REFERENCES: C 61-21, AC61-23, Airplane
Handbook and Flight Manual
P 1. Explain aircraft systems and operation.
Ex Controls, flaps, trim, engine, instruments, landing gear,
engine, propeller, fuel system, hydraulic system, electrical system,
environmental system, icing, navigation and communications, and
vacuum system.
The ailerons, elevators, and rudder are usually moved via a
system of cables and pulleys connected to a yoke or stick. In
some instances a system of push rods may be used. Flaps and spoilers
may be operated by push rods or electrically. Trim may be manual,
electrical or both. High performance aircraft may have hydraulic
or electric boost systems to aid the pilot.
Flight instruments have several modes of operation. The compass
is magnetic. The ball is gravitational and inertial. The needle
or turn coordinator is usually electric gyro driven. The attitude
and heading indicators are usually gyro driven by vacuum pressure.
The altimeter, airspeed indicator, and vertical speed indicator
are functions of outside air pressures. Examiners have been known
to cover or otherwise disable instruments.
Landing gear, fixed or retractable, have shock absorbing springs,
air/oil struts, or rubber in combination to take the shock of
landing. Retractables may operate manually or electrically with
visual or lighted indicators as to gear position. Higher insurance
and maintenance costs go with retractables.
Brakes are usually hydraulically operated shoes clamped to
the brake disk attached to the wheels. Hydraulic cylinder connected
to the top of the rudder pedals allows toe pressure to operate
the brakes. Retractable gear has similar braking systems. Aircraft
tires are usually of natural rubber and have a four-ply rating
but only two plies. This means that when you can see the beginning
of cord in a tire it is absolutely time to quit using it. The
nose wheel regardless of its suspension system allows the application
of foot pressure on pedals and brakes to provide ground steering.
Good operational techniques would use the nose wheel only during
the very slowest part of takeoff and landing.
Most light aircraft engines are four stroke, (intake, compression,
power and exhaust), horizontally opposed, and gasoline fueled.
Each cylinder has a spark plug on top and bottom, which obtain
an igniting, spark from dual magnetos. Each cylinder has an upper
and lower spark plug. The magneto serving the top right plugs
services the lower left plugs.
Spark plugs fouling from fuels with lead would be caused at
low power settings where the internal cylinder temperature was
not high enough to vaporize additives. Small lead pellets would
form in the lower plugs and cause preignition. When unleaded fuels
are used the deposits are calcium like particles that cause preignition
(knocking in automobiles) by shorting out the spark plugs. Avoid
low power descents and power off operations. During taxi be assure
to lean so as to avoid lead fouling. At shut down the rpm may
be increased momentarily so as to facilitate removal of any accumulated
fouling. Preignition is shown by engine roughness, backfiring
and high cylinder head temperatures. Detonations occur as a result
of ignition of unburned combustible material by pressure or temperature.
1. Copper runout or lead fouling = excessive heat;
2. Carbon and lead bromide deposits = low temperature and excess
richness.
3. Oil fouling shows piston ring problems and wear.
4. Other than brown/gray deposits = incomplete combustion
5. Cracked porcelain = preignition
6. Carbon fouling = valve guide or ring wear and oil burning.
The controls for the engine are few. The throttle moves a wire
connected to the butterfly valve of a carburetor engine and controls
the airflow drawing fuel to the engine. Pumping the throttle can
fill the carburetor as a priming method. Over use of this priming
can cause the fuel to over flow and start an engine fire. The
fuel injected engine throttle performs a similar function but
provides better fuel distribution. A fuel-injected engine cannot
be primed by pumping the throttle.
The venturi effect of a carburetor air intake can cause any
moisture in the air when mixed with fuel to form ice and adhere
to the interior of the venturi. This ice can choke off the flow
of air to the carburetor. This is most likely to occur at low
power settings but can occur at any time even on very warm days.
The symptoms of carburetor ice are insidious but start with unexplained
loss of RPM or manifold pressure accompanied by roughening engine
operation. Since this condition arises from conditions outside
the aircraft, correction rather than prevention is the control
method.
Application of carburetor heat opens a diversion gate in the
heater- exhaust system and cuts off the outside air intake while
diverting hot air into the carburetor. The hot air causes an additional
drop in RPM or manifold pressure and a rise as the ice melts.
Removal of carburetor heat will give an additional rise in RPM
and manifold pressure. Fuel injected engines do not have carburetor
heat controls.
Air and fuel are mixed by weight. About 16 pounds of
air to 1 pound of fuel gives best power. An engine can intake
only so much air depending on the volume of its piston cavity.
As the density of the air decreases with altitude the air molecule
intake into the engine decreases. The 16 to 1 air fuel ratio becomes
over-rich with fuel and power decreases. The mixture control allows
the pilot to adjust the air/fuel mixture for the best power for
the air available. Even so the power of a normally aspirated engine
decreases with altitude. It is possible to install an air pump
called a turbocharger which will pressurize the air being taken
into the cylinders and make possible more fuel consumption and
greater power.
Most light aircraft have a fixed pitch propeller, which is
a compromise pitch between a climb or cruise propeller. A constant
speed propeller has an additional cockpit control, which allows
the pilot to use oil from the engine to adjust the pitch for best
climb or cruise. The setting of the control causes the propeller
to maintain a constant RPM.
The airplane can operate much like a lawn mower. Just turning
the propeller can give the electrical spark needed for operation.
It is this feature which makes ground operation so dangerous.
A shorted magneto or fuel left in the carburetor could cause any
small movement of the propeller to start the engine. For these
reasons the engine shut down should include a magneto check and
fuel starvation. The checking of the magnetos prior to takeoff
should be as recommended in the POH (Pilot's Operating Handbook).
Checking at a lower RPM may cause a higher than normal magneto
drop, giving a false indication of trouble. A minimal or nonexistent
drop should raise suspicions of a "hot" or shorted magneto.
Hot magneto checks should be done at RPMs less than 800. A momentary
turn to "off" should show whether the engine is going
to stop (as it should) before returning to "both".
Gasoline is the fuel for airplanes. The fuel is enclosed in
metal or rubber tanks, which have cockpit gauges to indicate either
weight or quantity. The safest method to judge fuel is by time.
All low wing aircraft have electric fuel pumps as a backup for
the engine driven pump. High wing aircraft do not usually have
auxiliary pumps since the gravity flow is considered adequate.
All aircraft have a cockpit operated shutoff valve for gasoline
to the tanks. Most aircraft have fuel tank selector valves associated
with the shutoff valve. Low wing aircraft normally select single
tank operation while high wing aircraft select both tanks.
Every aircraft engine is designed for a specific grade of fuel.
Only this grade or a higher grade should ever be used. All grades
of fuel have different colors. The mixing of grades may give a
colorless mixture. The smell of the also colorless jet fuel is
an important safety check. Since the fumes of gasoline are very
explosive the aircraft should be grounded during fueling to prevent
and static electrical discharges. It is very possible to get widely
varying amounts of fuel into an aircraft tank depending on the
how level the ground. A level engine can make a difference of
1/2-quart reading in the oil level.
Fuels were once available as 80/87, (red) 91/96 (blue) and
115/145 (green) octane. The first two of these have been replaced
by 100LL (blue)(low lead). With some changes in maintenance low
compression engines can use 100LL with no problem. Because of
exhaust valve damage and valve guide wear of 100/130 (green) can
only be used with lead scavenging additives. Where carburetor
icing is a problem, certain anti-icing additives are available
to be used only after consulting aircraft manufacturer as to compatibility
with fuel tanks.
Automotive fuels must have STC (supplemental type certificate)
for the specific aircraft and engine before use. Such fuels may
cause preignition, detonation, vapor lock and valve problems.
Specific brands of fuel differ in their properties and composition.
Aircraft filler openings must be marked as to minimum grade to
be used.
The hydraulic system of most small aircraft applies mostly
to the brake system. Since brake application puts very high pressures
on the lines and hoses it is vital that the preflight check for
any hydraulic fluid leaks. These leaks are best noted by the accumulation
of oily dirt.
The engine also may have accessories. A battery-powered starter
can turn the propeller. An engine driven generator or alternator
will give enough electricity for lighting, radios and auxiliary
motors. At low power a generator may be inadequate but it will
function without a battery. An alternator needs battery voltage
through its field coil. Then it will function even at very low
engine power.
Each electrical circuit in the airplane will have a fuse or
circuit breaker for protection. If something fails to work properly
first confirm the switch position and then the fuse or breaker.
The ampere meter will show the proper functioning of the electrical
system and sometimes the load imposed. Many aircraft have an external
battery plug, which will allow an external battery to be used
to start the engine. The alternator will still require at least
a partially charged battery.
Adjustable air vents can be set to admit outside air into the
cockpit. The engine exhaust system has a heater muff, which can
conduct hot air into the cockpit. If there is a leak in the exhaust
system carbon monoxide can enter as well. Always mix heater air
with fresh air as well has having a detector disk.
There is no reason for the small aircraft to be exposed to
structural ice. Do not fly in or into weather conditions conducive
to icing. The only ice prevention device on a small aircraft might
be the pitot heat on the airspeed system. This should be turned
on when in precipitation as a preventative measure.
The vacuum system usually runs off an engine driven pump. The
cockpit has a vacuum pressure gauge that should read between 4.5
and 5.4 for normal operation. This pressure is used to operate
to attitude and heading indicators. Other things may work from
this as well. At vacuum pump failure the heading indicator will
begin to spin and the attitude indicator will begin to tilt and
remain tilted. If in IFR conditions, cover up any failed instrument.
The aircraft radio is VHF FM, which reduces interference but
operates essentially on line of sight from 118.0 to 135.975 kHz.
The current 720 possible frequency selections can be as selective
as 25/1000ths of a kHz, such as 122.725, and 122.975 which are
the 1992 additions to UNICOM frequencies. 122.72 and 122.97 may
be assumed to have the additional 5 to the thousandth place. Many
aircraft have an avionics master switch to reduce the frequency
of radio on/off switch failure. It is best to make an initial
setting of the radio volume and leave it. Use the panel switch
to turn off the speaker or phones.
The navigation side of the radio goes from 108.0 to 117.9 MH
FM. There is an additional switch, which allows a .05 sideband
to increase the reception of navigational aids operational verification/identification
code. No NAVAID should be used without such identification. The
use of the NAV side to receive voice from an FSS is now obsolescent.
