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Engine Systems
Contents:
The Engine; ...Getting Old.;...Engine Systems; ..Engine operations; ...Pilot training; ...Preflight Neglect; ...Engine maintenance; ...Trend monitoring; ...Engine savers; ...Ways of Losing Power; ...Engine Warnings; ...Engine Operation; ...Factors reducing engine power; ...Valves; ...Breaking in an Engine; ...Exhaust System; ...Shock Cooling; ...Engine Heat; ...Magnetos; ...P-Lead; ...Distributor; ...Spark Plugs; ...Ignition Problems ; ...Carburetors; ...Air Intake; ...Vacuum Pump; ...Engine Monitor; ...Oil; ...Oil Pressure Gauge; ...Oil Temperature Gauge; ...Cylinder Head Temp Gauge;...Carburetor Temperature Gauge; ... Tachometer; ...Engine Isolators;
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.
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.
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.
--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.
Engine TBO
1. Exceeding TBO will accelerate wear.
2. Part 121 and 135 operations cannot exceed TBO
Engine operation
--Thermal shock damage caused by failure to warm up engine
prior to takeoff.
--Shock cooling damage 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. 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.
--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.
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
Avoid Engine operations that involve:
Rapid throttle movement
Wide differences between manifold pressure and engine speed
Excessive RPM and power
Sudden cycling of the propeller
Sudden stoppage
Magnetos cause fewer that one accident a month. Most magneto failures are due to neglected maintenance and serving. 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.
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.
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. One in an effort to stop the engine
do 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.
Consider:
--Excessive leaning may cause a cylinder operating so poorly that
full power will not be produced. The vibration dampers will not
function properly.
--You will not be able to feel or hear uneven piston operation.
A six-cylinder engine may run smoothly even on five working pistons.
--Don't expect to hear or feel detonation.
--Injected engines do not always distribute fuel evenly.
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 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. 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.
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.
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.
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.
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.
Drag and cooling
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.
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.
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° and oil temperature between 165 and 200°. Recommendation is to lean to 100° 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°F changes per minute. Watch temperature instruments.
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 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.
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 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 nonarcing 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.
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.
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.
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.
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° 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° 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 Heat
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.
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 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.
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. A small tube comes from
a small port in the engine to the oil gauge. 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.
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.
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.
The Electrical System
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Contents
Electrical System; ...Alternators; ...Generator; ...Ammeter; ...Voltage Regulator; ...Electrical Failure; ...Position Lighting; ...Starters; ...Precipitation Static;
The ignition system is independent of the rest of the electrical system. The ignition system is traditionally totally redundant. Thus, any problem with the electrical system is not an emergency requiring immediate action from the pilot.
The master switch or one-half of a split-master is a safety device to keep the high battery amperage used in starting out of the cockpit. The master activates a battery solenoid and relay. This relay lets electricity go to the primary bus. The primary bus is a relatively heavy metal bar capable of carrying heavy current loads. Each electrical circuit is connected to the bus bar as a separate terminal. The master switch makes it possible to use battery power to start the plane, but has nothing to do with keeping it running. The magneto system that runs the engine is independent of the master switch and battery. With the magnetos on, just turning the prop as little as 1/8 turn can cause the engine to start.
In line with this circuit is a fuse or circuit breaker which is sized to the wire used. Aircraft wiring is specially coated and covered to prevent corrosion. Automotive wire is not suitable for aircraft. The purpose of a fuse of circuit breaker is to protect the wiring from catching fire. Co-incidentally equipment is protected. Some fuses are slowblow. This allows the fuse to accept a momentary overload without blowing. Some breakers can be pulled off (gear) but every breaker can be reset. No breaker should be held closed. At most, reset a breaker only once before getting expert advice/maintenance.
If the battery relay is not activated because of low voltage in the battery, there is no way for the alternator field coil to develop the voltage needed to recharge the battery or run the electrical system. When the battery is low some of the alternator energy is used for recharging. This recharging battery-load is bad for the electrical system. Having a cigarette lighter voltmeter is desirable.
Aircraft use lead-acid batteries made of a series of such cells to get the desired voltage. To reduce weight the battery casings are made lighter and smaller for aircraft. A 12-volt battery has six lead-acid cells giving two volts output each. Positive plates of lead dioxide and spongy lead are kept separated from the negative spongy lead plates in a sulphuric acid/water electrolyte. Battery capacity is measured in ampere-hours. Over its there to five-year life the battery capacity will decrease. A charged battery has an unbalance of electrons between the plates. When an electrical circuit is completed the excess electrons from one plate flow through the electrolyte to the other. This flow creates lead sulfate, which will eventually result in a 'dead' battery. Aircraft batteries can be run down in just a few minutes at the high amperage required for starting. Capacity and charge determine endurance. A load test is used to determine capacity. The airworthiness of an aircraft battery is about two years. A dead or weak battery is unairworthy.
The density of its electrolyte determines the charge of a battery. This state is checked with a hydrometer that measures specific gravity as it is floated in the electrolyte. Hydrometer readings are taken before adding water. Electrolyte is normally 30% acid by volume. Never add water to a battery unless it is below the plates. Charging will cause the level to rise. A specific gravity reading below 1.24 is considered low for an aircraft because of high load requirements. Such a battery needs to be charged or replaced. Normal range of specific gravity is from 1.26 to 1.3. A three-amp charge rate will be best to preserve the battery and prevent overheating. A weak battery is unairworthy. A dead battery puts a great load on the electrical system. Such loads are the number one cause of electrical system failure. All battery maintenance should be recorded in the airframe logbook per FAR 43.9 as to work performed.
Either current or voltage methods can do aircraft battery charging.
A battery must be removed from the aircraft for charging. Remove
and attach the negative cable first and last. Quick chargers will
reduce battery life expectancy. The constant current method requires
more time and can result in overcharging if excess time is used.
Vent caps, which allow the battery to breathe, should be loosened
but not removed during charging. Avoid sparks, which may ignite
the hydrogen and oxygen being vented during the charging process.
The aircraft alternator/generator system has a constant voltage
due to its voltage regulator. An exploding battery is a terrifying
experience. The battery may be allowed to become warm during charging
but not hot. Allow the battery to rest for several hours and give
final specific gravity test before reinstallation.
An aircraft battery is kept in a metal box for electrical and
mechanical shielding. Each battery cell is sealed in hard rubber
with a non-spill vent cap on top. A lead weight seals the vent
during inverted flight. The hydrogen gas vented from a battery
is highly explosive if ignited. No smoking around batteries. The
battery box has an exhaust tube through which battery gases are
vented. In some installations, intake air and a drain sump neutralizes
the gasses before venting. If a cable is loosely connected or
corroded, or if there is a direct shorting of the battery terminals
the current may burn off the terminal and start a fire.
Rapid discharge or charging ruins a battery and shortens its life.
Heat and cold cause shortened life. Keeping fluid levels correct
in the summer is important. Batteries have a life potential of
thousands of charge
and discharge cycles. Longer periods between starts extend life.
A battery with a low charge can be permanently damaged. The greatest
killer of batteries is over-charging. The colder the temperature
the greater charge that can be applied.
A newer type nickel-cadium (NiCd) is much more expensive but
require less maintenance and have a longer service life. Voltage
of the NiCd is constant up to 90% of its discharge cycle. NiCds
use potassium hydroxide electrolyte, which as a base instead of
acid requires the use of special tools and techniques. NiCds must
be bench checked with special equipment.
A still newer type called a recombinant gas (RG) type is designed
so it can't leak, doesn't require a sealed box, has greater capacity
and gives more power. It uses a glass mat to soak up the acid
and hold it in suspension. The mats can be packed closely with
more lead plates per cell. The RG battery has greater capacity
and is heavier for its size. It can be safely shipped by UPS.
Battery maintenance can be done as part of pilot's authority under FAR 43.3(g). For removal always remove the negative lead first and replace it last. Do not charge a battery while it is on the aircraft. A charging battery will release gases that can be exploded by a spark. Do not add water prior to charging. Terminal corrosion can be prevented by coating with a terminal sealer after reconnecting.
Alternator failure is the most frequent cause of electrical system failure. If you note alternator failure, shut down as much of the system as you can. My choice would be to leave only the transponder operating. A popped circuit breaker will indicate that an overload has occurred. Reset one time only. Resetting the breaker must be done with a very firm push. It is reset when the low-voltage light goes out. Know where this breaker is and what it feels like when set.
Belt or gear driven. Gear driven alternators can contaminate engine oil on failure as in C-150. Proper installation is important. Life 700 hours +. A belt driven alternator must have the belt at the proper tension. This means about 1/2" flex in the belt when pushed with a finger. Too tight will burn out the bearings of the alternator. Too loose will cause the belt to slip under load.
An alternator requires a "priming" voltage in the field coil before it can begin to produce electricity. For this reason, starting an aircraft with a totally dead battery will not enable it to produce electricity for the radios and lights unless it is an older aircraft with a generator. A quick battery charge of only a few minutes may be sufficient to activate the alternator field coil.
An alternator may work normally until it is subjected to a full load. At maximum load a failed diode may cause total failure. Turning everything on prior to shut down so see what happens could make an alternator check of this condition.
Older aircraft may be equipped with a generator to provide electrical power to the systems when the engine in running. The generator does not need battery power to begin functioning as does the alternator. This is its only advantage. The generator will provide insufficient electricity during low power engine operations such as pattern work. The battery must provide the difference and will become discharged in an hour of such operations. The increased electrical requirements of modern aircraft often exceed the generating capacity of the generator, hence the alternator.
There are two types, the charge/discharge and the load. The charge -discharge will remain centered so long as the system output can meet the system demand. Beyond this point the needle will indicate a discharge and use of battery power. The load type of ammeter will begin near zero and rise as more electrical load is put online. The voltage warning light will indicate if the load requirement is beyond the alternator's ability to produce. The load meter reflects the actual electrical load as it is turned on. A load ammeter at zero or discharge is saying that you are using battery power The ammeter is an essential element of any pilot's instrument scan. It should be a part of the pretakeoff, prelanding, and checkpoint checklists.
Alternator failure checklist
1. Master off
2. Minimumize electrical load
3. Check/reset alternator breaker
4. Master on
You should know that radios use .5 amp when receiving and 5 amps
when transmitting. Nav radios and electric gyros use l amp. Transponders
use about 2 amps. You can get an idea of usage by the ammeter
reading according to type. As the electrical load increases the
alternator tension belt becomes ever more important. Under load
a loose alternator belt will begin to slip and the alternator
will fail to produce the required electricity. Suspect a loose
belt if you get unusual ammeter indications. You can tell you
have a voltage problem if you can't hear the transmitter relay.
In the circuit with the alternator is a voltage regulator.
It adjusts the field coil electrical flow to provide an output
commensurate with the load applied.
If the voltage regulator malfunctions it may activate a high/low
voltage light. This is most likely to occur at low power settings.
If a circuit breaker "pops", reset only once. If an
over-voltage occurs it is important that the alternator be turned
off by switch or by circuit breaker. The circuit breaker may be
included as part of the master switch. Shut down (except the engine)
and start over with the alternator. If the warning light comes
on, shut down the alternator. Run off the battery. After the alternator
cools down you might try again.
There are several forms of electrical failure. The safest form
is the blowing of a fuse or the popping of a circuit breaker.
Never make more than one replacement or reset before getting on
the ground. These protective circuits don't always function
The next likely electrical problem will first appear as a burning
odor followed by smoke, an annunciator light coming on, or a failure
of some sort. Wiring and wiring connections are the most likely
causes of failure. Your preflight should note any loose wiring
in the cockpit. and when they don't fire is not far behind. Shutting
off the master switch is the emergency first step.
Have an electrical failure sequence checklist such as:
Master switch off
All switches off
Try battery switch (ammeter indication)
Try alternator switch (Voltage light)
Try one radio and light at a time to isolate problem
Run on minimum equipment
In the event of smoke, shut everything off and get on the ground.
Run through failure sequence only as second choice. If you have
a split master switch, you might use just the battery side and
the transponder to 7600. Don't reset circuit breakers or replace
fuses more than once.
Electrical failure
1. Get to VFR conditions
2. Get on the ground
3. Know where you are
4. Don't navigate on limited power
Wingtip navigation lights can be seen in a 110° arc from front to sides and vertically for 180°. The rear light need not be on the tail but must be visible 70° to either side of center. The use of low-powered pulsed landing lights as recognition aids is a $200 self-defense system. Such lights must not affect pilot vision or position lights. Wing tip landing lights give greater longevity but make some taxiing more difficult. Part 91 flight at night does not require landing lights. Aircraft switches are on when up or in, off then down or out.
The position lights are angled so that they have non-intersecting arcs. At no time can three position lights be seen at the same time except from above and below. Two lights can be seen from head-on, 110° to the left rear, and 110° to the right rear.
For collision avoidance if you see a red light give way. If you see a green light expect traffic to yield but be prepared if he doesn't. If you see a white light break right to clear traffic you are passing. If you see red and green you are head-on to traffic, break right.
All lights, position, strobe, and landing lights lose their intensity over time. Periodic replacement is a good idea.
A student should have knowledge as to the starting mechanism. A key switch activates the starter motor. The initial spins of the starter usually (Not C-150) uses a bendix spring to engage the starter gear with the engine flywheel gear as with automobiles. Once the engine starts the gear disengages. Occasionally the lack of lubrication or battery power will prevent the gears from engaging. Get help if after four tries the gears do not function.
Starters usually die from overheating while starting hard to start (cold) engines. The Bendix will stick if dirty and fail to retract. Since it remains engaged with the flywheel the Bendix drive will soon fail. Lack of use affects the Bendix, brushes and rusts the gears. These defects require more battery current to operate the starter and cause overheating. Should be part of engine TBO (time before overhaul) cycle.
As an aircraft flies through air a static charge can be created on the aircraft. Static dissipation wires are often placed on aircraft trailing edges to remove this static. When such static exists, the radios may make an undesirable noise.