raising
the costs of rentals can never keep up with the costs of poor
piloting.
1. Know total usable fuel aboard. Ignore unusable fuel.
2. Check and sump drain tank for proper color, debris, feel and
liquidity.
3. Check vents especially fuel cap vents.
4. Confirm operation of fuel selector in off position as well
as for fuel flow for selected tanks (Four minutes run time).
Fuel
problems
1. Water due to condensation, filler caps, rain, and
service personnel.
2. Wrong type or octane fuel.
3. Bladder tanks with defects.
Item:
A dead battery will show that you are out of fuel.
Fueling
1. Know exactly how much fuel you have prior to departure.
2. Know exact consumption rate
3. Know time in tanks
4. Develop fuel performance chart
5. Always plan to land with at least minimum reserves.
Fueling
Grounding is a procedure to prevent the flow of electricity
that is capable of producing sparks capable of igniting fuel.
By grounding at least three minutes prior to refueling you give
electricity time to balance the charge between the aircraft and
the fuel service be it truck or pump makes no difference.. Once
the balance is achieved the flow of fuel will not change the electrical
equilibrium. When fueling over the wing be sure to touch the nozzle
to an unpainted part of the fuel cap PRIOR to opening the cap.
Difficulties are simple and easy to understand. Prevention
of occurrence is not so easy. Running out of fuel is not an expected
event. The pilot is usually very much aware of the situation but
lacks the mental discipline to choose a safe option.
1. Misinformation
POH numbers for capacity, consumption, fueling
2. Mismanagement
Tank selection errors, fuel caps, sump leaks,
3. Mechanical difficulty
Fuel pump, leak in pump, selector, electric pump, bladder problem
or gauge problem.
Survival rate after engine failure is 94%.
Maintenance related accidents are only 20% of total.
Two planes a week run out of fuel.
Not refueling when the opportunity exists is viewed by the FAA
as careless operation.
Engine
TBO (Time Before Overhaul )
An aircraft engine that is flown non-commercially can be operated
beyond the TBO recommended by the manufacturer. Once the choice
is made a program of regular oil analysis should be in place.
Oil analysis will give trend indication on engine wear and type
of wear. The bottom end of the engine, consisting of crankshaft,
camshaft, bearings and gears can be monitored by oil analysis.
The top end, consisting of cylinders and their components are
monitored by both oil analysis and compression checks.
1. Exceeding TBO will accelerate wear.
2. Part 121 and 135 operations cannot exceed TBO
Engine
operation
Thermal shock damage is caused by failure to warm up engine prior to takeoff. Shock cooling damage is caused by failure to prevent rapid cooling. Hot or cold temperature damages engines. A normally aspirated engine can be throttled from idle to full power without damage. Detonation is impossible if proper fuel is used. Don't expect to hear or feel detonation. Aircraft should be climbed at full power and maximum rpm. Engine temperature is the controlling factor. At high power settings overly rich fuel flows are unnecessary and wasteful. Injected engines do not always distribute fuel evenly.
Excessive leaning may cause a cylinder operating so poorly that full power will not be produced.. The vibration dampers will not function properly unless occasional full power is used. You will not be able to feel or hear uneven piston operation. A six-cylinder engine may run smoothly even on five working pistons.
Engine operation is a fire triad made up of air, fuel and ignition.
Without all three in appropriate amounts the engine will
not function either at all or appropriately. To stop the engine
we eliminate one of the three. Icing of the air intake may
unintentionally cut off air. Fuel is commonly cut off by filling
the tanks with air, and the turning of the magneto key to off
shorts out the P-lead of the magneto thus grounding out the current
that triggers the spark plugs.
Air is controlled by a butterfly valve in the carburetor. This
is valve is moved open and shut by the throttle control. A special
form of air is made available to the carburetor by the carburetor
heat control. Many aircraft have an additional alternate air source
for the engine. The durability and life expectancy of an engine
is build into the engine. How well it reaches these levels is
keyed into the suggested operating parameters, fuels, lubricants,
and maintenance recommendations.
Fuel flows through a collection of pipes and valves from the tanks
to the carburetor. Contamination is one of the causes of
a fuel failure. Water is a common contaminant. The fuel selector
and cut-off valve allows fuel to flow, change directions and cut-off.
I have been 'surprised' on the ground by discovering that an occasional
cut-off setting does not cut-off. Low-wing aircraft have an auxiliary
electric pump while high-wing aircraft may rely just on gravity
feed. For ease in starting a primer pump is available to spray
fuel into the induction system.
There are two ways engine operation makes a critical difference
in flying. There is little that can be done about the 70% of
engine wear that occurs in the first 30 seconds of after start
operation. You must avoid operation over 800 rpm. Proper
operation allows us to get the published performance and it prevents
damage caused by overheating, shock cooling and
stress. Smooth power changes is the first requirement. With fixed
propellers we enrich before power increases and reduce power before
leaning. The POH has a cruise performance table that covers a
variety of settings of RPM, pressure altitudes, temperatures and
RPMs to be used for selected TAS and fuel consumption expressed
for a percentage of horsepower.
Proper leaning can set engine power parameters for best range,
maximum endurance, best economy, best speed, or anything in between.
Best economy runs the engine at peak power. This is the top of
the EGT scale. The best power is slightly richer by about 100
degrees EGT temperature. This uses more fuel with a slight improvement
in airspeed. Modern engines can be operated with manifold pressures
of square or even over square (manifold pressure higher than RPM)
without harm. Cruise power settings lower than 75% can be flown
at peak EGT.
The standard leaning procedure is to lean by pulling the mixture
until the engine runs rough, and then turn it in until it smooths
out. If using an EGT, pull the mixture until the EGT gauge read
as high as it will go. You may need to reposition the EGT needle
if it goes all the way to the top. Once it has stabilized at a
high point that the engine has gone rough, enrich the mixture
to get at least a fifty-degree drop. Proper leaning will coincide
With a throttle back to full idle, fuel flow is set
by the idle adjust screw. Full idle is usually about 600 RPM in
light aircraft engines. At idle you cannot taxi nor keep the engine
running if the weather is cold.
Typically, 800-1200 RPM are the ground operating speeds. At these
power settings leaning has noticeable effect. You should lean
regardless of the fuel used. At these powers leaning can be done
until the engine coughs; then enrich a bit. An engine that dies
when you add power is a good indicator of correct low power leaning.
You cannot damage the engine by leaning It is possible to foul
the plugs if you do not lean enough, regardless of the fuel used.
By operating an engine at low temperatures with a rich mixture you are sure to reap a harvest of carbon deposits. With leaded fuel you will harvest lead deposits. Proper leaning will solve the carbon-fouling problem but will not prevent lead fouling. Only by keeping the engine temperatures high enough to keep all the lead vaporized can you prevent lead fouling. The leaner the mixture the hotter the engine and the less fuel to supply lead needing vaporization.
One of the most common operating problems is on starting. The
throttle is in so far as to cause the engine to operate at
relatively high power before the oil has had a chance to circulate.
A surging engine start will cause excessive wear throughout the
engine. Abusive operation may involve taxiing at high power and
holding the brakes on to keep the speed down. This can use 50
hours of steel off the camshaft and scuff the pistons against
cylinders. Keep the starting rpm low and let the oil work before
moving. If it is cold the oil will be so thick that some oil passages
will be plugged, you will get squirts instead of sprays and there
won't be much splash for the splash-lubricated parts. High starting
oil pressure is indicative of plugged passages.
If you have ever taken an air filter and cowling off and observed
the action and location of the throttle (accelerator) pump
vs the prime (look where the primer activated fuel goes in) you
will see the wisdom of NOT pumping the throttle to prime.
Also it is important to prime and then IMMEDIATELY turn the starter,
as even he primer fuel will drain down to the carb
and intake box. The intake box is where you DO NOT want the fuel
to be. I am talking about typical Lycoming carb on
the bottom trainers and the like.
Abusive engine operation creates thermal shock to the engine
and its parts. Ideally every engine would be preheated. 110V preheat
systems now exist. Pre-oilers are in existence. Every cold engine
start has metal to metal parts scraping each other without oil
film separation. Abrupt throttle operation, cowl flaps, and rpm
adjustments cause variations in heat and cooling sufficient to
damage even the most rugged of engines. Do what it takes to keep
the engine warm within its operating temperatures and avoid extremes
of heat and cooling.
Keeping an engine log can prevent some failures. Keep a record
of fuel flow, oil pressure, temperatures, and electrical
readings. Record oil consumption and changes. Have the oil lab
checked for metal to determine what and how much
wear is occurring. Some is normal but too much of one kind is
significant. Poor lubrication causes excessive wear.
You can take better care of your engine if you avoid those throttle
changes that cause sudden heat changes. Abrupt throttle movements
cause cylinder head, exhaust header and turbo cracks. A sudden
power application or sudden shut down (As when stopping) can cause
bearings to coke up, overheat and seize. Counterweights can be
de-tuned by sudden throttle movement.
Under certain conditions an engine may be unable to obtain fuel
to the carburetor due to vapor lock. If the fuel lines have
curves that allow the formation of hot air pockets the fuel may
be unable to force its way through the blockage. A hot engine
compartment may make it impossible to start the engine and under
certain conditions the blockage may exceed the ability of a pump
or gravity to move the fuel. A dent or a crimp in a fuel line
can decrease fuel flow ability. A dent in a fuel injector line
is particularly dangerous where takeoff performance is required.
A pilot who has predetermined power and configuration to be used
in a journey is reducing his piloting workload. He knows ahead
of time the Vy/Vy speeds to be used in climb. He has picked an
altitude for winds and fuel efficiency, he
pre-plans his descent to make the best use of altitude for airspeed,
and arrives at the pattern for an efficient arrival. Additionally,
he knows the trim, mixture and power changes before the needs
occur.
The bible of engine operation is the POH but each aircraft model
year varies so only a POH specific to the aircraft and year should
be used. When an aircraft is a mixed breed such as having a later
engine different than that of the aircraft year, you must develop
your own POH.
Lycoming has some very specific suggestions for high performance
aircraft. Don't lean below 5000' when using climb power. Don't
reduce manifold pressure over five inches at one instance. Better
to do one-inch every one-minute. Maintain 15 inches during descent
with rpm set to lowest cruise to prevent piston flutter. Fixed
propeller operations should be limited to 400-rpm reductions at
a time. Descents faster than high cruise and more than 1000 fpm
are not recommended.
Leaning is performed to get the maximum power with the best
air to fuel mix. Excess fuel will cool the engine, but too much
can dilute the oil needed to lubricate the cylinder wall. This
causes excessive engine wear. An excessively rich mixture tends
to foul plugs in smaller engines. A lean mixture will gives better
power, but excessive leaning will cause higher temperatures in
the combustion chamber.. Excessive leaning over a period of time
can cause serious engine problems and lack of power. Aircraft
with an exhaust gas temp gauge, (EGT) gives positive indicators
in the leaning needed to attain best operation. Without the EGT
lean by incrementally pulling the mixture until the engine noticeably
loses power and then put the mixture in until you get the max
rpm back. This setting is correct for a specific altitude for
that time. Any other
altitude or time requires new settings.
The knob at the front of a constant speed propeller (inside the spinner) is an oil piston that is controlled by the operation of the propeller knob (blue). This knob controls the amount of oil allowed into the piston. The piston sets the blade pitch according to the amount of oil allowed into the piston.
The twisting moment of the blade resulting from centrifugal forced tends to decrease blade angle. This effect is offset by the oil pressure from the prop control governor which acts against the linkage to the blades.
Adjacent to the piston is a type of flywheel which is a combination of weights and springs. This flyball is rotated by engine crankshaft gears that also drive the governor. Should the propeller change speed for some reason the flyball will open or close oil passage accordingly to the piston. This will change the blade angle to reestablish desired speed. One of the best ways to see this in action is do face at right angles to a stiff wind. The propeller will cycle repeatedly in this situation.
Propeller is always increased in rpm prior to addition of power as when beginning a climb. Power is reduced before the propeller when setting cruise. With constant speed propellers we enrich and bring up the RPM before adding power. Reduction of power reverses the process, power back, set the RPM and then lean.
Primer
Primer is a hand operated fuel injection system. It injects
fuel into the engine while bypassing the carburetor. The plunger
has a pin that is keyed to hold the primer in place during engine
operations. If the key is not properly locked in place the
engine will draw fuel through the primer and run excessively rich.
I know of a situation were the pilot failed to lock the
primer and several hours of mechanic time was spent trying to
find the problem.
Engine
maintenance
--Reliability is related to maintenance of oil/filter changes,
air/fuel filters, magneto timing, spark plugs, ignition harness,
baffling and seals.
--Any control cable housing should be clamped every 12 inches
to prevent and repeated flexing that will ultimately cause a
break. Pilot should be taught not to force any control into making
such a flexing pattern.
--Propeller ground strikes will not only damage the propeller
but the internal engine parts connected to the crankshaft.
Taxiing across a shallow drainage ditch while taking the runway
can easily flex the shock absorbers sufficiently to
cause a propeller strike. There are those who make a living on
poor pilot techniques.
--Muffler internal failure can be abrupt or extremely gradual.
The former is easy to detect. The latter can be so insidious as
to evade any detection until aircraft performance degrades as
much as 30%. Adequate inspections every 100 hours
should resolve this problem.
--Within limits most propeller damage can be repaired. A qualified
mechanic or shop should do all repairs of even the
most minor dressing out of nicks and scratches. Limit any pushing
or pulling of the aircraft by holding as close to the
spinner as possible. Pulling from further out on a blade can give
sufficient leverage to create a stress riser (crack).
--A hard starting or 2-minutes rough engine usually indicates
a sticking valve. This is a major repair problem that if
neglected will lead to a total stoppage.
--The consistent fouling of spark plugs may indicate worn rings,
improper plug heat range, a shorted ignition harness, a
cracked cigarette (connects to top of plug), wrong fuel, or incorrect
timing. More likely it will be improper leaning
during low power operations and idle. Typically such fouling can
occur during full rich gliding descents where
excessive cooling occurs.
--Unfiltered air ingested into the engine will cause severe damage
to the cylinder walls. The proper fit of the air filter is
essential. Use of carburetor heat on the ground should be limited
to removal or ice and heat checks
--Cylinder compression checks require replacement if readings
are below 60/80 psi. The ideal is 80/80. An experienced
mechanic either through the exhaust or at the air cleaner can
hear air leaks.
--Induction filter in good condition will screen out 98% of impurities.
--Mixed with dirt, oil becomes a powerful abrasive.
--Induction filters are best changed or cleaned every 25 hours.
--Some aircraft have alternate air doors that open if the filter
becomes clogged. Enter unfiltered air.
--As engine cools water vapors will condense and mix with the
oil in the crankcase.
--Corrosive exhaust products become acidic when mixed with water.
--Oil additives are blended into the oil to inhibit and neutralize
these acids.
1. A trend monitoring can predict a failure mode before it
happens.
2. Internal engine deterioration can be determined by oil analysis.
3. Keeping a record of compression checks will track cylinder
condition.
4. Accessories such as magnetos, harness, spark plugs, exhaust,
alternator, belts, hoses, pumps should be removed for inspection
and testing.
5. A trend is only as valid as the RPM, oil pressure, oil temperature,
cylinder head temperature, EGT, fuel gauges and manifold gauge
are in
giving accurate measurement.
Starting a cold engine is just about the worst think you can
do to it. The colder it is the more damaging the start. The culprit
is lack of lubrication. Very little oil gets to the cylinder walls
during the first few critical revolutions. The cause is lack of
lubrication so that metal to metal contact results in excessive
bearing and ring wear, oil burning, and piston slap.
An engine seldom flown with long non-operation periods between
is subject to corrosion. Lycoming recommends over haul after 12
years regardless of use factors..
1. Use cylinder head temperature gauge. Gradual power reductions
2. Use slight power reductions for cruise descents
3. Avoid touch and goes on cold days
4. Carry power throughout landing approach
5. Be as slow and smooth on the throttle while doing airwork as
possible.
6. Keep mixture leaned until leveling off in pattern
7. What is good for the cylinders is good for the engine.
8. Use power only as required.
9. Minimize full power operations
10. Use oil analysis program
11.Follow manufacturers recommendations
Engine
operations to avoid:
--Rapid throttle movement
--Wide differences between manifold pressure and engine speed
--Excessive RPM and power
--Sudden cycling of the propeller
--Sudden stoppage
--Avoid inactivity over one week.
--Use low quality oil.
--Not keeping the air filter clean.
--Minimize ground running.
--Climb at speeds sufficient to cool engine.
--Avoid high speed descents.
--Practice good leaning procedures for all operations.
--The older the engine, to more it shows wear.
--Running past TBO increases wear rate.
Using the Engine
--Aircraft engines are reliable but expensive to repair and
replace
--Improper care and poor operational practices keep aircraft engines
from making it to TBO.
--Engines wear out more from lack of use than from frequent and
heavy use.
--Rust and corrosion damage bare metal surfaces that do not get
the lubrication of frequent use.
--Oil, over time, will drain away from high to low and eventually
into the crankcase.
--After seven days of non-use the moving components will make
their first moves without oil.
--After fourteen days of non-use rust and corrosion occur.
--Rubber parts lose resiliency and become permanently deformed
when not flexed often
--Many non-metal parts require oil to 'wet' them to prevent drying
out.
--The engine has openings that allow hot gases to escape and allow
moisture into the unused engine.
--A minimum of 30-minutes at full cruise every seven days is required.
--Ground runups do more harm than good. The engine never gets
hot enough to purge moisture.
Ground running
--Air cooling takes place by way of the pressure cooling of the
relative wind.. Propeller is a poor second.
--The lack of cooling by ground operations is damaging the more
it is done.
--Facing the wind, minimize runup, no high power runups, low idle
before shutdown
--Propeller at fine pitch for all ground operations except when
cycling.
--Always open cowl flaps on the ground.
--Never run engines at high power with the cowling removed.
--Engine accessories are damaged by excessive ground running.
Cold Weather Operation
--Aviation fuel is not blended for seasons. Excess priming
will wash cylinder walls of oil.
--Maintain idle speeds until engine warms.
Power-off Descents
--High speed descents with rich mixtures can and will damage cylinders.
--A windmilling propeller will drive the engine and detune the
counterweights or create internal flutter.
--Maintain enough power in descents to maintain temperature at
least to the bottom of the green.
--Lean mixture gradually during descent
Magnetos cause fewer that one accident a month. Most magneto
failures are due to neglected maintenance and servicing. As much
as 15% of engine power is lost through the application of carburetor
heat. The heat enriches the mixture thus causing the loss. For
every 10 degrees of heat above standard (59F) there is a one-percent
power loss. Application of preventative carburetor heat during
conditions conducive to carburetor ice requires leaning the mixture.
A dead or weakened magneto costs at least 3 percent of your available
power. Electrical leaks from the sparkplug harness will cause
a loss of power. This type of loss is more likely at altitude.
Plugs are connected to the harness by a "cigarette' like
connector. A dirty "cigarette" or plug barrel can cost
you two percent of your power. Never touch a clean "cigarette".
Loose intake manifolds let in air and lean the mixture when richness
is required as during takeoff and climb. If the carb heat flapper
valve in the air box does not make the proper seal it will cost
you power.
Engine Warnings
30% of single engine aircraft accidents involved a loss of
engine power. As an engine ages it loses compression. All cylinders
do not lose compression equally. When piston rings, cylinder walls
and exhaust valves fail to perform within design limits, it is
time for an overhaul. Any one of the above failings is not going
to cause an accident. Carburetors wear out and require overhaul.
Fuel pumps have an average life expectancy. Oil pumps have a limited
pressure adjustment. Cylinders operate at widely varied temperatures
due to uneven fuel flow. You have no way of knowing about this
without instrumentation.
Compression checks can be used to determine if the valves, and
rings, are producing adequate sealing. Excessive pressure escaping
reduces available power. The compression check is done with the
engine at operating temperature with readings made of all cylinders
using the starter to turn the engine. A 15-pound variation in
cylinder pressures is the maximum limit. Valve leakage is the
most common weakness. In addition to the compression check the
condition of the spark plugs are indicative of problems. See spark
plugs.
Low oil pressure is symptomatic of problems. If a bearing moves
from its proper position, more than the required oil will pass
by it and reduce the amount of oil for other parts. High-tech
oils may be too slippery for an older worn engine. Difficulty
in starting may be due to sticking valves that have warped due
to heat.
The sounds of the engine or its accessories from clanks to grinds
to whines are like cries of pain. A change in sound not caused
by throttle movement warns of greater problems to come later.
Get on the ground. You ability, as a pilot, to detect subtle changes
in performance, sound, or feel will give you time to get down
successfully. When the aircraft has engine-monitoring gauges,
learn what is normal. Any change from normal is a warning. A change
in an exhaust gas temperature gauge (EGT) or a cylinder head temperature
(CHT) gauge is indicative of combustion problems in the cylinder.
One way to check the condition of an engine is by several different
compression checks. Applying 80 pounds of pressure to the cylinder
while measuring the pressure retained makes a differential compression
check. A 60/80 reading means that 60 pounds of pressure is retained
for the 80 pounds applied. The leakage can located by listening
at the exhaust pipe, induction air intake, breather tube or oil-filler.
A 20/80 reading or worse signifies a probability of a real problem.
A dynamic compression check has the plugs removed to allow starter
to attain a maximum speed. If one cylinder checks 15 pounds below
another there is a problem. Engine power is the result of good
maintenance. Total time on engine is not necessarily, indicative
of engine capability. Frequent oil changes are essential to engine
life.
Top overhaul does not include rocker arms, pushrods and tubes plus everything in the bottom of the engine-bearing, cams, gears, bushing, and crankshaft. The top does not include the alternator, starter, and other accessories. Top overhaul costs less than half of a major overhaul. Lack of use is the most damaging factor in engines. Without use engines die from corrosion and dry starting. Excessive leaning is likewise bad on engines, since the engines run hotter and heat is damaging to engines. Signs of needing a top may be seeping oil, low cylinder pressure, oil consumption or poor performance. The FAA does not consider a top as affecting the TBO.
TBOs are best case and more often than not never reached. Engines worked hardest seem to last longest. Optimum use is about 500 hours per year. Heat ruins engines so running too lean can cause a heat problem. Break-in should be conducted at highest allowable power setting with minimum at 75%.
1. Carburetor heat/alternate air (-10%)
2. Temperature 1% loss for every 10 degrees above standard.
3. Rich mixture
4. Altitude (For each 1000' a 25% increase in takeoff distance.)
5. Single magneto operation causes 3% power loss.
6. Wet air merits additional 10% required takeoff distance.
The exhaust valves are most likely to give notice to the pilot that there has been a heat, corrosion or stress problem. Even though such values are made from nichrome, silchrome or cobalt-chromium steel, pilot abuse can cause a failure of the designed lubrication and resulting sticking or bending/breaking. The valve face and seat are made from stellite for increased resistance to the factors just mentioned while the valve stem is likewise hardened. The valve is lifted and allowed to re-seat by a cam on the engine driven camshaft and a return spring.
It is most likely that piloting practice has caused a valve problem. Oil quality/quantity, leaning practices, and climb/descent procedures affect engine cooling. Try to start an engine as slowly as possible. This allows time for lubrication to build up pressure and penetrate into the valve guides. Climb power uses excess fuel for cooling but other flight regimes should be leaned so that excess fuel does not put lead-sludge into the oil. Cylinder fins, fuel, baffles, and throttle usage.
Valves usually stick in the valve guide. This shows to the pilot by an engine hesitation or miss. Something is making it so the valve cannot move freely. These may be bits of carbon or cooked oil. Early detection and correction of sticky valves is important.
An engine or a piston that is new should not be run at low power. The proper seating of the piston rings requires sufficient pressure to seat into the cylinders. The cylinder walls need to be given a sheen that will allow the oil film to separate the wall from the ring's surface. Any area that is not separated, lubricated and cooled by oil will become hot. Heat is required for engine operation, excessive heat is the greatest enemy of continuous and successful operation. A proper break-in is essential if an engine is to reach its design time before overhaul (TBO). If the break-in allows lacquer and varnish to accumulate on the surfaces of moving parts, you will have problems of oil consumption and engine temperatures. Multi-viscosity mineral-based oils are less likely to form lacquer and varnish deposits.
The way an engine is broken in has much to do with its ability to reach its TBO. Many engines were run to failure to determine a mean time before failure. (MTBF) This time was cut by 50% to set an initial TBO time. TBO could be extended in experience proved it safe. Most important factor in reaching or exceeding TBO has to do with use and maintenance. Unused engines will accumulate water, which mixes with the acids of combustion to cause corrosion to the engines internal parts. Normal flight use will clean the engine and allow the oil to carry off the corrosive acids and will evaporate the moisture. The less an airplane is flown the more frequently the oil should be changed. TBO is an average that many well used engines can be expected to exceed. The principle reason for changing oil is to remove oil containing suspended impurities. An engine can be run past the TBO, but only in 100-hour increments. This could result in a more expensive overhaul since wear increases with age.
Given enough time heat, friction and load will wear even the best-maintained engine down. The manufacturers estimate of engine life is just that, an estimate of expected service life. An engine that has regular changes of oil and filters may live an extended life if flown regularly. Keeping records and track of the oil condition helps. Baffles are essential to proper engine cooling. Check their condition at every opportunity.
Well-maintained engines will start promptly. I quit cranking if after six-blades the engine is not running. Cranking time is the most trying time for an engine. Keep the engine filter tight and in good condition and minimize C.H. on the ground. Lean for taxi, and whenever you can to prevent excessive rich mixture dilution of the oil. Don't force the engine to takeoff power before it has a chance to warm up.
Throttle changes should be smooth and gentle. Avoid disuse that can cause corrosion c
The system is made as light as possible and formed to minimize space at the lowest possible cost. Exhaust systems are subjected to extreme temperatures, temperature changes, and vibration. The exhaust takes away hot engine gases to prevent damage to engine components and provide maximum breathing capability for the engine. The aircraft manufacturer makes some systems while the engine manufacturer makes others.
For smaller aircraft engines the system is made of 321 stainless alloy. The average life of a system is about 1000 hours. Reparability goes down with age. Important inspection areas require removal of shields. Damage directly related to condition of engine isolators (rubber doughnuts on engine mounts) and out of balance propellers. Baffles of interior are weak spot. Best repaired by specialist shop. White stains at cylinder heads indicate leaks. Critical inspection is around the heater muff. Cracks in the muff can cause carbon monoxide to enter the cabin via the heater. Internal parts of the muffler baffle can break lose and plug the exhaust breathing capability. Only a pressure check can determine if a muffler is internally o.k.
Where the exhaust meets the cylinder is a flange that requires
a gasket to keep gases from escaping. This is an area requiring
frequent visual inspection. Problems are cause by bolts not being
tightened correctly. Certain parts of the system have slip
joints that are lose only when the engine is cold. Age is the
determining factor. The older the system the more internal erosion,
fatigue, and stress will lead to ultimate failure. Just like people.
Cockpit heating is done via a heat exchanger welded around the
exhaust manifold. Engine heat is transferred to the ducted
air from outside and this heated air is again ducted into the
cabin. Any obstruction of this system is likely to reduce engine
power. If every, you sense unexplained power loss and greatly
reduced fuel consumption with smooth engine operation
while leaned, suspect that the baffles of the muffler have come
apart inside. In some aircraft it is possible to inspect this
condition with a flashlight.
Cooling created by the aircraft engine baffles and deflectors make up as much as 30% of an airplane's total drag. This cooling is required because of climbs in hot conditions. Cooling is predicated on worst case conditions. If the cooling system fails, the lubrication system will fail. Air is the most common cooling method because it is so available and so inexpensive. There is no need for a recovery system and it adds no weight. Air can cool simply by its velocity over the engine or can be 'pressurized' by the design of the air intakes, baffles, and low-pressure escape routes.
The design of the cylinders with cooling fins allows easier heat transfer. Additional cowl flaps can be used for additional cooling during climb. The fuel air mixture can be used for additional cooling. Pointing the aircraft into the wind during runup is a good operational practice. Care should be taken not to cool the engine too fast by rapid descents with the power off. Shock cooling can be avoided by reducing the throttle in stages every thirty seconds or so.
If the cooling system fails, the lubrication system will fail.
Air is the most common cooling method because it is so available
and so inexpensive. There is no need for a recovery system and
it adds no weight. Air can cool simply by its velocity over the
engine or can be 'pressurized' by the design of the air intakes,
baffles, and low-pressure escape routes.
The design of the cylinders with cooling fins allows easier heat transfer. Additional cowl flaps can be used for additional cooling during climb. The fuel air mixture can be used for additional cooling. Pointing the aircraft into the wind during runup is a good operational practice. Care should be taken not to cool the engine too fast by rapid descents with the power off. Shock cooling can be avoided by reducing the throttle in stages every thirty seconds or so.
An aircraft engine spends much more time developing near full power than does an automobile engine. The wear on an aircraft engine is made shorter through negligent operation, non-operation, corrosion, and the shocking effect of hot and cold cycles. Shock heating cycles the metals of an engine just as much as does shock cooling.
Heat shock can be reduced by starting the engine at idle leaning to reduce oil dilution by excess fuel and then allowing the oil pressure to rise before aggressive leaning. The start of an engine its most damaging cycle of operation.
A sudden reduction of engine power after a period of increased power causes a rapid reduction of engine heat being generated. This heat change inside the cylinders is reflected in the heat released by the cooling fins and increased cooling airflow through the engine plenum. The result is called shock cooling. Lycoming says that shock cooling results in worn piston grooves, broken rings, warped exhaust valves, bent pushrods, and plug fouling. Recommended cooling rate is no greater than 50-degrees per minute.
Shock cooling occurs when the pilot reduces power to off and goes into a descent. The effect of this is to suddenly reduce the internal heat of the engine and greatly increasing the cooling effect of the air over the cooling fins of the engine. This may be a damaging shock to the bimetallic cylinder blocks. The principal effects of shock cooling are cylinder-head cracking, valve seat to valve seat, plug to plug. Anywhere inside the engine where tool marks, sharp edges and other stress points are liable to damage. Any engine operation that makes it possible for the valve guide to shrink faster than the valve will cause sticking. Valves stick open and the pushrod bends. A raised valve hits the piston dome, breaks and is redistributed throughout the engine and turbo if any. This situation often occurs when poor navigational planning causes the pilot to arrive over his destination at several thousand feet too high. Never make descents that will shock cool the engine. It may not fail on your but it will on some pilot down the road.
To prevent all these bad things from happening to your engine keep some power on the engine, re-lean during altitude changes to keep the EGT near cruise values. If you have CHT on all cylinders maintain a controlled (slow) decrease rate. Use of factory CHT on one cylinder is a very poor second. Regardless, always reduce power in increments so that engine temperature changes will be gradual. Retard the throttle during descents. Do not enter a descent that will both give a throttle reduction and an increase in engine cooling air. Use landing gear and flaps to keep the speed down. control the thermal changes of the engine to limit temperature and cooling related damage.
When on the ground, take advantage of any cooling wind, lean the mixture, open cowl flaps on the ground and during climb. All engines should be run for at least two or three minutes on the ground after a long flight to allow the oil to carry heat away from the engine. In hot weather or with a turbo engine allow more time. Before killing the engine run it up to 1200 and lean to but not into roughness for 20 seconds. This will clean the plugs from any residue of lead or carbon.
Heat makes an engine work. An engine burning ten gallons of gasoline an hour gives off as much head as would be required to boil 750 gallons of water. Less than 40% of this engine heat produces work. Over 60% is wasted and must be taken away. Controlled heat makes it work better and longer. If removal fails to take place the engine will fail in short order. Allowing oil to come up to operating temperatures removes trapped moisture and insures oil coating of the engine parts.
Nearly 50% of the energy of an aircraft engine is wasted out the exhaust stacks. 30% of the fuel is used in cooling, pumping and friction factors. Only 27% of the fuel's chemical energy turns into horsepower.
Most of the working heat is removed through the exhaust system. The rest remains in the metal of the engine waiting to be removed by circulating oil and cooling air. The engine has numerous well-placed holes, sprays, and reservoirs to facilitate the flow of oil. Oil lubricates but cooling is a big part of its function. The engineering of the engine and the aircraft controls the airflow over the engine. The engineering of an engine has cylinder fins and baffles placed to dissipate the engines operational heat continuously and evenly. Even the propeller spinner is a factor in cooling. Cowl flaps, air intakes, cowling openings and baffles are designed so that aircraft and engine are mated to provide the cooling the engine requires.
Recent aircraft/engine combinations have used exhaust augmenters to provide a pressurized flow of air over the engine. This eliminates flaps and allows a more streamlined aircraft with smaller cowl openings. The latest propellers have a cooling projection near the hub
To achieve maximum service life, Lycoming recommends limiting power to 65% instead of the more common 75%. Cylinder head temperature should be below 400-degrees and oil temperature between 165 and 200-degrees. Recommendation is to lean to 100-degrees F rich for best power; peak EGT for best economy. Engine roughness is caused by EGT or traditional leaning that causes one cylinder to fail first. Always enrich for smooth operation. Below 5000' density altitudes takeoffs require full rich mixture. Whenever mixture is adjusted, rich or lean, it should be slowly in increments with pauses between. Do not increase power settings without slowly setting mixture to full rich. To reduce shock cooling, avoid power/mixture changes that cause greater than 50-degrees;F changes per minute. Watch temperature instruments.
Magnetos
The magneto is a self-contained voltage amplifier designed
for constant rpm. It provides its own electrical energy. Magnets
on the rotor shaft set up a magnetic field in the coil. through
timing the right spark is distributed at the right time. Three
internal circuits are involved. The primary is of relatively heavy
wire and few turns. This primary coil has 1-200 turns. Combined
with a condenser and a powerful magnet this circuit produces a
high current (amperage). At a precise moment in the cycle the
primary circuit is broken and the electro-magnetic flux field
collapses and cuts through the thousands of thin wire coils in
the secondary system. The secondary coil induces voltage into
the 15,000 turn secondary coil. This sends a 20,000-volt surge
toward the spark plug. This produces a critically timed high voltage
but low current surge, which arcs across the points of the spark
plug. It uses a mechanical spark advance, is independent of the
electrical system, and is driven by the engine.
The faster you pass a magnet past a wire the higher the current. The magneto secondary coil is used to create a current that cuts across the primary coil of numerous fine wires. The secondary coil greatly multiplies the voltage and it is delivered to the spark plugs. The timing of the voltage to the plugs is done by a rotating magnet, which makes a brief contact that allows the high-voltage to leave the coil and reach the spark plug.
Starting the engine with the starter causes the magneto to initially retard the spark and activate a spring loading device called an impulse coupling that supercharges the initial starting spark to the plugs. The magneto generates the electricity needed by the spark plugs and engine operation. For ease of starting, two different systems exist. The impulse coupling would allow the magneto to rotate faster than the engine. This increased the voltage and thereby the size of the spark to the plugs. The timing is adjusted (retarded) to provide maximum starting opportunity. The second method used a vibrating relay to create a rapid series of sparks to the plug during the start. This extended the ignition spark exposure time to the fuel.
Bendix and Slick are the major manufacturers. A mechanic can repair Bendix. The latest Slicks are sealed units not for local repair. Many ADs and SBs on magnetos exist. Points and gap inspections required at 100 and 500 respectively. Bendix recommends dissembling and inspection every 500 hours. Lack of use causes interior deterioration. A special non-arcing bearing lubricant is required for magnetos. Change magneto and harness at engine TBO cycle. Magnetos are usually only checked for operation at 100 hour and annual inspections. Never, never hand turn a magneto to watch the spark during overhaul and avoid shocks that will degrade the magnets. Since 1985, magnetos are cited as cause/factor in 92 accidents. 130 reports of deficiencies (cracks, arcing, leaking) in magneto ignition coils have been filed since 1993.
The magneto uses a permanent magnet coil, a condenser, and timed-gapped points to generate a high voltage (25-30,000 volts)/low amperage spark. This spark is sent through a rotor to the spark plugs. The arc of the spark across the tip of the spark plug ignites the fuel/air mixture. The timing of the ignition of air/fuel is, when once set, an effective and simple method of running an engine. Failure of a magneto is gradual over a period of time. Failure of an electronic magneto is instantaneous.
This mechanical system does not age in the manner of electronic systems. The latest electronic systems are piggybacked on the magnetos and give 10% greater fuel efficiency because the timing and spark can be varied by engine requirements. The electronic microprocessor can detect a fault or electrical failure and allow the magneto system to take over engine operation.
Early magnetos were unreliable so aircraft were equipped with two parallel systems. Now dual magnetos are used for better fuel combustion. When properly functioning, the dual system gives better fuel efficiency. The slight drop in rpm when switching between the dual systems is due to loss of this combustion efficiency. One spur gear running both magnetos reduces reliability (PA32, C-182RG, and Mooney 201). Gearing rotates the magneto at engine speed (1.5 x for 6 cylinders).
Dual magnetos produce prolonged ignition (microseconds) that starts two flame fronts when the piston is well advance of top-dead center. A single system retards the combustion and prolongs the burning. The magneto points and rotor cap send the voltage to the plugs in the correct sequence. 12 sparks per second in 6-cylinder engine) Prolonged burning causes higher combustion temperature and detonation. Not a problem at 75% power but under takeoff and climb can cause engine damage. A frequently flown aircraft will be less likely to have a magneto problem.
One of the hindrances to improved ignition systems is that such a development might suggest to the legal profession that the existing system is less than safe. Magnetos are quite wasteful in their operation. Only during full power operations are magnetos operating at top efficiency. The timing of the magnetos remains the same even at low power operations such as taxiing. The magneto and engine operation, once started, is completely independent from the aircraft electrical system.
The electronic systems of automobiles are designed for variable operations not as important to aircraft. The relatively low rpm of aircraft engines do not require electronic ignition or multi-barrel carburation. The spark advance of the magneto is not set to optimum, it is fixed, and it decreases power and fuel economy. Starting is made easier if the aircraft has an impulse coupling which can vary the speed of the magneto. Magneto timing is adjusted by varying its mounted position.
Magnetos are designed to last as long as the engine. Magneto maintenance is normally only done when a failure occurs. Points out of adjustment on spark plugs and fouling are factors that place heavy loads on magnetos for which they are not designed. When the magneto has difficulty firing the plugs, the voltages try to find another way. This other way is usually inside the magneto itself through the insulation of the secondary coil. Inline noise filters on magneto leads will create an electrical imbalance resulting in advanced timing, points burning and subsequent weakening of the magneto.
A rough engine during run-up means that one cylinder is not firing. Plug fouling is the problem. With the 100LL fuel now being used in engines designed for 87 octane this is a common problem caused by failing to lean during taxi. Fouling can be 'cured' by leaning the mixture. First increase the RPM to 2000+ and then slowly pull the mixture. This will increase the cylinder internal temperature sufficient to vaporize the lead/carbon deposits. Reduce power to 1700 after a minute and check magnetos again. It is proper to lean an aircraft engine any time the power is 75% or less.
Magneto maintenance should at least consist of setting the spark plug gaps every hundred hours. The wrong gap can cause the magneto's high voltage seeking other routes through the wiring insulation. It is well every 500 hours to clean the inside of the magneto case and check the breaker point gap. Internal corrosion can occur if the inside of the case is not vented for fresh air and the removal of moisture.
Leaning for taxi and low power operations helps keep the plugs clean. Check the P-lead for its ability to shut down the engine every few shutdowns. Do this by turning the switch to off to confirm that the engine will actually stop running. Switch the engine on before it dies completely. If it dies, let it remain dead and make a normal restart. A damaging backfire is possible if the magnetos are turned on too late.
When the key is in the off position the expectation is that
the magnetos are shorted out electrically so that any turning
of the engine or propeller will not start the engine.
Opinion on Magneto Check
From the Sky Ranch Engineering Manual by John Schwaner: "To
check for a hot magneto, reduce RPM ti idle and turn switch to
OFF to see if engine dies. If it keeps running, beware of a hot
magneto."
The "Top End" volume of Light Plane Maintenance's "Firewall
Forward" library has a long discussion of checking for hot
mags...too long to quote, but it goes along with the "turn
it to OFF" procedure.
The thing to remember is that if the engine quits when the key
is turned OFF, it should be left there, not turned back to either
L or R, and the airplane towed back to the shop. If that is not
feasible, use the normal starting procedure and get it to a mechanic
as soon as possible.
Bob Gardner
The P-lead is part of the primary coil of the magneto. If the magneto will not stop the engine it means that the P-lead is not grounding out the magneto. Voltage can go to the secondary coils of the magneto and thence to the spark plugs only through induction from the primary coil. If the P-lead is unable to ground through the starter switch 'off' position the engine will continue to run as long as fuel is provided. Occasionally make a magneto check for a defective contact or broken P-lead. When the magneto switch is to OFF the system is supposed to be grounded so any turning of the propeller will not activate the magneto voltage and start the engine.
The harness of the engine consists of multi-layered insulated wiring from the distributor to the spark plugs. The positioning of the harness protects it from engine heat and weather. The construction of the harness reduces electrical radio interference and other magnetic effects.
The spark plug is made up of a ceramic insulator, which protects the electrode and acts as a heat sink to cool the plug. the outer casing of the plug is made of machined steel threaded to fit into the cylinders. A copper washer completed the pressure seal. The electrode carries the voltage from the harness to the gap sized to produce the maximum arc size and heat.
The spark plug gets the burst of high voltage produced by the magneto via the distributor and harness at a timed moment to produce an arc of flame that will ignite the fuel air mixture in the cylinder. A propeller approaches 2500 rpm nearly 20 arcs at 30,000-degrees F cross every plugs electrode every second. The cylinder gas pressures will exceed 2000 pounds per square inch.
Spark plugs must be matched to the engine according to the desired and required heat range. It is a violation of the FARs to use a plug other than specified for aircraft engines. A hot plug may be used if your engine runs cool. A cold plug is used if the engine runs hot. A colder plug is subject to combustion deposits of carbon and lead. It is only at temperatures below 800 degrees F that these deposits are likely to form. The burning off of these deposits during runup occurs when a rise of just 100 F degrees by leaning will vaporize the deposits. It is always best to taxi with a leaned mixture. Fouled or burned plugs make an engine hard to start. Iridium tipped plugs cost twice as much but give three times the life. Greater care is required in cleaning iridium plugs.
The best way to avoid lead fouling is by using one ounce of Alcor TCP per gallon of fuel. TCP is known as a lead scavenger. By avoiding rich mixtures and sudden full movements of the throttle you give the spark plug temperature to increase with the increase in power. Lead fouling may not be removed by leaning when the plug temperature is above 1300 degrees. Removal may be the only solution. Carbon fouling is most likely to occur at low power settings with the plug temperatures below 800-degrees as when first starting or taxiing. Added power during runup or shutdown can be used to raise the plug temperature to burn off the carbon. The best preventative is to lean the mixture at every opportunity to keep the plug in the proper heat range that will keep carbon fouling away. When all top plugs show wet oil it means excessive wear on all cylinders and guides.
Improper engine operation will cause lead fouling of the plugs. This results in a rough engine and low power. High voltages will seek out the weakest insulating point of the harness resulting from wear or moisture. Every time a spark plug fires the electrode erodes slightly. At some point the erosion affects engine operation, efficiency and starting. Coils can burn out, condensers short out and points burn and wear.
Carburetors allow the pilot to meter fuel into the engine. A part of this system is the idle system, which is rich, low power but separate in operation and adjustment from the other systems. Its richness cools the engine when airflow is least over the engine. Extensive operation at idle will foul the plugs unless leaning is an operational practice.
The accelerating system of the carburetor provides extra fuel when the throttle is moved . If this system is not properly adjusted the engine will hesitate with quick throttle movement. Overly abrupt movement can still cause the engine to hesitate. This system can be used to prime the engine during mile weather by giving the throttle a couple of rapid pumps. Any pumps beyond two can cause excess fuel to flow into the air intake. This is hazardous because the exhaust can ignite the fuel and create a fire in the engine compartment.
The existence of an engine start fire requires the pilot to immediately apply full throttle and pull the mixture This will allow the propeller to extinguish the fire and the engine to use all carburetor fuel very quickly. If the fire exists and the engine has not started the mixture should be pulled and the engine cranked to suck any fire up into the exhaust and air intake. Shut off the fuel selector, evacuate the cockpit and locate a fire extinguisher on one of the light poles. Part of your preflight should be to locate the nearest fire extinguisher.
The fuel-air proportions are adjusted by the mixture control. This adjustment is done based on temperature and altitude, both of which affect atmospheric density. Carburetors use either mechanical or back-suction methods to control fuel flow. The mechanical method restricts fuel flow. The back suction allows a low pressure to enter the carburetor, which reduces the pressure differential and draw caused by piston movement.
A full extension of the mixture control activates the idle cutoff. the idle cutoff stops all fuel flow and is used to stop the engine and reduce the probability of a propeller accident. The mechanical control shuts off the fuel while the back suction cuts off fuel by reducing the pressure differential to zero.
In most operations some fuel is used to cool the engine. This
additional fuel is added by the economizer system which applies
more to the engine than to fuel savings. The economizer system
operates by increasing or decreasing supplementary fuel flow in
conjunction with movement of the throttle.
Liquid aviation fuel will not burn in its pure condition. The
carburetor's functions to put air and fuel to the engine any where
from an 8:1 through 16:1 parts of air to one part of fuel, by
weight. 12:1 gives best power. The mixture is richest at idle
and goes from there through a range of leanness until it becomes
rich again at full power. The rich mixtures use some of the fuel
for cooling. Lean mixtures burn slower but hotter. Lean mixtures
can make an engine backfire when the fuel is still burning as
the intake valve lets the next draw of fuel to enter. Rich mixtures
cause an after-fire when unburned fuel is ignited in the exhaust
system.
The fuel mixture is pulled into the engine by the intake stroke of the pistons. This stroke creates a low pressure in the carburetor, which sucks fuel and air through the carburetor venturi. The carburetor has a constricted tube (venturi throat) for the air intake from either outside air filter or from the heater muff around the exhaust system. The constriction is a high airspeed/low pressure area that draws fuel from the carburetor fuel tank.
The proportion of the fuel is maintained as proportional to the air by a "butterfly" valve in the air intake throat of the carburetor. The "butterfly" is directly linked to the throttle. The less the "butterfly " blocks the air by moving toward a knife-edge the more air, fuel and power to the engine. Lycomings have a different intake system from Continentals, which make them less susceptible to carburetor icing.
The carburetor flows from the bottom to the engine. Water in
the fuel will settle to the bottom. Two ounces of water are enough
to make your engine quit. Contaminated fuel causes more accidents
that bad magnetos, blocked air filters and mechanical failures
combined. One test for water in the fuel required two people.
One person should hold the tail down while the other drains the
sumps. I did this after a plane had been flown all day and was
able to get water in the sump cup. The larger the aircraft the
more likely it is to occur. Try it.
The fact that airplanes normally operate at relatively high and
constant revolutions per minute (RPM), are not subject to rapid
throttle changes and power smoothness requirements means the carburetor
does not need automotive vacuum advances and throttle pumps. Aircraft
carburetors are single barrel. The spark from a magneto or an
added vibrator (shower of sparks) does the job in an airplane.
Having a variable mixture in an aircraft also helps the starting.
The intake manifold still makes certain cylinders to run richer
than others do. The system, for aircraft, is simple and reliable
and still in use in a fuel injection age.
During WWII the Germans used fuel injection in aircraft. This
gave them superiority during many aerobatic situations over carburetor
equipped aircraft. Today, fuel-injection systems are aimed at
fuel efficiency (5%). A mechanical pump parcels out the fuel evenly
to each cylinder. Timing is still by an "old-fashioned"
magneto. Fuel pump operations with fuel-injection engines must
be according to POH since, in some instances, the pump can kill
the engine with too much fuel. Fuel injection engines are subject
to choking caused by impact icing. This can be corrected by application
of alternate air before ice can freeze door shut. Automotive electronic
ignitions and injection systems are primarily for emissions control.
The operation of this system does not, at present, hold advantages
for aircraft.
Carburetor revisited:
A float type carburetor uses the vacuum produced by the
air drawn in through the venturi of the carburetor to lift fuel
into
the engine. This vacuum sucks the fuel through a jet with a metering
pin in it (adjust with the mixture knob) and into the
airstream created by the intake stroke of the piston. A throttle
plate (butterfly valve) which is down stream from the fuel
intake port meters the airflow. When you open the throttle plate
you momentarily reduce the level of vacuum and thereby
fuel flow until the RPMs pick up to bring the vacuum back up which
will suck the proper amount of fuel to maintain that
power level.
The carburetor has a plunger pump in it connected to the throttle
cable so when you push on the throttle it literally pumps in
a metered amount of fuel in the airstream to make up for what
isn't being sucked up by the vacuum. This is the accelerator
pump. Being too quick in pushing in the throttle can cause the
engine to sputter and then pick up. Smooth movements of
the throttle work better. Additionally, when the throttle is in
the last half inch of movement as for takeoff and climb, a
separate fuel jet allows excess fuel to flow, the sole purpose
of this jet is to provide additional cooling for the engine.
Airplanes have updraft carburetors where the carburetor sits
on the bottom of the engine. To get fuel to the cylinders, you
are advised to use the primer pump. The primer which pumps fuel
through a set of lines to some fittings which are screwed
into some holes which lead directly into the intake manifold of
the cylinders where it can go into the cylinder where it is used
to prime the engine. Using this primer reduces the likelihood
of an engine start fire. Using the accelerator pump with the
throttle can cause a fire by overfilling the carburetor.
Fuel Injection
With fuel injection, the throttle cable is connected to a
throttle body, which meters the air with a plate much as with
a
carburetor. This mechanism is connected with a rod to a unit known
as a fuel controller. The fuel controller meters fuel
through a valve that is controlled both by the throttle body link
and the mixture control. This fuel is then pumped up to
what is known as the spider which is the round thing you see sitting
on top of the engine with all the metal lines coming
out of it going to the cylinders. The spider distributes the fuel
to the injector nozzles, which are screwed into the intake
ports in the cylinder heads.
The fuel is sprayed into the airstream for use in the cylinder. The fuel pump pumps more fuel than will be pumped through the nozzle at any given pressure and more than the engine can use. For this reason there is a return line at the nozzle which returns excess fuel to the tank. To maintain the pressure where it needs to be, the return is metered through a jet sometimes called "The Pill". You can prime the engine with the fuel pump, while controlling the amount of the prime you use the throttle and mixture controls along with the pump.
Both the throttle via the butterfly valve and the mixture controls the ratio of air to fuel in the carburetor. Carburetor icing when it clings to the butterfly valve, which will decrease the airflow and decrease the venturi effect drawing fuel from the carburetor fuel jet. On the application of carburetor heat the warmer air will cause the mixture to become richer. It is possible that with a Carburetor Heat Temperature gauge that some leaning could be done to offset the effect of the warm air on the fuel/air ratio.
Leaning the mixture can reduce some of the additional roughness caused by the use of carburetor heat. This leaning will also increase the operating temperature of the engine and thereby increase the amount of carburetor heat available.
The air intake below the propeller has a filter that is much smaller and less effective than that used on automobiles. Accidents occur every year when the air filter is installed backwards and parts of it are ingested into the carburetor system. A stationary aircraft with the engine running has a propeller vortex that is putting dirty air into the air intake. For this reason you want to minimize operations where such dust is possible. Dust is like sandpaper once it gets into the engine and oil.
The cowl flaps allow an escape route for engine intake air
while allowing a directed flow over the hotter (upper) engine
parts during low speed operations. This is called downdraft cooling.
Small changes in the flexible baffling material can have large
effect on cooling efficiency. Airflow is supposed to make The
baffles flex into a tight seal, thus redirecting airflow to where
it is needed most.
Vacuum Pump
Poor life expectancy of dry type pumps characterized by instantaneous failure. The wet (oiled) pump functions better but is uncommon. They slowly fail over a period of time. Contamination of air in system is the greatest cause of failure. Low pressure should not be corrected by increasing pump setting. Replace or find cause for decrease. Pump must work harder at altitude. Such operations increase wear and frequency of failure. Tight cowlings and dirty filters are next causes of limited life. Pumps that last 20 hours usually last a thousand. Average life is 400-500 hours. Vacuum pump manufactures say that turning the propeller backwards also causes the vacuum pump to work backwards and is damaging. It is for this reason that it is best not to turn a propeller backwards. There is some question about this.
An additional question related to vacuum is how much will be available in the event of engine failure. A windmilling propeller may not turn fast enough to make the vacuum pump keep the gyros of the HI and AI functioning. In IFR conditions you might be well advised to cover both the HI and AI for this reason.
The effective use of an engine monitor depends first on being able to set the normal operational parameters. Secondly, you need to pick up abnormalities as they occur in your flight and ground operations.
The engine preflight with the magnetos confirmed off and the P-lead shorting out the magneto says that you pull the propeller through to listen for air leaks. The intake valves leak if the air sound comes from the air intake. Sounds coming from the exhaust indicate exhaust valve leakage. Hissing from the engine casing says you have a piston ring problem.
If on startup or even later, the engine backfires or is rough this should be interpreted as it's not being warm enough for full power operations. If the oil is too cold to allow a rise in oil pressure the engine can be damaged.
Oil
Oil seals, cools, lubricates and cleans. The viscosity of
a multi-grade oil allows these activities to be accomplished with
lighter molecules with much less of the lacquers and varnishes
that cause deposits.
All oils seem to lubricate equally well. An ashless-dispersant (AD) oil will prevent carbon build up by suspending wear particles for removal in the oil filter. AD oils give you a cleaner engine with unclogged oil passages. Price or additives are not good measures of oil. Oils are about equal although some are different. Any additive that is supposed to do a particular job will be wasted money unless present in sufficient quantity to do the job. You have no way of knowing if the job is being done.
You can learn what is going on inside your engine by reading
what oil can tell you. You will need to use your senses of sight,
sound, smell and feel.
--How soon oil gets black indicates the amount of blowby past
the rings.
--The acidic mixture of oil residue and water can only be removed
by oil changes.
--Black oil will lubricate but must be heated to 180 degrees to
boil off moisture.
--Hot oil that smells like exhaust indicates engine problems unless
over 25 hours old.
--Listening for air sounds at the dipstick or breather tube while
turning propeller by hand checks rings blow-by.
--Oil dripping from breather outlet is blow-by indicator
--Oil leaks must be bad before they are serious.
--Max oil burn: (.006 x HP x 4)/7.4 … A 200 H.P engine can
burn .65 qt. per hr.
--Track oil consumption rate.
--Cut open oil filter to study metal in oil.
Oil Filter
Best filtering has most restriction, which causes greater pressure
drop. Filter may have an adjacent valve to open when pressure
difference reaching a certain point of about eight pounds differential.
Any time idle oil pressure drops by 10 psi…check the filter.
Some of these valves have an associated cockpit light.
Oil is another area of change. Pilots and mechanics have rejected
the use of multigrade oils. There are now aviation grade multiviscosity
oils that retain viscosity at 250 degrees as do single grade oils
and become thicker at 300 degrees. Multigrade oils do not wear
out nor do they drain off engine parts sooner than 100 weight
oils. Single grade oils will be out performed by the multigrade
oils.
The purpose of oil analysis is to use the history of an engine's oil to detect small problems before they become large ones. Oil analysis uses a spectrometer to measure the particular colors and their proportions when a specimen is burned. The history of the colors (different substances) and a change can be indicative of a change in engine wear. By asking the right questions about oil analysis you can avoid unexpected large expenses by accepting smaller expenses earlier.
This is the most important instrument of engine operation. In normal conditions oil pressure will be indicated within 30 seconds of starting. the gauge measures engine resistance to the flow of oil. Oil pressure indications can be quite variable and unreliable in cold conditions. get the engine plenty warm when taking off in cold weather.
In trainers the oil gauge is mechanical. Gauge has small line to engine. Interior of gauge has Bourdon tube, which unwinds under pressure. Attached needle give reading of oil pressure. A reducer lets a limited amount of oil into a watch-spring shaped Bourdon tube. Oil pressure unwinds the tube and moves the oil pressure indicator needle. Pricy aircraft use an electric system. The oil pressure line is very small so that it will read zero after a break long before the engine oil is lost. This allows you time to note the reading and still more time to get down before the engine quits.
If the oil temperature goes beyond the red line but the oil pressure remains in the green the problem may be in the temperature gauge and not the engine. High temperature and low oil pressure calls for an immediate engine shut down. If you smell hot oil, shut down the engine. Too high pressures are indicative of too heavy oil grade or oil not warmed enough for high power operation.
Any sudden rise in temperature along with engine roughness is good cause to get on the ground. Rise in temperature without roughness is sign of low oil level. Good pressure and high temperature is sign of gauge error. Any internal engine cooling that takes place is done by oil. The size of your oil supply is the determining factor. No oil, no cooling.
The oil temperature gauge is a pressure gauge that uses a Bourdon tube. Inside the tube is liquid methyl chloride that expands when heated. This allows the Bourdon tube to wind and unwind with changes of oil temperature. Gauge is usually electric and usually accurate. High temperature readings can be confirmed by hot oil smell.
The gauge on a Cessna is a dc meter movement connected to a variable-resistance sender mounted in the crankcase near the varitherm. Its resistance decreases as the temperature rises, causing more current to flow, and thereby causing the meter reading to increase.
The wire between the meter and the sender were to fail, the meter reads cold. If a short occurs then the meter would read to the highest temperature. Meter movements do not become more sensitive on their own. So its much more likely that when an oil temp gauge shows hot, it is hot.
This is installed after flight testing on the hottest cylinder
unless it
is installed on all. This gauge gives a faster more accurate indication
of
engine temperatures. Detonation and preignition produce a rapid
rise in CHT to be followed by rising oil temperature. The engine
may be damaged in this situation very quickly.
This device uses a temperature probe to determine if the carburetor is subject to icing. A color-coded scale of green, yellow, and red indicates probability of ice. Adjust heat to keep needle in the green.
Green is normal; red is never exceed limit. Gauge is usually mechanical and prone to low readings. Numbers are usually high at low speeds and low at cruise speeds. False tachometer readings are major cause of inaccurate fuel calculations. The tachometer should be used to avoid exceeding the redline operational limit. Forces on the engine parts increase greatly as engine speed rises. The tach is used to check magneto operation and adjustment during runup. The tach shows when plug fouling has occurred and when it has been corrected. A falling tachometer is the first visual indication of carburetor ice except in constant speed propeller aircraft.
A flexible shaft geared to the engine drive system mechanically drives trainer tachometers. The drive cable has a rotating magnet on the end that drives the tachometer dial. Tachometers are relatively inaccurate and read too high at the low rpm and too low at the high rpm settings. This inaccuracy makes it wise to plan fuel consumption on the safe side. Age affects the tachometer accuracy and may cause a pilot to operate at the very speeds he should be avoiding. Only AC Type ST-640 grease should be used on Cessnas. This is a G.M. product.
RPM
Because tachometers are so inaccurate, the only way you can fly
with any assurance that you are using the same rpm over two given
same courses is by flying at full power. Your tachometer usually
reads low.
These are thick rubber pads that go between the firewall and the engine mount. There are used to reduce the engine/propeller vibration transmitted to the aircraft frame.
Manufactured of rubber compounds and have limited service life. Rarely last 10 years or to TBO of engine. Harden with age and do not provide protection. 100-hour inspections are a good time to rotate to even wear. Isolators are subject to damage from oils and fluids, which cause swelling and loss of elasticity. Heat will age and crack. They should be checked by finger to see if hard or spongy. They have limited shelf life and are dated at manufacture. Don't use undated isolators.
Any time the nose wheel shock system is not functioning properly every jar from the ground is transmitted to the rubber engine isolators. As these in turn lose flexibility the ground shocks are sent direct to the engine where cumulative damage occurs.
Engine
Shutdown by Lycoming
"Prior to engine shut-down the engine speed should be
maintained between 1000 and 1200 RPM until the operating temperatures
have stabilized. At this time the engine speed should be increased
to approximately 1800 RPM for 15
to 20 seconds, then reduced to 1000 to 1200 RPM and shut-down
immediately using the mixture control."
Fuel
Efficiency
You can reduce fuel consumption just by slowing down. Vz is
the speed of lowest consumption. At 7500' you are close to the
optimum altitude at which an engine can develop 75% power for
the highest true airspeed.
In a C-172 operating at Vz +5% you are at 50% power with at TAS of 97 knots getting 16 nautical miles per gallon and operational costs are about 10% less than normal cruise. Slowing down saves you $12 per hour on a flight that takes a half-hour longer. The only aircraft more efficient are the two seat Cessnas and the next level of efficiency is all the Mooneys. If you want to fly for maximum endurance select the speed that is halfway between Vx and Vy.
Leaning for maximum efficiency requires that you fly at altitude where the engine is capable of developing no more than 75% power. You lean as far as will keep a smooth running engine and still maintain EGT and CHT in limits. A new engine should be run rich to seat rings.
Aggressive leaning can only cause detonation in higher-powered engines, turbos or fuel injection systems where one injector is plugged. The lower the power the less likely is detonation or damage. Learning to lean both in climb and descent can save considerable fuel. The wear of an engine occurs from the relative speed of the moving parts. Temperature, material and lubrication are the determining wear factors. The piston/cylinder fit on aircraft is much looser than for automobiles. This is done deliberately to give better reliability though at the cost of greater oil consumption. Low power operations can cause the cylinders and pistons to glaze. Using full power on takeoff should be enough to remove and keep removed this problem.
By flying lighter you do the most for fuel economy most remarkably when flying slow. You can fly higher and slower more easily when light. The faster you fly the less difference weight makes. For every 10% your weight is below gross you can reduce Vz by 5%. With the advent of LORAN and GPS we have made available a major reduction in fuel consumption. Flying a straight line is becoming an easier way to save gas.
Goodyear made the first airplane tires in 1909. These tires were mounted on bolted rims. The tires had implanted wire into the rubber to better secure the tire to the rim. Internal reinforcement to the tire used leather strips. B.F. Goodrich built a Palmer Aircraft Tire as a four ply continuous cord embedded in rubber. This tire was used by Glenn Curtiss to set his early speed records. Military airplane tires were developed in 1911. In the 1920 Goodyear made a streamlined tire to reduce drag.
During the 1920's tires caused numerous accidents. A balloon tire to give softer landings was developed by Goodyear called the Airwheel in 1928. It has a pressure between 10 and 15 psi. Landing shock was greatly reduced by this tire.
In the 1930s diamond tread and the deep ribs still in use today
took over. Today aircraft tires are tested and retested to
meet the safety, quality and reliability required by the FAA.
Any new tire must meet or surpass the 200 dynamic load,
speed, and time requirements of the FAA test. Then an eight-day
endurance test on a flywheel must be matched with a
specific aircraft's performance requirements.
Basics
of Tire Construction
Materials have a dozen different components each made from
up to fifteen ingredients. Rubber is the largest single element.
Specialties are softeners, reinforcing agents, tackifiers, plasticizers,
anti weather materials, anti-oxidants and vulcanizing
agents. Each of these is specially blended to make the nine distinct
parts of an aircraft tire.
The main part of the tire is called the CARCASS. The
carcass includes the rubber, fabric and bead wire. The
UNDERTREAD is a layer of rubber that lies between the tread
and the top of the carcass. Tubeless tires have a
LINER that is a layer of dense rubber that acts an air
sealant like an inter tube. The SIDEWALL is a layer of
rubber
that extends over the outside of the tire from tread to the bead.
The foundation of a tire is called the BEAD. The bead
gives the tire strength and bead is a number of cotton or other
fiber cords that are wound parallel and diagonally to the
circumference and centerline of the tire. The fabric provides
the tire stability, bruise resistance, flexibility and weight
carrying ability. Nylon is now used instead of cotton or rayon.
Because of nylons characteristics two plies of nylon are
given a 4-ply rating. Remember this the next time you see cord
showing through the rubber.
The CHAFER is added as a layer of fabric and rubber
about the beads of the tire where it holds to the wheel. The
APEX STRIP is a triangular insert to shape the tire from
the bead to the sidewall. The FLIPPER is a fabric addition
that circles the bead and apex strip and holds them for ease of
manufacture and to give the tire a more rigid structure.
Tire
Classification
Tire types are either III, VII or VIII that still exist at
the present from the eight that existed over time. Type II is
low
pressure and high volume. Type Vii is extra-high pressure for
military and civil jets and prop jets. Type VIII is a low
profile, high pressure for very high takeoff speeds.
Tire
Care
Proper inflation extends tire life. Only an accurate
needle gauge should be used at least once a week. Under-inflation
causes a tire to overheat on landing, taxiing and takeoff. Over-inflation
causes a tire to wear unevenly and excessively.
Sharp turns should always have the inside tire rolling. The more
a tire flexes the hotter it gets. Touchdown speeds
should be as slow as the aircraft performance will allow. The
tire goes from dead still to full speed in an instant on landing
touchdown. This transition should be done as slow as possible.
Excessive braking speed is a sure way to tire failure.
The tire tread is essential to give the tire stability and
resistance to sideloads and hydroplaning. Move your tires every
few days to distribute the weight and weathering that takes place
naturally.
Avoid:
---Excessive inflation that causes the tread center to wear excessively.
---One sided tire wear is caused by excessive camber of the tire
out or in.
---Weather checking is acceptable unless the chord is exposed.
---Tires can be retreaded if wear is stopped soon enough.
---Any cut through 50% of a rib is cause for removal.
---Braking hard will damage the bead making removal necessary.
---Rough runways cause excessive tire wear.
---Severe braking can and will cause flat spots on tires.
---The use of improper tire tools during installation will damage
tires.
---Tire abuse of many kinds can cause tread separation.
---Running on a flat tire can and will cause irreparable tire
damage.
Flying
with Pressure
---Absolute pressure, such as manifold pressure, is pressure
relative to a total vacuum.
--Gauge pressure is the difference between atmospheric pressure
and the measured pressure. An example would be
differential cabin pressure measured as the difference between
static pressure (outside pressure) and inside cabin
pressure.
---Differential pressure is measured between two separate pressures
as that sealed inside an altimeter bellows and the
atmospheric pressure.
Airplanes have several possible fluid pressures such as hydraulic,
oil and cooling (rare). Nonfluid pressures such as
vacuum, manifold, pitot and static are part of most every aircraft.
Hydraulic pressure operates brakes, struts, shimmy
dampers and some flaps. Oil pressure keeps moving metal parts
apart and inline engines are liquid cooled. Vacuum
pressure drives most HI and AI. This pressure can be obtained
from a venturi or a suction pump. The difference
between pitot and static air pressure gives us our airspeed reading.
A coiled Bourdon tube is used to measure the hot-oil pressure
of engines. The tube is like the coiled tail of a monkey that
unwinds to move the indicator needle. The manifold, altimeter,
VSI and airspeed are relatively low pressures, which use
a bellows to move the indicator needles over a carefully calibrated
scale. Higher temperatures are usually measured
electrically such as by means of a dissimilar metal thermocouple.
Cabin and outside temperatures can be measured by
mercury or spring calibrated thermometers
Converting raw pressure in. Hg. to millibars.
29.92 inches of mercury = 1013.2 millibars
1 inch of mercury = 33.86 millibars
Calculating station pressure (Psta) when I have the raw pressure
in. Hg,
Thermometer reading and the correction (in. Hg.).
Calculating sea level pressure (Pslp) when I am given raw pressure
(in.
Hg.) and station altitudes above sea level.
Pressure decreases at about 1 in. Hg per 1,000 increase in altitude.
Altimeter
Readings
Flying over a mass of air as our reference and the altimeter is
reading correctly. We weigh the air mass and then heat it and
it expands. The density decreases and the mass decreases. Colder
temperatures increase the air density. The higher density air
will give the same pressure/altimeter reading at a lower altitude.
By flying into colder air you are probably lower than your altimeter
indicates. Look out below,
Every cubic foot of air it now weighs less. The heated air no
longer fits in the same space. Air does have mass. The altimeter
is a pressure device, not a density device. When gravity is the
force applied to mass, it produces weight, which in turn produces
pressure. So the altimeter provides pressure change data.
In warm air the pressure change with altitude is less than in
cold air. You must climb higher to see the same drop in pressure.
The air expands when it is warm, and the pressure lines are separated
farther apart. You will be higher than what your altimeter tells
you. This has nothing to do with density being lower in warm temperatures.
Altimeter is a pressure instrument, not a density instrument.
Hot to cold, High to low... Look out below"
Rusting
out
Most pilots rust out before they wear out. The same is true
of aircraft engines. The more the engine is used the more
likely
it is to avoid the various ailments that come from inactivity.
Most important is the used engine will have frequent oil changes
and hence clean oil. Only oil changes remove the acid residue
resulting from moisture and oxidized oil. The active engine
will be at the higher temperatures required to evaporate out the
moisture and inhibit the formation of acids. Regardless of
operation, oil should be changed every four months.
Preservation of an inactive engine is a complex procedure that
may or may not work. Special spark plugs can reduce
moisture. Plugging all orifices will help. Do not move the propeller
during storage. Ground running is not a substitute for
flying. It takes 30 minutes of 180-degree temperatures to remove
all moisture. Shell oil has developed a special type of
oil that acts as a preservative.
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