Figure 7-32. Aviation fuel color-coding system.
80
AVGAS
100
AVGAS
100LL
AVGAS
JET
A
AVGAS
100LL
AVGAS
80
AVGAS
100 JET A
RED GREEN BLUE COLORLESS
OR STRAW
Fuel Strainers, Sumps, and Drains
After leaving the fuel tank and before it enters the carburetor,
the fuel passes through a strainer that removes any moisture
and other sediments in the system. Since these contaminants
are heavier than aviation fuel, they settle in a sump at the
bottom of the strainer assembly. A sump is a low point in a fuel
system and/or fuel tank. The fuel system may contain a sump,
a fuel strainer, and fuel tank drains, which may be collocated.
The fuel strainer should be drained before each flight. Fuel
samples should be drained and checked visually for water
and contaminants.
Water in the sump is hazardous because in cold weather the
water can freeze and block fuel lines. In warm weather, it
can flow into the carburetor and stop the engine. If water is
present in the sump, more water in the fuel tanks is probable,
and they should be drained until there is no evidence of water.
Never take off until all water and contaminants have been
removed from the engine fuel system.
Because of the variation in fuel systems, become thoroughly
familiar with the systems that apply to the aircraft being flown.
Consult the AFM/POH for specific operating procedures.
Fuel Grades
Aviation gasoline (AVGAS) is identified by an octane or
performance number (grade), which designates the antiknock
value or knock resistance of the fuel mixture in the engine
cylinder. The higher the grade of gasoline, the more pressure
the fuel can withstand without detonating. Lower grades of
fuel are used in lower-compression engines because these
fuels ignite at a lower temperature. Higher grades are used
in higher-compression engines because they ignite at higher
temperatures, but not prematurely. If the proper grade of fuel
is not available, use the next higher grade as a substitute. Never
use a grade lower than recommended. This can cause the
cylinder head temperature and engine oil temperature to exceed
their normal operating ranges, which may result in detonation.
Several grades of AVGAS are available. Care must be
exercised to ensure that the correct aviation grade is being
used for the specific type of engine. The proper fuel grade is
stated in the AFM/POH, on placards in the flight deck, and
next to the filler caps. Automobile gas should NEVER be
used in aircraft engines unless the aircraft has been modified
with a Supplemental Type Certificate (STC) issued by the
Federal Aviation Administration (FAA).
The current method identifies AVGAS for aircraft with
reciprocating engines by the octane and performance number,
along with the abbreviation AVGAS. These aircraft use
AVGAS 80, 100, and 100LL. Although AVGAS 100LL
performs the same as grade 100, the “LL” indicates it has
a low lead content. Fuel for aircraft with turbine engines is
classified as JET A, JET A-1, and JET B. Jet fuel is basically
kerosene and has a distinctive kerosene smell. Since use of
the correct fuel is critical, dyes are added to help identify the
type and grade of fuel. [Figure 7-32]
In addition to the color of the fuel itself, the color-coding
system extends to decals and various airport fuel handling
equipment. For example, all AVGAS is identified by name,
using white letters on a red background. In contrast, turbine
fuels are identified by white letters on a black background.
Special Airworthiness Information Bulleting (SAIB)
NE-11-15 advises that grade 100VLL AVGAS is acceptable
for use on aircraft and engines. 100VLL meets all
performance requirements of grades 80, 91, 100, and 100LL;
meets the approved operating limitations for aircraft and
engines certificated to operate with these other grades of
AVGAS; and is basically identical to 100LL AVGAS. The
lead content of 100VLL is reduced by about 19 percent.
100VLL is blue like 100LL and virtually indistinguishable.
Fuel Contamination
Accidents attributed to powerplant failure from fuel
contamination have often been traced to:
• Inadequate preflight inspection by the pilot
• Servicing aircraft with improperly filtered fuel from
small tanks or drums
• Storing aircraft with partially filled fuel tanks
• Lack of proper maintenance
Fuel should be drained from the fuel strainer quick drain and
from each fuel tank sump into a transparent container and
then checked for dirt and water. When the fuel strainer is
being drained, water in the tank may not appear until all the
fuel has been drained from the lines leading to the tank. This
indicates that water remains in the tank and is not forcing the
fuel out of the fuel lines leading to the fuel strainer. Therefore,
drain enough fuel from the fuel strainer to be certain that
fuel is being drained from the tank. The amount depends on
the length of fuel line from the tank to the drain. If water or
other contaminants are found in the first sample, drain further
samples until no trace appears.
Water may also remain in the fuel tanks after the drainage
from the fuel strainer has ceased to show any trace of water.
This residual water can be removed only by draining the fuel
tank sump drains.
Water is the principal fuel contaminant. Suspended water
droplets in the fuel can be identified by a cloudy appearance
of the fuel, or by the clear separation of water from the colored
fuel, which occurs after the water has settled to the bottom
of the tank. As a safety measure, the fuel sumps should be
drained before every flight during the preflight inspection.
Fuel tanks should be filled after each flight or after the last
flight of the day to prevent moisture condensation within the
tank. To prevent fuel contamination, avoid refueling from
cans and drums.
In remote areas or in emergency situations, there may be no
alternative to refueling from sources with inadequate anti-
contamination systems. While a chamois skin and funnel
may be the only possible means of filtering fuel, using
them is hazardous. Remember, the use of a chamois does
not always ensure decontaminated fuel. Worn-out chamois
do not filter water; neither will a new, clean chamois that is
already water-wet or damp. Most imitation chamois skins
do not filter water.
Fuel System Icing
Ice formation in the aircraft fuel system results from the
presence of water in the fuel system. This water may be
undissolved or dissolved. One condition of undissolved
water is entrained water that consists of minute water
particles suspended in the fuel. This may occur as a result of
mechanical agitation of free water or conversion of dissolved
water through temperature reduction. Entrained water settles
out in time under static conditions and may or may not be
drained during normal servicing, depending on the rate at
which it is converted to free water. In general, it is not likely
that all entrained water can ever be separated from fuel under
field conditions. The settling rate depends on a series of
factors including temperature, quiescence, and droplet size.
The droplet size varies depending upon the mechanics
of formation. Usually, the particles are so small as to be
invisible to the naked eye, but in extreme cases, can cause
slight haziness in the fuel. Water in solution cannot be
removed except by dehydration or by converting it through
temperature reduction to entrained, then to free water.
Another condition of undissolved water is free water that
may be introduced as a result of refueling or the settling of
entrained water that collects at the bottom of a fuel tank. Free
water is usually present in easily detected quantities at the
bottom of the tank, separated by a continuous interface from
the fuel above. Free water can be drained from a fuel tank
through the sump drains, which are provided for that purpose.
Free water, frozen on the bottom of reservoirs, such as the
fuel tanks and fuel filter, may render water drains useless
and can later melt releasing the water into the system thereby
causing engine malfunction or stoppage. If such a condition
is detected, the aircraft may be placed in a warm hangar to
reestablish proper draining of these reservoirs, and all sumps
and drains should be activated and checked prior to flight.
Entrained water (i.e., water in solution with petroleum fuels)
constitutes a relatively small part of the total potential water
in a particular system, the quantity dissolved being dependent
on fuel temperature and the existing pressure and the water
volubility characteristics of the fuel. Entrained water freezes
in mid fuel and tends to stay in suspension longer since the
specific gravity of ice is approximately the same as that of
AVGAS.
Water in suspension may freeze and form ice crystals of
sufficient size such that fuel screens, strainers, and filters
may be blocked. Some of this water may be cooled further as
the fuel enters carburetor air passages and causes carburetor
metering component icing, when conditions are not otherwise
conducive to this form of icing.
Prevention Procedures
The use of anti-icing additives for some aircraft has been
approved as a means of preventing problems with water
and ice in AVGAS. Some laboratory and flight testing
indicates that the use of hexylene glycol, certain methanol
derivatives, and ethylene glycol mononethyl ether (EGME)
in small concentrations inhibit fuel system icing. These tests
indicate that the use of EGME at a maximum 0.15 percent
by volume concentration substantially inhibits fuel system
icing under most operating conditions. The concentration
of additives in the fuel is critical. Marked deterioration in
additive effectiveness may result from too little or too much
additive. Pilots should recognize that anti-icing additives are
in no way a substitute or replacement for carburetor heat.
Aircraft operating instructions involving the use of carburetor
heat should be adhered to at all times when operating under
atmospheric conditions conducive to icing.
Refueling Procedures
Static electricity is formed by the friction of air passing
over the surfaces of an aircraft in flight and by the flow of
fuel through the hose and nozzle during refueling. Nylon,
Dacron, or wool clothing is especially prone to accumulate
and discharge static electricity from the person to the funnel
or nozzle. To guard against the possibility of static electricity
igniting fuel fumes, a ground wire should be attached to the
aircraft before the fuel cap is removed from the tank. Because
both the aircraft and refueler have different static charges,
bonding both components to each other is critical. By bonding
both components to each other, the static differential charge is
equalized. The refueling nozzle should be bonded to the aircraft
before refueling begins and should remain bonded throughout
the refueling process. When a fuel truck is used, it should be
grounded prior to the fuel nozzle contacting the aircraft.
If fueling from drums or cans is necessary, proper bonding
and grounding connections are important. Drums should be
placed near grounding posts, and the following sequence of
connections observed:
1. Drum to ground
2. Ground to aircraft
3. Drum to aircraft or nozzle to aircraft before removing
the fuel cap
When disconnecting, reverse the order.
The passage of fuel through a chamois increases the charge
of static electricity and the danger of sparks. The aircraft
must be properly grounded and the nozzle, chamois filter,
and funnel bonded to the aircraft. If a can is used, it should
be connected to either the grounding post or the funnel.
Under no circumstances should a plastic bucket or similar
nonconductive container be used in this operation.
Heating System
There are many different types of aircraft heating systems that
are available depending on the type of aircraft. Regardless of
which type or the safety features that accompany them, it is
always important to reference the specific aircraft operator’s
manual and become knowledgeable about the heating system.
Each has different repair and inspection criteria that should
be precisely followed.
Fuel Fired Heaters
A fuel fired heater is a small mounted or portable space-
heating device. The fuel is brought to the heater by using
piping from a fuel tank, or taps into the aircraft’s fuel system.
A fan blows air into a combustion chamber, and a spark plug
or ignition device lights the fuel-air mixture. A built-in safety
switch prevents fuel from flowing unless the fan is working.
Outside the combustion chamber, a second, larger diameter
tube conducts air around the combustion tube’s outer surface,
and a second fan blows the warmed air into tubing to direct
it towards the interior of the aircraft. Most gasoline heaters
can produce between 5,000 and 50,000 British Thermal Units
(BTU) per hour.
Fuel fired heaters require electricity to operate and are
compatible with a 12-volt and 24-volt aircraft electrical
system. The heater requires routine maintenance, such as
regular inspection of the combustion tube and replacement of
the igniter at periodic intervals. Because gasoline heaters are
required to be vented, special care must be made to ensure the
vents do not leak into the interior of the aircraft. Combustion
byproducts include soot, sulfur dioxide, carbon dioxide, and
some carbon monoxide. An improperly adjusted, fueled, or
poorly maintained fuel heater can be dangerous.
Exhaust Heating Systems
Exhaust heating systems are the simplest type of aircraft
heating system and are used on most light aircraft. Exhaust
heating systems are used to route exhaust gases away from
the engine and fuselage while reducing engine noise. The
exhaust systems also serve as a heat source for the cabin
and carburetor.
The risks of operating an aircraft with a defective exhaust
heating system include carbon monoxide poisoning, a
decrease in engine performance, and an increased potential
for fire. Because of these risks, technicians should be aware
of the rate of exhaust heating system deterioration and should
thoroughly inspect all areas of the exhaust heating system to
look for deficiencies inside and out.
Combustion Heater Systems
Combustion heaters or surface combustion heaters are often
used to heat the cabin of larger, more expensive aircraft.
This type of heater burns the aircraft’s fuel in a combustion
chamber or tube to develop required heat, and the air
flowing around the tube is heated and ducted to the cabin.
A combustion heater is an airtight burner chamber with a
stainless-steel jacket. Fuel from the aircraft fuel system is
ignited and burns to provide heat. Ventilation air is forced
over the airtight burn chamber picking up heat, which is then
dispersed into the cabin area.
When the heater control switch is turned on, airflow, ignition,
and fuel are supplied to the heater. Airflow and ignition are
constant within the burner chamber while the heater control
switch is on. When heat is required, the temperature control
is advanced, activating the thermostat. The thermostat (which
senses ventilation air temperature) turns on the fuel solenoid
allowing fuel to spray into the burner chamber. Fuel mixes
with air inside the chamber and is ignited by the spark plug,
producing heat.
The by-product, carbon monoxide, leaves the aircraft through
the heater exhaust pipe. Air flowing over the outside of the
burner chamber and inside the jacket of the heater absorbs
the heat and carries it through ducts into the cabin. As the
thermostat reaches its preset temperature, it turns off the fuel
solenoid and stops the flow of fuel into the burner chamber.
When ventilation air cools to the point that the thermostat
again turns the fuel solenoid on, the burner starts again.
This method of heat is very safe as an overheat switch is
provided on all combustion heaters, which is wired into
the heater’s electrical system to shut off the fuel in the case
of malfunction. In the unlikely event that the heater fuel
solenoid, located at the heater, remains open or the control
switches fail, the remote fuel solenoid and/or fuel pump is
shut off by the mechanical overheat switch, stopping all fuel
flow to the system.
As opposed to the fuel fired cabin heaters that are used
on most single-engine aircraft, it is unlikely for carbon
monoxide poisoning to occur in combustion heaters.
Combustion heaters have low pressure in the combustion
tube that is vented through its exhaust into the air stream. The
ventilation air on the outside of the combustion chamber is
of higher pressure than on the inside, and ram air increases
the pressure on the outside of the combustion tube. In the
event a leak would develop in the combustion chamber, the
higher-pressure air outside the chamber would travel into the
chamber and out the exhaust.
Bleed Air Heating Systems
Bleed air heating systems are used on turbine-engine
aircraft. Extremely hot compressor bleed air is ducted into
a chamber where it is mixed with ambient or re-circulated
air to cool the air to a useable temperature. The air mixture
is then ducted into the cabin. This type of system contains
several safety features to include temperature sensors that
prevent excessive heat from entering the cabin, check
valves to prevent a loss of compressor bleed air when
starting the engine and when full power is required, and
engine sensors to eliminate the bleed system if the engine
becomes inoperative.
Electrical System
Most aircraft are equipped with either a 14- or a 28-volt direct
current (DC) electrical system. A basic aircraft electrical
system consists of the following components:
• Alternator/generator
• Battery
• Master/battery switch
• Alternator/generator switch
• Bus bar, fuses, and circuit breakers
• Voltage regulator
• Ammeter/loadmeter
• Associated electrical wiring
Engine-driven alternators or generators supply electric
current to the electrical system. They also maintain a
sufficient electrical charge in the battery. Electrical energy
stored in a battery provides a source of electrical power for
starting the engine and a limited supply of electrical power
for use in the event the alternator or generator fails.
Most DC generators do not produce a sufficient amount of
electrical current at low engine rpm to operate the entire
electrical system. During operations at low engine rpm, the
electrical needs must be drawn from the battery, which can
quickly be depleted.
Alternators have several advantages over generators.
Alternators produce sufficient current to operate the entire
electrical system, even at slower engine speeds, by producing
alternating current (AC), which is converted to DC. The
electrical output of an alternator is more constant throughout
a wide range of engine speeds.
Some aircraft have receptacles to which an external ground
power unit (GPU) may be connected to provide electrical
energy for starting. These are very useful, especially
during cold weather starting. Follow the manufacturer’s
recommendations for engine starting using a GPU.
The electrical system is turned on or off with a master switch.
Turning the master switch to the ON position provides
electrical energy to all the electrical equipment circuits
except the ignition system. Equipment that commonly uses
the electrical system for its source of energy includes:
• Position lights
• Anticollision lights
• Landing lights
• Taxi lights
• Interior cabin lights
• Instrument lights
• Radio equipment
• Turn indicator
• Fuel gauges
Figure 7-33. On this master switch, the left half is for the alternator
and the right half is for the battery.
• Electric fuel pump
• Stall warning system
• Pitot heat
• Starting motor
Many aircraft are equipped with a battery switch that
controls the electrical power to the aircraft in a manner
similar to the master switch. In addition, an alternator switch
is installed that permits the pilot to exclude the alternator
from the electrical system in the event of alternator failure.
[Figure 7-33]
With the alternator half of the switch in the OFF position, the
entire electrical load is placed on the battery. All nonessential
electrical equipment should be turned off to conserve
battery power.
A bus bar is used as a terminal in the aircraft electrical system
to connect the main electrical system to the equipment using
electricity as a source of power. This simplifies the wiring
system and provides a common point from which voltage can
be distributed throughout the system. [Figure 7-34]
Fuses or circuit breakers are used in the electrical system to
protect the circuits and equipment from electrical overload.
Spare fuses of the proper amperage limit should be carried in
the aircraft to replace defective or blown fuses. Circuit breakers
have the same function as a fuse but can be manually reset,
rather than replaced, if an overload condition occurs in the
electrical system. Placards at the fuse or circuit breaker panel
identify the circuit by name and show the amperage limit.
An ammeter is used to monitor the performance of the aircraft
electrical system. The ammeter shows if the alternator/
generator is producing an adequate supply of electrical power.
It also indicates whether or not the battery is receiving an
electrical charge.
Ammeters are designed with the zero point in the center
of the face and a negative or positive indication on either
side. [Figure 7-35] When the pointer of the ammeter is
on the plus side, it shows the charging rate of the battery.
A minus indication means more current is being drawn
from the battery than is being replaced. A full-scale minus
deflection indicates a malfunction of the alternator/generator.
A full-scale positive deflection indicates a malfunction of
the regulator. In either case, consult the AFM/POH for
appropriate action to be taken.
Not all aircraft are equipped with an ammeter. Some have
a warning light that, when lighted, indicates a discharge in
the system as a generator/alternator malfunction. Refer to the
AFM/POH for appropriate action to be taken.
Another electrical monitoring indicator is a loadmeter.
This type of gauge has a scale beginning with zero and
shows the load being placed on the alternator/generator.
[Figure 7-35] The loadmeter reflects the total percentage of
the load placed on the generating capacity of the electrical
system by the electrical accessories and battery. When all
electrical components are turned off, it reflects only the
amount of charging current demanded by the battery.
A voltage regulator controls the rate of charge to the battery
by stabilizing the generator or alternator electrical output. The
generator/alternator voltage output should be higher than the
battery voltage. For example, a 12-volt battery would be fed
by a generator/alternator system of approximately 14 volts.
The difference in voltage keeps the battery charged.
Hydraulic Systems
There are multiple applications for hydraulic use in aircraft,
depending on the complexity of the aircraft. For example, a
hydraulic system is often used on small airplanes to operate
wheel brakes, retractable landing gear, and some constant-
speed propellers. On large airplanes, a hydraulic system is
used for flight control surfaces, wing flaps, spoilers, and
other systems.
A basic hydraulic system consists of a reservoir, pump
(either hand, electric, or engine-driven), a filter to keep the
fluid clean, a selector valve to control the direction of flow,
a relief valve to relieve excess pressure, and an actuator.
[Figure 7-36]
Figure 7-34. Electrical system schematic.
Low-voltage
warning light
ALT
To wing
flap circuit
breaker
Magnetos
Starter
contactor
Clock
Ammeter
Alternator
control unit
Circuit breaker (auto-reset)
Circuit breaker (push to reset)
Circuit breaker (pull—off,
push to reset)
Fuse Diode
Resistor
Capacitor (Noise Filter)
To inst
LTS
circuit
breaker FUEL IND.
BCN PITOT
PULL
OFF STROBE
RADIO FAN
LDG LTS
FLAP
INST LTS
STBY VAC
RADIO 1
RADIO 2
RADIO 3
NAV
DOME
RADIO 4
G
F
B
Alternator
B
Master
switch
Pull off
Alternator field
circuit breaker
Flight hour
recorder
Oil pressure
switch
To fuel quantity indicators
To flashing beacon
To pitot heat
To radio cooling fan
To strobe lights
To landing and taxi lights
To ignition switch
To wing flap system
To red doorpost maplight
To low-voltage warning light
To instrument, radio, compass
and post lights
To oil temperature gauge
To turn coordinator
To low-vacuum warning light
Switch/circuit breaker to
standby vacuum pump
To white doorpost light
To audio muting relay
To control wheel maplight
To navigation lights
To dome light
To radio
To radio
To radio or transponder
and encoding altimeter
To radio
P
R
I
M
A
R
Y
B
U
S
A
V
I
O
N
I
C
S
B
U
S
Low volt out
Power in
Sense (+)
Field
Sense (-)
Ground
Starter
Battery
A
L
T
B
A
T
+ 60- 60
-30 +300
A M P
L R
Battery
contactor
Ground service
plug receptacle
CODE
The hydraulic fluid is pumped through the system to an
actuator or servo. A servo is a cylinder with a piston inside
that turns fluid power into work and creates the power needed
to move an aircraft system or flight control. Servos can be
either single-acting or double-acting, based on the needs of
the system. This means that the fluid can be applied to one
or both sides of the servo, depending on the servo type. A
single-acting servo provides power in one direction. The
selector valve allows the fluid direction to be controlled.
This is necessary for operations such as the extension and
retraction of landing gear during which the fluid must work
in two different directions. The relief valve provides an outlet
Figure 7-35. Ammeter and loadmeter.
+ 60- 60
-30 +300
A M P
AL T AMPS
0 30 60
Ammeter Loadmeter
Figure 7-36. Basic hydraulic system.
Pump
BOTH
LEFT
RIGHT
OFF
Selector valve
System relief valve
Double acting cylinder
Motion
Hydraulic fluid supply
Return fluid
Hydraulic pressure
Figure 7-37. The landing gear supports the airplane during the
takeoff run, landing, taxiing, and when parked.
for the system in the event of excessive fluid pressure in the
system. Each system incorporates different components to
meet the individual needs of different aircraft.
A mineral-based hydraulic fluid is the most widely used type
for small aircraft. This type of hydraulic fluid, a kerosene-like
petroleum product, has good lubricating properties, as well
as additives to inhibit foaming and prevent the formation
of corrosion. It is chemically stable, has very little viscosity
change with temperature, and is dyed for identification. Since
several types of hydraulic fluids are commonly used, an aircraft
must be serviced with the type specified by the manufacturer.
Refer to the AFM/POH or the Maintenance Manual.
Landing Gear
The landing gear forms the principal support of an aircraft on
the surface. The most common type of landing gear consists
of wheels, but aircraft can also be equipped with floats for
water operations or skis for landing on snow. [Figure 7-37]
The landing gear on small aircraft consists of three wheels:
two main wheels (one located on each side of the fuselage)
and a third wheel positioned either at the front or rear of the
airplane. Landing gear employing a rear-mounted wheel is
called conventional landing gear. Airplanes with conventional
landing gear are often referred to as tailwheel airplanes. When
the third wheel is located on the nose, it is called a nosewheel,
and the design is referred to as a tricycle gear. A steerable
nosewheel or tailwheel permits the airplane to be controlled
throughout all operations while on the ground.
Tricycle Landing Gear
There are three advantages to using tricycle landing gear:
1. It allows more forceful application of the brakes during
landings at high speeds without causing the aircraft to
nose over.
2. It permits better forward visibility for the pilot during
takeoff, landing, and taxiing.
3. It tends to prevent ground looping (swerving) by
providing more directional stability during ground
operation since the aircraft’s center of gravity (CG)
is forward of the main wheels. The forward CG keeps
the airplane moving forward in a straight line rather
than ground looping.
Nosewheels are either steerable or castering. Steerable
nosewheels are linked to the rudders by cables or rods, while
castering nosewheels are free to swivel. In both cases, the
aircraft is steered using the rudder pedals. Airplanes with a
castering nosewheel may require the pilot to combine the
use of the rudder pedals with independent use of the brakes.
Tailwheel Landing Gear
Tailwheel landing gear airplanes have two main wheels
attached to the airframe ahead of its CG that support most of
the weight of the structure. A tailwheel at the very back of the
fuselage provides a third point of support. This arrangement
Figure 7-39. Fixed (left) and retractable (right) gear airplanes.
Figure 7-38. Tailwheel landing gear.
allows adequate ground clearance for a larger propeller
and is more desirable for operations on unimproved fields.
[Figure 7-38]
With the CG located behind the main landing gear, directional
control using this type of landing gear is more difficult while
on the ground. This is the main disadvantage of the tailwheel
landing gear. For example, if the pilot allows the aircraft to
swerve while rolling on the ground at a low speed, he or
she may not have sufficient rudder control and the CG will
attempt to get ahead of the main gear, which may cause the
airplane to ground loop.
Diminished forward visibility when the tailwheel is on or near
the ground is a second disadvantage of tailwheel landing gear
airplanes. Because of these disadvantages, specific training
is required to operate tailwheel airplanes.
Fixed and Retractable Landing Gear
Landing gear can also be classified as either fixed or
retractable. Fixed landing gear always remains extended
and has the advantage of simplicity combined with low
maintenance. Retractable landing gear is designed to
streamline the airplane by allowing the landing gear to
be stowed inside the structure during cruising flight.
[Figure 7-39]
Brakes
Airplane brakes are located on the main wheels and are
applied by either a hand control or by foot pedals (toe or heel).
Foot pedals operate independently and allow for differential
braking. During ground operations, differential braking can
supplement nosewheel/tailwheel steering.
Pressurized Aircraft
Aircraft are flown at high altitudes for two reasons. First, an
aircraft flown at high altitude consumes less fuel for a given
airspeed than it does for the same speed at a lower altitude
because the aircraft is more efficient at a high altitude.
Second, bad weather and turbulence may be avoided by flying
in relatively smooth air above the storms. Many modern
aircraft are being designed to operate at high altitudes,
taking advantage of that environment. In order to fly at
higher altitudes, the aircraft must be pressurized or suitable
supplemental oxygen must be provided for each occupant.
It is important for pilots who fly these aircraft to be familiar
with the basic operating principles.
In a typical pressurization system, the cabin, flight
compartment, and baggage compartments are incorporated
into a sealed unit capable of containing air under a pressure
higher than outside atmospheric pressure. On aircraft powered
by turbine engines, bleed air from the engine compressor
section is used to pressurize the cabin. Superchargers may
be used on older model turbine-powered aircraft to pump
air into the sealed fuselage. Piston-powered aircraft may use
air supplied from each engine turbocharger through a sonic
venturi (flow limiter). Air is released from the fuselage by