Page a9 C.H.
Carburetor Heat; ...Occurrence; ...Three kinds of Carburetor Ice; ...Recognition; ...Using carburetor heat; ...Application; ...Results; ...Questions and Answers;
Somewhere on the front of the engine is an air intake for the carburetor. The air enters through a mesh filter. Every pilot, in the pre-flight should check security of this filter and its mounting. The air flow through the carburetor is controlled by the throttle moving a circular flat rotating plate. Air passing by this butterfly valve is accelerated by a narrowing throat called a venturi.
This narrowing reduces pressure and sucks cooling fuel into the air. The combination of a lower pressure, gasoline, and moisture in the air can so cool the metal parts of the intake that the moisture adheres to the parts as ice. The greater the air's humidity the more ice will be formed. As the air flow is restricted by the ice, the excess fuel so enriches the mixture that the engine to flood and misfire. The initial signs of this icing is a gradual decrease in rpm and an increase in engine roughness. Avoidance of carburetor ice can be improved by avoiding long, low power descents and by leaning to make the engine run hotter. Lycoming engines are less susceptible to ice than are Continental engines since the carburetor is attached so as to benefit from the heat of the engine itself.
Float type carburetors can create ice any time. Moisture in the air increases the possibility that the cooling effect of the atomizing of gasoline in the venturi (narrow throat) of the carburetor will cause vapor expansion and cooling to create ice from any moisture present. When the ice adheres to the parts of the carburetor it cuts down the flow of air and 'chokes' the engine. Carburetor ice onset is dangerous and insidious. The first indication of ice is a drop in rpm. This drop may be accompanied by engine roughness and eventual stoppage. You must sensitize yourself to rpm changes that you have not induced.
There are important training aspects related to carburetor ice and the use of carburetor heat. The pilot must train to be aware of the weather conditions that have a proclivity for causing carburetor icing. Being aware of the likelihood is the primarily aspect of the anticipation required. Just where icing occurs in the carburetor is a variable. Icing can occur before the butterfly, in the carburetor intake, on the butterfly valve or afterwards. When impact icing blocks the air intake filter as induction icing, then the carburetor heat source serves as alternate air for the engine. The different design of carburetors, engines, and induction systems all make a difference however indeterminate. Some cowling designs are better than others in reducing the occurrence of carburetor ice.
We know how/why ice forms and how to use CH as preventative but we no reliable predictive ability. You are more likely to get carburetor ice with warm temperatures because warm air has the ability to hold more moisture. Fahrenheit temperatures between 20 and 70 accompanied by atmospheric moisture usually trigger the required venturi temperature drop. Anything less than full CH application is potentially dangerous. Low fuel pressure and contaminated fuel can give symptoms similar to carburetor ice. By keeping both fuel and induction air clean we can avoid these as causes of unusual engine behavior. Carburetor icing occurs when the air/moisture/temperature in the carburetor is modified after ingestion so as to be capable of freezing moisture and adhering it to the internal parts of the carburetor. Carburetor heat also performs the function of an alternate air door in case of induction icing covering the engine air filter.
There is no FAA carburetor icing probability chart. In given circumstances carburetor icing can occur at any temperature. Some aircraft models and engines are more susceptible to icing than others. Carburetor ice will get you when you least expect it. It has occurred on cloudless days and temperatures up to 100 F. The ambient air temperature in a carburetor can be reduced at its verturi by as much as 70F. The 10 seconds that it takes C. H. to take effect can be the longest of your life. Don't make it the longest 12 seconds by failing to immediately apply C. H. Any unexplained power reduction is a red flag notice. Make all descents with partial power to retain as much C.H. potential as the situation allows. Aircraft with an EGT will show a decrease in EGT readings at the onset of carburetor ice. It is an early warning system.
Most likely icing is at 50%+ humidity from 20 to 90 degrees F. (Some texts give 80% humidity between 40 and 70 degrees.) Ice is caused by absorption of heat from air/fuel vaporization as it enters low pressure ventrui of carburetor at high speed. Can cause drop of 60 degrees causing ice to adhere to "butterfly" and verturi throat. Even with no visible moisture, ice can form in the throat of the carburetor due to adiabatic cooling as the air passes through the venturi by the throttle plate.
The carburetor on a Lycoming engine is mounted at the very bottom of the engine in such a way that any heat it gets is from direct contact with the engine and very little from elsewhere. The engine's putting out enough heat at higher power where the use of carburetor heat will prevent the formation of ice in the carburetor and given time it'll melt any existing ice. Even the position of the butterfly valve helps. Carburetor ice can occur at any power setting, it is most likely to occur in the green arc.
Continental's carburetor is suspended below the engine so that there is no residual heat transferred between the engine and the hanging carburetor. Lycomings, on the other hand, have the carburetor secured to the oil pan. The carburetor is heated by the hot engine oil. For this reason Piper recommends carburetor heat be used only as necessary. The pilot determines just where and when necessary exists. Lycoming engines are much less susceptible to carburetor icing than Continentals because of their design. However, because of pilot complacency, the icing of a Lycoming is going to be more traumatic.
Many unexplained engine failures are probably due to carb ice. When humidity is more than 50% and temperatures range from 20 to 90 degrees Fahrenheit ice will form as the internal carburetor temperature can drop 60 degrees.
Carburetor icing during takeoff is not as rare as some would like to believe. The best preventative is to apply C.H. on leaving the run-up area and removing it at just before full power is applied. every descent made at reduced power should be done with full carburetor heat on. the use of C.H. decreases the power of the engine slightly less than 10% and causes the mixture to be over-rich. Leaning is advised for best engine operation and to maintain the required head for C.H. No use of leaning or C.H. is advised for engine operations over 75% of maximum power.
C. H. Reference List
--Ice can form at full power
--Always apply full carburetor heat
--If cruising with C.H. lean the mixture.
--Always use C.H. during descents
1. Impact ice is caused by moist air on the air filters and
air intakes are impacted as rain, snow, or sleet. Forms from 15
to 32deg;F but is worst at 25F.
2. Fuel ice is caused by vaporization where fuel enters the manifold
system. Will occur when relative humidity is 50% and between 40
and 80deg;F.
3. Throttle ice forms in the carburation system on the throttle
valve or butterfly or on the interior of the venturi system. The
water vapor from the air intake freezes due to the venturi effect.
Effective temperature drop is about 5deg;F and ice is most likely
between ambient temperatures from 32 to 37deg;F.
On first start, there may not be sufficient heat to either prevent
or melt any carburetor icing. Leaning will raise engine temperature.
The more moisture in the air, the greater the likelihood of icing.
A humid hot day is just as likely to cause icing as is a cold
day with water on the ground. When conditions indicate that icing
is likely, the prudent procedure is to apply carburetor heat in
anticipation rather than as reaction. Using the carburetor heat
every time you reduce power is a good operating procedure and
much safer than the POH suggestion for use when required.
Most of my carb icing encounters have been while taxiing. The
explanation of what occurs deserves repeating. I demonstrate the
cooling effect of gasoline on moving air by placing some gas on
the back of a student's hand and have him wave it. Under certain
atmospheric conditions and power settings it is possible for the
blending of fuel and air in the carburetor venturi to cool any
moisture present to freezing. Automotive fuel is more likely to
cause ice because of its vapor point. (Venturi effect can be demonstrated
by holding two pieces of binder paper vertically about three inches
apart and blowing between them.) This can adhere to the metal
parts of the carburetor particularly the butterfly valve which
is the throttle control. This ice will restrict the flow of air
through the venturi and cause an initial reduction in rpm and
subsequent engine roughness to final failure.
At idle power, in the air or on the ground an aircraft can ice
up in a very short time. There is no logical safety reason behind
the concept of removing carburetor heat on short final as a go-around
safety measure. There is nothing so urgent about a go-around that
makes it necessary to remove carburetor heat prior to landing
as a time saving or safety procedure. The closer to the ground
you are when initiating the go-around the greater will be the
ground effect and aircraft acceleration. The go-around is initiated
first with a mixture check, full throttle, and finally with carburetor
heat. Always first with the most. These forward movements can
be accomplished nearly simultaneously in one motion.
Recognition is important but it comes after the fact. Depending
on the circumstances, after the fact, may be too late. As with
pitot heat, prevention is the name of the winning game. Waiting
five minutes for pitot heat to remove ice can be done, not easily
but 'do-able'. Carburetor heat will not allow the time because
as you lose power you are losing the engine's ability to produce
the required heat. Planning your options now is better than a
spur of the moment decision that can be wrong.
I am no longer surprised when a pilot does not show awareness
of carburetor ice. The onset can be very subtle. The effects very
subdued, and the wrong reaction very common. Contrary to some
manuals, the National Transportation Safety Board is now recommending
that heat be applied for all power reductions below cruise. I
teach my students to keep their right hand and indexing finger
on the throttle at all times below 1000' except when trimming
or making radio changes. This helps determine if any power change
can be classified as 'unexplained' hence attributable to carburetor
ice.
Using carburetor heat
1. Even in warm weather
2. All or nothing at all, not just a song.
3. Use before takeoff; not during takeoff
4. The engine talks; you listen and feel.
5. Not for continuous operation on the ground.
6. Check proper operation and maintenance
When you pull the aircraft carburetor heat you are moving a
diverter panel which has been taking external air through the
nose air filter to change to taking unfiltered air through the
heat exchanger of the exhaust system. The heat exchanger air is
usually warm enough to both affect the engine power (reduced up
to 15%) and melt any ice that may have accumulated on the carburetor
venturi or butterfly valve. This melting may not occur if the
engine has cooled off so don't waste any time pulling the handle.
Taxiing lean tends to keep the engine hotter. If pulling C.H.
during taxi or run-up raises the rpm the aircraft is improperly
leaned. Better yet use C.H. as a preventative to pre-heat the
system and if in doubt continuous use may be required.
There are no complex operations in flying for which there are
not several simple, straight forward, and WRONG ways to perform.
One such combination of operation and solutions is the use of
carburetor heat. The warning indicators for the need of carburetor
heat are deceptive, variable, and inconsistent. The actual application
of carburetor heat will produce results that are deceptive, variable,
and inconsistent. Every student pilot is goings to have opportunities
to make carburetor heat mistakes. The learning process will consist
of both successes and mistakes in the use of carburetor heat.
The training process is designed to reduce the probability of
a mistake resulting in an unpleasant event. It takes an act of
faith to stay with your initial application of carburetor heat
when it only makes things worse. Very often the worst thing that
can happen to a pilot is to 'get away' with a mistake. This applies
to use of carburetor heat as well as any other aspect of flying.
Carburetor heat is intended as a preventative rather than a cure.
Heat should be applied early and fully. Carburetor heat is best
used in anticipation of carburetor ice. Put it on before a potential
icing situation occurs. Going into a slow flight configuration
will increase the effectiveness of carburetor heat by decreasing
the cooling air over engine. In addition, carburetor heat makes
it possible for the engine to draw its air from inside the engine
compartment. If the engine air intake filter under the propeller
is being blocked by impact ice or snow, the use of engine compartment
air via the carburetor heater will bypass the blockage and allow
continued engine operation.
Don't move the throttle. The ice may break loose and cause instant
stoppage. Apply full carburetor heat and allow the diverter valve
to bring heat in the form of hot air to enter the venturi so as
to melt the ice. This air is unfiltered. It passes through the
heat exchanger and flows to the carburetor as hot air. The engine
will react as follows. The already ice reduced rpm will be further
reduced by the application of heat. (Hot air is less dense and
reduces power about 15%.) As the ice melts there will be a gradual
rise in rpm because of increased air flow. A further rpm increase
occurs when carburetor heat is removed. Carburetor heat admits
unfiltered air into the engine. This unfiltered air can contain
particles harmful to the engine. This is particularly true close
to the ground. For this reason we limit the time carb heat is
applied on the ground. (I find my personal allergic reaction to
pollen ceases above 500') Carburetor heat should not be used when
maximum power is required such as on takeoff.
The application of carburetor heat changes the flow of engine
air from the outside air intake to unfiltered hot air from the
heater muff. This hot air causes an additional loss of power.
Normally the power loss is 1% for every 10 degrees of hot air
differential. With icing this loss can reach 15%. Due to lack
of ram air and a usual 100 degrees of heat above standard the
runup loss can reach 13%. On hot days the heat differential is
less so the apparent drop in power is less. If you have leaned
for taxi there is no need to enrich the mixture during runup since
you are not going to full power and will check carburetor heat.
Carburetor heat enriches the mixture. If you intend to fly with
carburetor heat on you should also plan to lean your mixture since
with carburetor heat the engine will be running rich. In icing
conditions always retain enough power to keep the engine warm.
Without a warm engine you will not have carburetor heat. Avoid
extended low power or no power descents.
Using the carburetor heat every time you reduce power is a good
operating procedure and much safer than the POH suggestion for
use when required. Some aircraft are equipped with a carburetor
air temperature gauge to warn if the internal temperature of the
carburetor is conducive to icing. This serves notice to use carburetor
heat as the icing preventative which is its primary purpose. Carburetor
heat effects engine operation and power only as does a higher
density altitude. No harm to the engine occurs beyond that which
may occur through the ingestion of unfiltered air. It is never
wrong to use carburetor heat as an icing preventative prior to
any power reduction. The use of carburetor heat is too late if
the engine has become cooled to the point where it is unable to
melt any existing ice. If you need as much engine heat as you
can get, set up a climb attitude even if you cannot climb. This
will be a much better option than one that would increase speed
and engine cooling. Aggressive leaning and use of the magneto
switch to cause a backfire are emergency measures.
I often use carburetor heat as a cruise descent device. It will
decrease power by about 200 rpm and allow a fuel saving leaner
mixture throughout the descent. You can reduce some power as well.
Don't mess with the trim. You will probably be leveling off at
pattern altitude on arrival and by just removing carburetor heat
or adding power you are already trimmed for level. When conditions
warrant, I use carburetor heat when rolling into position on the
runway for takeoff. There are exceptionally cold times when you
should not use carburetor heat. Partial heat applications are
never recommended. Carburetor heat use should be limited to operational
checks while on the ground as a standard procedure since air going
to the carburetor will be unfiltered and allows dust and abrasives
into the internal engine. However, some ground temperature dewpoint
conditions may require the use of carburetor heat on the ground,
regardless. Having a carburetor air temperature gauge is highly
recommended to increase awareness and accuracy in the use of carburetor
heat.
Some POH do not suggest or recommend CH application at power reductions.
NTSB recommends use on power reduction regardless of POH. A substantial
number of engine failures occur because of failure to recognize
carburetor ice and apply heat immediately and fully. Once the
engine has stopped the rapid cooling caused by the descent limits
the effectiveness of any latent heat remaining in the system.
If your life's ambition is to become an old pilot, I would become
sensitized to the conditions causing carburetor icing; sensitized
to the first unexplained drop in rpm; and start using anticipatory
carburetor heat. If you think any loss of power is due to ice
you should apply heat and try to enter a climb with any remaining
power to conserve and create as much heat as possible.
Any suggestions regarding the use of carburetor heat should be
qualified by reference to the specific model of aircraft. Cessna
usually recommends carburetor heat in operations below the green
arc, Piper does not; instead, Piper's usual recommendation is
to use only when indicated. The reason behind the operating differences
has to do with the way the carburetor is mounted below the engines
and operational experience.
If engine failure seems imminent induce a backfire by turning
the magneto switch to off and then back on. A backfire may be
further induced by leaning. The backfire can jar any ice loose.
Use carburetor heat in high moisture conditions just prior to
takeoff while entering the runway. Always use full C.H. since
partial applications can actually cause carburetor ice. Any time
you have carburetor ice you also have a rich mixture which of
itself will cause a rough engine. The situation permitting lean
the mixture aggressively until the engine begins to fire again.
A running engine will begin to produce the heat necessary to allow
the C.H. to melt the intake ice.
The concept of relative safety extends itself to all matters
of flying. The emergency procedures for engine failure in all
carburetor aircraft includes the application of carburetor heat,
immediately and fully. Required pilot knowledge should be knowing
why you use carburetor heat in this way, what the effect will
be in the near term and what future benefit is to be expected.
The initial effect will be an even greater loss of power, soon
to be followed by a rough running engine, and an increase in rpm
as ice is removed as water. The final increase occurs when carburetor
heat is removed. Failure to understand what is happening and what
to expect can be a fatal deficiency.
Removal depends on availability of hot air from alternate air
intake system. FAA requires that CH be able to provide 90 degree
air down to 30 degrees outside temperature with engine at 75%
power. If you allow engine temperatures to drop, as during descent,
this heat may not be available for ice removal. In a C-150 the
air intake for C. H. is on the right side of the engine cooling
intake as seen from the cockpit. The intake on the left is for
cabin heat.
Symptoms can be varied but involve initial loss of power, engine
roughness, and stoppage. If ice is suspected do not move the throttle.
Such movement can cause ice to break loose and further clog the
Venturi If possible enter a climb attitude to lessen the flow
of cooling air over the engine. Power loss is shown by lower RPM
in fixed pitch and lower manifold readings in constant speed propellers.
It make take 15 or more seconds to clear the ice. Throttle may
be difficult to move. If the pilot is not sensitive to engine
sounds the power loss may occur quickly enough to result in stoppage.
I insist that my trainees keep their hands on the throttle below
1000' but never recommend constant removal regardless of altitude.
You need to know if any power loss is due to throttle movement.
Any unexplained loss of power should be assumed to be due to carburetor
ice. Apply full carburetor heat.
Removal of ice requires application of full CH for as long as
it takes to have the engine rise above its additional lower RPM
and roughness due to the introduction of hot air. Removal of CH
will cause an additional RPM rise. Use of partial heat may make
it possible for any moisture to re-freeze. Use of throttle prior
to allowing carburetor heat to become effective may cause the
butterfly to jam the ice and stop the engine.
When carburetor heat is turned on there is normally a slight drop
in rpm because heater muff heats the air going into the carburetor.
Hot air has a lower density which means less oxygen is getting
into the engine. The mixture of air and fuel is made richer. On
occasions when the outside temperature is quite near that of the
engine, no drop in rpm may occur. This is normal because the expected
drop in rpm is due to a marked difference in outside air temperature
and engine heated air. Some pilots will add power prior to the
use of carburetor heat so maintain engine temperature and power
should a sudden ingestion of water occur. More conventional wisdom
indicates that such power involves movement of the carburetor
butterfly and may result in sudden blockage and engine failure.
Pilots have been surprised by sudden engine roughness or stoppage
in what they considered to be non-icing conditions. The temperature
inside the carburetor can drop to freezing level in the carburetor
venturi making ice a possibility in all but the driest air conditions.
The initial effect of adding carburetor heat will be an even greater
loss of power, soon to be followed by a rough running engine,
and an increase in rpm as ice is removed as water. The final increase
occurs when carburetor heat is removed. A pilot should be aware
that a reversal of the results of carburetor heat application
can and does occur at altitude, when leaned for taxi, and on takeoff.
In these situations any increase in rpm caused by application
of carburetor heat is indicative of improper leaning.
Tests have shown that cruise power and full C.H. is not damaging
to the engine. The drop of power and even increased roughness
often frightens a pilot into taking off the heat. This is a no-no.
Don't remove the C.H. until the engine smoothes out even though
at reduced power. Since an iced carburetor is running rich, leaning
will improve engine operation until C.H. melts the ice. After
removing C.H. be sure to readjust the mixture.
A NTSB study shows that there are a number of 'unexplained' engine
failures every year. By the time the airplanes are inspected there
is no visible 'explanation' for the engine stoppage. NTSB suspects
carburetor ice. Over 18 out of the 35 stoppages cause accidents
annually
--What is carburetor ice?
May form at the fuel discharge nozzle, in the venturi, on or around
the butterfly valve, or in the passages from the carburetor to
the engine.
--Why is carburetor ice dangerous?
It restricts the power output and may stop it by depriving it
of air.
--What causes carburetor ice?
Forms during vaporization of fuel. Expansion of air causes sudden
cooling of the air fuel mixture. If water is present it may be
deposited as frost or ice inside the carburetor.
--What are ideal carburetor ice conditions?
Usually 68F or less where visible moisture exists. When partial
throttle is being used. Most critical operational periods are
during partial cruise or descent. Apply C.H. ahead of such operations
and
maintain sufficient power to retain reserve of engine heat.
--How is carburetor ice detected?
First sign is loss of rpm or drop in manifold pressure (no drop
in rpm}. Engine roughness occurs. Carburetor air temperature in
the yellow.
--How is carburetor ice prevented?
Carburetor heat is a preventative. It may not melt existing ice.
Periodic checks advisable when favorable conditions exist. Always
apply full heat. Always apply full heat during partial throttle
situations.
--What is the most important reason for not taxiing with carburetor
heat on?
C. H. allows induction air to bypass the air filter. Unfiltered
air contains abrasives that can enter the engine and cause damage.
--Why do Lycoming engines often say carburetor heat is optional
to the extent that ice must be recognized before applying
carburetor heat?
The mounting of the carburetor to the Lycoming oil pan allows
the engine heat to reach the carburetory. Contenintal
engines have their carbuetor separated from the engine soengine
heat does not give this beneficial side effect. Since
Continental have a cooler carburetor they tend to get more power
than an equal displacement Lycoming.
Contents
Airspeed indicator; ...Airspeed;
...Pitch vs. Power; ...Pitch
for speed; ...Airspeeds; ...V-
Speeds; ...Uncommon V Speeds;
...V Speeds and Flaps; ...Standard
performance profiles; ...Pitch; ...Climbs; ...Power curve;
...Pattern speeds;
...Variations on a Theme
The airspeed indicator uses the differences between the static pressure and the pitot pressure to display airspeed. The pitot tube takes ram air pressure (not a flow of air) from aircraft motion to drive the diaphragm of the airspeed indicator. The static air hole(s) takes the ambient pressure of the aircraft and registers this pressure on the altimeter, the vertical speed indicator and the side of the airspeed diaphragm opposite the pitot's ram air side. The IAS requires both the pitot and the static to operate. The pitot tube usually has a heater element around it to melt ice as an obstruction. It would be useless against an embedded insect. Static stystem failure component by component or total is nearly impossible to detect during preflight. The functioning of the alternate static souce is best checked during climb and descent for detecting a partially blocked system.
The airspeed indicator is color coded to show certain ranges of flight operation. White is flap range with Vso at the slow end and Vfe the high end. Cumulative damage will occur if flaps are lowered at speed higher than this range. The green range is normal with Vs1 as the gross stall speed without flaps. Where the green meets the orange is Vno listed as the maximum structural cruise speed. The orange range is to be avoided in turbulence. The high end of the orange range is capped by the red line. All warranties of structural strength are voided when speed meet or exceed the redline. All marked color codes are based on indicated airspeeds.
The airspeed will read slightly slower when you climb and slightly faster when you go down. If the pitot tube is blocked the airspeed indicator will work like an altimeter. As altitude is gained the ias become s greater than would be expected. As with the ear the airspeed works best which air pressure is equalized. There is no required inspection of the pitot-static system. There is a required inspection of the static system which involves the altimeter only.
Many need to know speeds are not shown on the airspeed indicator. Va the maneuvering speed at which warranties are voided if abrupt full control movements are made. Airspeed indicators since 1976 have been standardized as to panel location and to reading in knots with mph as an inner line of readings.
Every airspeed is the end result of thrust overcoming drag. The relative movement of the plane to the earth and the air above is the end result of pilot settings of pitch and power. Airplanes need speed to fly. However, the recording of this speed does NOT use moving air; instead, it uses air pressure. You should know that moving air does not enter the pitot tube. Only air pressure is applied through the pitot tube.
The pitot tube pressure can be indicated in several ways but the most common is a differential pressure indicator that measures the difference between impact pressure and static pressure on different sides of a flexible air chamber. A movable arm is geared to the air chamber so that diaphragm movement is measured on the airspeed dial. The pitot tube measures impact pressure while the static tube measures undisturbed air pressure. A single static port has inherent errors that occur when uncoordinated flight disturbs the air at the static hole. The resulting speed measure, called indicated airspeed (ias) is uncorrected for the plumbing installation, air density, or instrument imperfections.
Over the years the markings of airspeed indicators have remained much the same. However, the source of the markings have varied. In the 1970 airspeed indicators were usually in miles per hour and in calibrated speeds. Calibrated airspeed is indicated airspeed adjusted for installation and interment error. Generally the airspeed correction tables have indicated airspeeds that show lower and higher than calibrated speeds. This is a built in safety factor in that when you are indicating a slow speed you are not quite as slow as you think you are. When you are fast you are not quite as fast as you think you are. Calibrated airspeed should always be used to calculate the 1.3 Vso approach speed and then converted to indicated airspeed for the actual approach. 1.3 Vso is the speed to use if no maneuvering is required on final. The final authority for any aircraft is the appropriate model and year matching the airplane to the POH.
Calibrated airspeed is different from indicated airspeed in that it makes corrections for installation, density, and instrument errors. The range of difference between indicated and calibrated airspeeds are shown on a chart in the POH.
The calibration of an airspeed system is based on standard conditions of pressure and temperature. As density decreases with altitude the speed of the aircraft must be higher in order to achieve the same instrument reading. While the indicated speed will decrease with altitude due to this decreased impact pressure, the true airspeed will increase. True airspeed is available in the POH for planning purposes. True airspeed can also be calculated in flight using E6B calculations.
If wind is not a deciding factor, it is always better to fly high to obtain the resulting higher true airspeed. The calibration of an airspeed indicator is based on standard pressure of 29.92 inches and 59 degrees F temperature. Indicated and true airspeed are identical. Above sea level your true airspeed will be faster than indicated because it takes more speed in thinner air to register the indicated speed. True airspeeds Are slightly faster the cooler the temperature but any increase is negated by the drag of the denser air.
Deviate from the manufacturers V speeds and you will have reduced performance in every flight regime. However, if you trust the performance figures in your POH you are an accident waiting to happen. The book figures are for a new aircraft. A ten year old plane with a mid-time engine may have a 20% performance deficit. Instead of the book we must trust our experience and judgment augmented by local experts. You could develop your own POH book for your aircraft and insert your ‘real’ performance figures. The Piper pitot/static air mast is not good over the full range of speeds and gives variations of static pressure.
Instrument aircraft often have an alternate air valve that allows cabin air to replace the blocked exterior static hole. When being used, the alternate static air causes the altimeter to read high, the airspeed to read high, and the VSI to show a climb for level flight.
For years old timers and the FAA have been arguing what controls airspeed and altitude. 'Stick and Rudder' pilots believe that elevator controls airspeed. The FAA demands that the elevator controls altitude with attitude controlling airspeed. The conflict has became one of theory against reality. Will an FAR soon legislate the laws of physics?
The altitude and airspeed performance is not independent of either attitude or power. The pilot is the controlling factor. What he does in the cockpit with the elevator set attitude and what he does with the throttle sets the power. One control or the other is a factor in all flight and only in some situations does one dominate the other. How well you know your airplane’s flight characteristics is as essential for all flight regimes not just landings. Every aircraft has idiosyncrasies your checkout must expose you to those. Otherwise, you become a test pilot.
In cruise flight and constant speed approaches the elevator dominates attitude and altitude while power sets airspeed. The ability of the elevator to exercise this control exists when power is both variable and available. It is only when power is not available or considered as a locked constant that the elevator can control airspeed. The FAA expects you to use the vertical speed indicator (VSI) (elevators) to maintain pitch and the throttle to keep airspeed. This works for flying the instrument landing system (ILS). If the throttle is set as a constant 1500 rpm then the trimmed elevator can control airspeed. The FAA accepts this idea that elevator controls airspeed when power is constant. The trimmed elevator gives greater control over the glidepath than power.
Power is a relatively coarse adjustment to the glide path. Massive reductions or increases in power produce illusions of change, especially, if airspeed changes occur simultaneously. Only by keeping the airspeed constant can the illusions of change due to power application be overcome. Mistakes, and corrections, in being high and low on approach will be an important part of your landing training. Reduction of power in increments of 100 rpm can allow slight and smoother changes in the approach path. Since the effects of power are so variable, due to inertia, most pilots chose to set power at predetermined levels and use the elevator to set the trim to give the attitude most likely to meet the airspeed sought. Why? Because it works.
We use pitch as a form of energy control. We can covert kinetic into potential and potential back to kinetic. With speed we can use the elevator control to create a climb at a cost of airspeed. With altitude we can use the elevator control to forgo altitude in exchange for airspeed.
In reality the pilot controls the situation by using a combination of elevator and power which through this combination determines airspeed and altitude. At any given moment the pilot will make a control decision between altitude or airspeed and use power or elevator, in combination, to meet the needs of that decision. Where ever the FAA does not have a recommendation as to procedure "...a coordinated combination of both pitch and power adjustments is usually required". The Flight Training handbook is being rewritten as of 1995. Hang on.
1. Best angle climb - obstacle clearance - most altitude over distance. Full power, level wing, set wing angle, hold nose, trim
2. Best rate climb - most altitude over shortest time. Full power, level wing, set wing angle, hold nose, trim
3. Cruise climb - most distance over time + altitude. Full
power, level wing, set wing angle, hold nose pitch attitude same
as slow cruise with reduced power, trim
4. Cruise - Most distance over time - select altitude carefully within 3000 feet above ground. Same as acceleration for takeoff, best glide at idle, slow cruise descent at 2000 rpm, power-on landing at 1500 rpm and approach, trim.
5. Cruise descent - distance over time minus altitude. Reduce power but leaving trim alone will give level flight again just by replacing the proper reduction. Called economy of effort.
6. Power-off landing and approach. Not recommended unless engine is cooled down first. Run at reduced power in gradual descent.
Indicated airspeed:
(common usage) What the airspeed indicator shows it is a raw value. The air pressing on the pitot tube is compared with the pressure on the static port and registered on a dial as miles per hour or knots per hour. (Post-1970 aircraft give performance figures (stall etc) as indicated airspeeds. Indicated airspeed can be used to obtain calibrated airspeed and then changed to true airspeed by correcting for temperature, pressure altitude and instrument installation error.
Indicated airspeed:
(uncommon usage) To get valid indicated" airspeed you must know what the instrument error of a given reading may be. Airspeed indicators allow up to 5 mph error on installation as new. Indicators tend to hang up or ratchet with age. An ever increasing amount of friction is also expected with age. This also applies to people.
Calibrated airspeed:
Indicated airspeed corrected for installation errors. A chart for calibrated airspeed vs. indicated airspeed is in the Pilots Operating Handbook. (Performance Speeds of pre-1970 aircraft are given as calibrated speed.) The indicated airspeeds tend to be lower than calibrated airspeeds on the slow end and higher than calibrated airspeeds at the fast end. Most accurate in mid-ranges.
True airspeed:
Indicated airspeed corrected for temperature and pressure as different from standard. True airspeed increases with altitude. True airspeed can be calculated on the E6B or in the cruise performance table of the POH. At best this is a rough estimate. GPS will give the most accurate true airspeed if the speed is determined over two reciprocal flight courses. The spread between indicated airspeeds and true airspeeds increase with altitude. Only Indicated airspeeds are used in taking off or landing an aircraft.
Ground Speed:
True airspeed corrected for wind effect gives ground speed. Groundspeed is determined on the E6B wind correction slide. Most radar services can give you a ground speed read-out. (See DME, Loran, GPS) At high altitude airports as much as 20% more ground speed will be required for takeoff. The same 20% increase in ground speed will exist at touch down.
These are the "Vital Velocities required for precision flight. Many V speeds vary with the aircraft weight and not always in the way you might expect. Va speed will decrease with weight, stall speed decreases with weight as does best glide speed and approach speed.
Va, Is the design maneuvering speed. This speed is thought of as having to do with control movement. Va, is the turbulent air penetration speed. A full deflection of the controls will cause the stall before it folds, spindles or mutilates. In normal category "clean" aircraft this load limit is 3.8 positive. The load limit of an aircraft can be exceeded by a PIO (pilot induced oscillation) and turbulence in a seconds. Get to or below Va even if you must stall. Va is based on weight, the heavier the weight the higher the Va. Reduce the maximum gross weight Va by a percentage equal to half the weight reduction. 10% weight reduction reduces Va by 5%. I find that thinking of it as driving over country railroad tracks. The lighter the car the higher the bounce. Know your Va for gross from the POH and how to compute it otherwise. Once way to determine the approximate Va at below gross weights is to change the Va by half the percentage of weight reduction. Find the actual weight reduction below gross as a percentage of gross. Increase the published Va by half of the weight reduction percentage. A 30% reduction of weight would result in a 15% increase in Va. Don't even "think" about what happens to Va in over gross situations. Essential knowledge: Va maneuvering speed decreases with decreasing weight. Va maneuvering speed is determined by aircraft weight. Vb Is the seldom used design speed for maximum gust intensity speed.
When an aircraft flies at a constant speed a straight-line relationship between any increase in the angle of attack and the G-load applied. At higher speeds an aircraft can reach a higher angle of attack. With G.A. aircraft by the time the angle of attack becomes great enough to crunch the aircraft with G-load, it will stall. It will not break at speeds below Va. The aircraft that is flown in turbulence must fly at slower speeds, specifically Va or lower, so that abrupt G-loads produced by turbulence angle of attack changes will not exceed the structural capability of the aircraft.
When you fly slower you must increase your angle of attack to maintain level flight at 1-G. A 3.8 increase in G-load is about 3.8 times the higher angle required for the slow speed. The aircraft will stall before reaching this angle. Hence, the value of Va in protecting the aircraft. Additional weight also requires a higher angle of attack for level flight at 1-G. So additional weight plus a slower speed compounds the G-load protection offered because they require a higher angle of attack. Flying lighter and faster decreases the protection. By reducing weight below gross by 2% we can reduced the Va by 1% .
Positive limit load factor for aircraft is +3.8Gs/-1.52 for
normal category, 4.4 Gs/-1.76 for utility, and 6.0/-3 for acrobatic.
When you consider that the 'zero' G is actually 1.0 the ability
of an aircraft to take G-loads is nearly the same in either direction.
Vr Rotation speed allows you to use the wheel axel to raise
the nose prior to lift off where you use the center of lift as
rotation axis for setting climb speed.
Vlof Lift-off speed
Vso is the stall speed in landing configuration. This speed is for level, unaccelerated, 1-G flight and a slow deceleration to stall. It is at the bottom of the white arc of the airspeed indicator at gross weight. At lesser weights it will be slower. The change tends to be proportional, a 5% lower speed for a 5% decrease in weight. The bottom of the white arc is the stall speed for maximum landing weight at the most unfavorable but allowable center of gravity location. Depending on year of aircraft this may be either calibrated or indicated airspeed.
Vs1 Stalling speed or the minimum steady flight speed
obtained in a specified configuration.
Vs 1 is stall speed in a clean configuration and is the bottom
of the green arc. Vs Stalling speed or the minimum steady flight
speed at which the airplane is controllable. This speed as well
as Vso is for level, unaccelerated, 1-G flight and a slow deceleration
to stall.
Vfe is at the top of the white arc maximum speed for
flap extension. The use of lower flap extension speeds reduces
the strain on the system. As aircraft age this reduction can be
important.
Vf Design flap speed
Vlo is the maximum landing-gear operating speed.
Vle is the maximum landing-gear extension speed and
is based upon the durability of the landing gear doors.
Vlo Maximum landing gear operating speed
Vne is the redline speed. This speed is found by diving
1.4 beyond Vno to attain Vd design diving speed. .9 times Vd =
Vne. Beyond Vne you become a test pilot.
Vmo Maximum operating limit speed determined by maximum
continuous power in other than level flight.
Vh Maximum speed in level flight with maximum continous
power
Vno is the top of the green arc known as structural
cruise speed.
Vc is the speed range of the green arc or design cruising
speed
It is used in turbulence that is different than Va; it is called
structural cruise speed or Vno. Unlike Va this is shown on the
airspeed indicator as the meeting point of the orange (yellow)and
green. This is a speed below cruise that is recommended for rough
air penetration. Vno does not offer the structural assurances
offered by Va. At Vno the aircraft, as certified, should not be
structurally damaged by a 35 knot vertical gust. This is not the
same protection given by Va.
Vx is the best angle of climb speed which gives the greatest altitude over horizontal distance. It is used to climb over FAA trees. (50')
Vy is the best rate climb speed. It gives the most altitude over time. It is used in noise abatement situations. Vy is greater than Vx and decreases with altitude, while Vx increases the only time they are equal is at the aircraft's absolute ceiling.
Vso Is the stalling speed or the minimum steady flight
speed obtained in the landing configuration
Vso is also referenced at every 10% reduction in weight gives
a %5 reduction in Vso as the full flap landing speed. This is
your ‘over the fence’ speed. It is a minimum normal
final approach speed. The hydroplaning speed is within a couple
knots of this speed using 7 times (smooth) or 9 times(treaded)
the square root of the tire pressure.
Vref This is a reference speed based on Vso. We can find the short final approach speed by multiplying Vso x 1.3 + 1/2 wind gust speed. Vmu Minimum unstick speed will get you off the ground in groundeffect but not allow climb.
Vd Design diving speed
Vdf Demonstrated flight diving speed
Vfc Maximum speed for stability characteristics
Vmc Minimum control speed with critical engine inoperative
Vtoss Takeoff safety speed for Category A rotorcraft
V1 Takeoff decision speed
V2 Takeoff safety speed
V2min Minimum takeoff safety speed
Vlof
Vfc
Vy decreases with flaps applied
Vx increases with flaps applied
Vx and Vy converge with flaps applied
Vx and Vy separations are greatest with flaps up
Vs is slower with flaps applied
1.3 Vs is slower with flaps applied
Climb rate decrease with flaps applied
Vx can be below Vs at maximum flap settings
1.3 Vs is slower than Vy (Behind the power curve)
1.1 Vs is less than Vx (Short field landings)
Sudden retraction below 1.3 results in stall
Flap use is restricted to 2G maneuvers
You can develop your own standard profile for flying or landing any aircraft. There are only a few flying profiles required. They are takeoff, climb with its variations, level cruise with its variations, descent and its variations, missed approach/go-around, and landing. For each phase there are variations of configuration and airspeeds which can be checklisted to provide constancy in procedure and performance. Once you have developed the procedures and profiles for one aircraft it is much the same process for other types. Standard profile development is needed if you expect to have IFR competency.
Fixed pitch aircraft at gross weight climb at full throttle at Vy or Vx from the POH. Simple, except that most of your flying is not done at gross. You can roughly figure that for every 10% decrease below gross that the Vx and Vy indicated speeds will decrease by 5%. For cruise speeds always let the aircraft reach the desired indicated speed before reducing power. If you reduce power before or afterwards you will begin a cycle of changing speeds, trim, and altitudes that lead to frustration. To make the required power change you must know the required power. Cycle through several climbs and cruise changes until you can anticipate your level off at 75% power by knowing where to set power and trim and the sequence required. Go through the procedure again to determine the requirements to maintain low cruise and slow flight. Follow the same process for going from level cruise, to low cruise, to slow flight and back again until you can make changes without hesitation. Make your own checklist.
By using the wing chord line as a pitch angle indicator a VFR pilot can note that there are several performance standards where all aircraft have very close to the same pitch angle. The variables being power and airspeed.
Best glide
Slow cruise
Cruise
Cruise climb
Best-angle of climb
Best-rate of climb
Only your computation of Vref can make more specific climb speeds that the POH. Regardless, Vx is slower than Vy until they meet at the aircraft’s ceiling. Vy is highest at sea level and decreases with altitude. The decrease is related to the decrease in excess engine power.
Any climb in excess of Vy is in front of the power curve. The rate of climb is determined by excess power. The angle of a Vx climb is set by excess thrust. One of the reasons Vx is lower than Vy is because thrust decreases with airspeed. Vx is on the backside of the thrust-required curve and in a region of reversed command that if pitch increases you will reach the power-on stall.
Any takeoff or climb at Vx is going to test your coordination abilities. At Vx you have reduced forward visibility and an increased need for proper rudder application. Rudder will still do what the ailerons cannot. Wrong amount of rudder and a Vx stall will initiate a spin. Aileron application will, however, provide the adverse yaw needed for a spin entry. The Vx climb and stall at several thousand feet has none of the inherent dangers of one at takeoff. You cannot trust the POH speeds any low level Vx flight should be practiced at altitude first.
An airplane can land in considerably less distance than it can takeoff. even your roll to a stop is a shorter distance than your accelerate to liftoff distance. An airplane has certain design capabilities. Age and attrition will reduce original ability. Knowing this, a pilot, should be able to determine an aircraft’s present capability and make the aircraft perform to that level.
The sum of an aircraft’s flying energy must add together the potential energy it has in altitude along with the kinetic energy due to its movement in air. These two energies can be exchanged within limits. There is a curved relationship between the airspeed and the power required from the engine. The limit of the curve is determined by available power. The power of the engine is changed into thrust via the propeller with some additional loss of efficiency. The faster you fly the lower the thrust because the faster the air moves past the propeller the less additional kick it can provide.
The fore-mentioned curve has a front side and a back side. The dividing line is the point at which a minimum amount of power can be used to maintain a minimum airspeed in sustained level flight. The back side is a slower speed while the front side is a faster speed. At this point there is a relationship reversal between power and airspeed. On the back side more power produces a lower airspeed only when the aircraft is first slowed; on the front side more power produces a higher airspeed. At some point there is insufficient power to fly any slower. Only by lowering the nose and losing altitude can a recovery be made. One of the saddest moments of my life was during WWII when an airplane crashed near me after getting too far behind the power curve.
The next major phase is the descent. By using the POH Vy speed, as adjusted for weight, and adding about 30 knots we have obtained the approach speed for that weight that will give a 500 fpm descent. for the desired 3° approach slope at 10 miles we should be a 3000 feet. At five miles you should be 1500 feet above touchdown. At your two mile report or downwind entry you should be a pattern altitude but no less than 600 feet. Every additional drag configuration will contribute a 10 knot reduction in speed. Appropriate trim adjustments must always be made to maintain the 3° approach slope. If power alone is used to make a descent, you will find that 500 rpm reduction will approximate a 500 fpm descent. Pattern descent power setting begin at the numbers. From full cruise the C-150 first has carburetor heat applied and the throttle reduced to 1500. You hold heading and altitude until reaching 60 knots. The same is done with the C-172 except the power is reduced only to 1700 rpm and heading and altitude maintained to reach 70 knots.
Airspeed control begins with knowing the power setting required for any flight condition. We climb, with full throttle, not with a range of speeds, we climb at a certain speed which the POH says will get us the highest in the least amount of time. Vy is that speed. It is 65 in the C-150 and 75 in the C-172. These are gross weight speeds. At less than gross a slightly slower speed by two or three knots would be Vy. Noise abatement requirements and safety say that you should always climb at Vy. Level cruise is 85 knots in the C-150 and 100 knots in the C-172. All speeds are indicated airspeeds. The last speeds are landing speeds flown in both types of aircraft with a power setting of 1500 rpm. The C-150 uses 60 knots in all configurations for normal approaches into the flare. The C-172 uses 70 knots until final, which is flown at 60 knots.
Full power in all fixed pitch climbs is the simplest application. When leveling off the power is kept fully applied until the C-150 has reached 85 knots and the C-172 has reached 100 knots. On reaching these speeds the power is reduced to 2450 rpm for level cruise flight below 5000 feet. This is very close to 75% power. Above 5000 the rpm can be advanced 100 rpm for every additional 2500 feet of altitude. Again, this is about 75% of power. Leaning for best operation can be done at all altitudes flown with constant power.
As a student you begin your sight profile development by getting the required POH speeds. You need to write in the changes as affected by weight variations of solo and dual. Apply these changes as required by your normal operating weight. Fly the proper Vref airspeed. A standard profile works for the pilot who can fly the aircraft to the speed and performance determined to be safe.
For every airspeed in level flight there is a power needed.
At every airspeed there is equal thrust and drag. Excess thrust results in acceleration. Whenever drag exceeds thrust deceleration occurs.
Power and speed are normal in front of the power curve.
Adding power while holding altitude will cause acceleration until thrust and drag are equal.
A level back-side airspeed can be increased with power but then the power needed to stay there is less than that required to fly slower.
Reducing power to fly a slower level back-side speed requires added power to maintain that speed. More power will be required for this slower speed.
Stall speed
Inverse effect on Va in turbulence
Absolute Altitude.
Vx is the best climb to get over an obstacle. Vy is the best climb to attain altitude over time. At altitude increases the Vy speed decreases while the Vx speed increases. At a given altitude where the two speeds are the same we have reached the aircraft’s absolute altitude.
Arrival; ...Radio log; ...Airport Information; ...Weather Information; ...Crosswind Information;
Don't takeoff until you know as much as possible about where you are going. A part of your preflight that may not be a part of your early training is preparation for your arrival. Statistics show that inadequate preflight combined with landings, comprise the highest cause and place factors for accidents. An arrival produces unexpected situations and conditions that can confuse even experienced pilots. The risk of any arrival can only be reduced by adequate research and information.
Regardless of density altitude you always fly approach speeds according to the POH. True airspeed and your apparent speed will be higher the higher the density altitude. If you are a lowland pilot, this effect can be quite disconcerting. Lean your mixture for best performance. Landing on any unfamiliar runway of less than 150% of the obstacle clearance distance for your aircraft is indicative of poor judgment. The additional distance is needed for any mistakes in judgment or performance.
You need to make an arrival information checklist to supplement the information available from the charts and airport guides. Abbreviate the following information on an airport card. A concise form 2x2 inches will contain most of the information and the other side should have a simple diagram of the airport, patterns and beacon. Airports and their surroundings are easier to locate when we know where to look.
Radio log
Com Nav
FSS VOR/code/radials
RADAR NDB/code
Tower
Ground
Unicom
CTAF
Name
Frequencies
Arrival call-up points/common/familiar/likely
Pattern entry/easy/efficient/likely
Altitude/Pattern altitude
Runway numbers available/Runway number probable
Density altitude
Wind favored runway
Runway elevation and length/required length
Lighting
Surface winds
Ceiling
Temperature
Based on expected runway and expected wind
Crosswind component Headwind component
to 30deg; off runway 1/2 velocity 3/4 velocity
to 60deg; off runway 3/4 velocity 1/2 velocity
to 90deg; off runway full velocity zero