Airframe

7
Airplane Components
The major components of an airplane are:
• the fuselage;
• the wings;
• the empennage (tail section);
• the flight controls;
• the landing gear (or undercarriage); and
• the engine and propeller.

Rear-facing Trim tab (on Elevator

position light (white) some types)
Rotating
beacon (red) Horizontal stabilizer (or tailplane)

EMPENNAGE Red position

Vertical light
Elevator Cockpit
stabilizer canopy Left
(or fin)
Rudder wing tip
Radio
Left aileron
antenna

LEFT
FUSELAGE WING Wing leading
edge

Wing
Wi Propeller
flaps ng
roo Engine
t cowling
Wing trailing edge
Spinner
Right aileron Landing light(s)
Nose wheel
RIGHT
and nose gear Oil cooler or air filter
WING
Green (Right) main wheel
position light and landing gear Engine exhaust
Right wing tip

Figure 7-1 Features of a modern training airplane.

Fuselage
The fuselage is the body of the airplane to which the wings, empennage, engine and
landing gear are attached. It contains a cabin with seats for the pilot and passengers
plus cockpit controls and instruments. It may also contain a baggage compartment.
The fuselage of many modern training airplanes is of semi-monocoque construction, a
light framework covered by a skin (usually aluminum) that absorbs much of the stress.
It is a combination of the best features of a strut-type structure, in which the internal
framework absorbs almost all of the stress, and a monocoque structure which, like an Figure 7-2
eggshell, has no internal structure and the stress is carried entirely by the skin. Fuselage.

Chapter 7 Airframe 203

 Figure 7-3 Composite construction.

Wings
The wings are designed to cope with the flight loads of lift and drag. They also may
support other external devices such as engines (on multi-engine airplanes) and flaps.
Wings generally have one or more internal spars which are attached to the fuselage
and extend to the wingtips.The spars carry the major loads, which are upward bending
because of the lift, and downward bending because of wing-mounted engines and fuel.
The wings in most airplanes also contain fuel tanks installed between the curved
upper and lower surfaces. This is an efficient use of the space available, and the weight
of the fuel in the tanks also provides a downward force on the wing structure that
reduces the upward bending effect of the lift forces.
In addition to the spar(s), some wings also have external struts connecting them to
Figure 7-4 the fuselage to provide extra strength by transmitting some of the wing loads to the
Wing with fuel tank
located inside.
fuselage.

Skin
Stringers Ribs

Skin
Rib Spar

Fuel tank

Flaps
Main spar
Aileron Slingsby T67 Firefly

Figure 7-5 Components of a wing.

204 Access to Flight The Airplane

 Ribs, roughly perpendicular to the wing spar(s), assisted by stringers running parallel
to the spars, provide the airfoil shape and stiffen the skin which is attached to them.
The ribs transmit loads between the skin and the spar(s).
Monoplanes are designed with a single set of wings placed so that the airplane
is known as a high-wing, low-wing, or mid-wing monoplane. Biplanes, such as the
Pitts Special, are designed with a double set of wings. The Cessna 172 is a high-wing Figure 7-7
monoplane; the Piper Warrior is a low-wing monoplane. Biplane.

A high-wing
A low-wing monoplane A biplane
monoplane with wing struts

Figure 7-8
Empennage.
Figure 7-6 Low-wing monoplane, high-wing monoplane, and biplane.

Empennage
The empennage is the tail section of the airplane. It is generally constructed like the
wings and consists of a fixed vertical stabilizer (or fin) to which is attached a movable
rudder, and a fixed horizontal stabilizer with a movable elevator hinged to its trailing
edge.
There are variations in design, some airplanes have a stabilator (all-moving tail-
plane), others have a ruddervator (combined rudder and elevator) in the form of a Figure 7-9
butterfly tail, and yet others have a high T-tail, with the horizontal stabilizer mounted V-tail.
on top of the vertical stabilizer.

Flight Controls
The main flight control surfaces are the elevator, ailerons and rudder. They are operated
from the cockpit by moving the control wheel and rudder pedals. In a typical airplane,
movement of the control wheel or rudder pedals operates an internal system of cables
and pulleys that then moves the relevant control surface. Turnbuckles may be inserted
Figure 7-10
in the cables to allow the cable tension to be adjusted by qualified personnel. Aileron.
There are usually stops to protect the control surfaces from excessive movement in
flight and on the ground. Stops in the flight control system may be installed to limit
control wheel movement.

Landing Gear
The landing gear (or undercarriage) supports the weight of the airplane when it is on
the ground, and may be of either the tricycle type (with a nosewheel) or the tailwheel
Figure 7-11
type. Most tricycle landing gear airplanes are equipped with nosewheel steering through Conventional
the rudder pedals, and almost all airplanes have mainwheel brakes. landing gear.

Chapter 7 Airframe 205

 Mainwheels
The mainwheels carry most of the load when the airplane is on the ground, espe-
cially during the takeoff and landing, and so are more robust than the nosewheel (or
tailwheel). They are usually attached to the main airplane structure with legs in the
form of:
• a very strong spring leaf of steel or fiberglass;
Figure 7-12 • struts and braces; or
Spring-steel strut. • an oleo strut.

Oleo Oleo
Spring-steel
strut

Brake unit

Figure 7-14 Various types of landing gear.

Figure 7-13
Retractable gear.

A squat switch is used on airplanes with retractable landing gear to prevent the
wheels from being inadvertently raised when the airplane is on the ground. With the
weight of the airplane pressing down on the wheel struts, the squat switch opens the
gear circuit so electricity will not flow to the hydraulic gear pump, even if the gear
handle is placed in the up position.

Figure 7-15 Micro-switch (squat switch).

The oleo strut acts as a shock absorber, and is of telescopic construction, with a
piston that can move within a cylinder against an opposing pressure of compressed
air. The piston is attached to the wheel by an oleo strut and the cylinder is attached
to the airframe.
The greater the load on the strut, the more the air is compressed by the piston.
While the airplane is moving along the ground, the load will vary, and so the strut
will move up and down as the compressed air absorbs the loads and shocks, preventing
jarring of the main airplane structure.

206 Access to Flight The Airplane

 Special oil is used as a damping agent to prevent excessive in-and-out telescoping
movements of the oleo strut and to damp its rebound action.
When the airplane is stationary, a certain length of polished oleo strut should be
visible (depending to some extent on how the airplane is loaded), and this should be
checked in the preflight external inspection. Items to check are:
• correct extension when supporting its share of the airplane’s weight;
• the polished section of the oleo strut is clean of mud or dirt (to avoid rapid wearing
of the seals during the telescoping motion of the strut); and Parked
• there are no fluid leaks.

Nosewheel
The nosewheel is usually of lighter construction than the mainwheels and is usually
Polished Torque
attached to the main structure of the airplane near the engine firewall. A torque-link section link
is used on nosewheel assemblies to correctly align the nosewheel with the airframe.
It links the cylinder assembly attached to the airplane structure with the nosewheel
assembly, and is hinged to allow for the telescopic extension and compression of the
oleo. In flight
Most airplanes have nosewheel steering, achieved by moving the rudder pedals which
are attached by control rods or cables to the nosewheel assembly, thereby allowing the
pilot greater directional control when taxiing.
Some airplanes have castoring nosewheels which are free to turn, but are not connected
by controls to the cockpit. The pilot can turn the airplane by using the rudder when
it has sufficient airflow over it (from either slipstream or airspeed) or with differential
braking of the mainwheel brakes.
Nosewheel oleo struts are prone to nosewheel shimmy, an unpleasant and possibly
damaging vibration set up when the nosewheel oscillates a few degrees either side of
On touchdown
center as the airplane moves along the ground.To prevent this, most nosewheel assem-
blies are equipped with a shimmy damper, a small piston-cylinder unit that dampens Figure 7-16
out the oscillations and prevents the vibration. If nosewheel shimmy does occur, it The oleo strut.
could be because the shimmy damper is insufficiently pressurized or the torque link
has failed.

Shimmy
damper
Oleo

Torque
link

Figure 7-17 Shimmy damper.

Chapter 7 Airframe 207

 Tires
Airplane tires must be inflated to the correct pressure for them to function as designed.
Vibration during taxiing, uneven wear and burst tires may result from a pressure that is
too high; damage to the tire structure and a tendency for the tire to creep with respect
OK
to the rim can occur if pressure is too low. Correct inflation is important in achieving
a good service life from a tire. Aircraft tires are unique in that they have to withstand
ballooning pressures on each landing.
Creep can occur in normal operations because of the stresses during landing, when a
stationary tire is forced to rotate on touching the ground and has to “drag” the wheel
around with it, and will also occur when the airplane is braking or turning.
To monitor creep, there are usually paint marks on the wheel flange and on the tire
Needs attention which should remain aligned. If any part of the two creep marks is still in contact, that
amount of creep is acceptable, but if the marks are separated, then the inner tube may
Figure 7-18 suffer damage and the tire should be inspected and serviced.This may require removal
Creep marks on the tire
and wheel flange
and reinstallation, or replacement.
enable visual checks Tire strength comes from its carcass which is built up from casing cords and then
for creep. covered with rubber. The ply rating is a measure of its supposed strength. Neither
the rubber sidewalls nor the tread provide the main strength of the tire; the sidewalls
protect the sides of the tire carcass, and the rubber tread provides a wearing surface at
the contact points between the tire and the runway.
Shallow cuts or scores in the sidewalls or on the tread, or small stones embedded in the
tread, will not be detrimental to tire strength. However, any large cuts (especially if they
expose the casing cords) or bulges (that may be external indications of an internal casing
failure) should cause you to reject the tire prior to flight.The condition of the tires should
be noted during the preflight external inspection, especially with respect to:
• inflation;
• creep;
• wear, especially flat spots caused by skidding;
• cuts, bulges (especially deep cuts that expose the casing cords); and
• damage to the structure of the sidewall.

Wheel Brakes
Most training airplanes are equipped with disc Left Right
brakes on the mainwheels. These are hydrau- toe-brake toe-brake

lically operated by the toe brakes which are

Brake fluid
situated on top of the rudder pedals. Press- reservoir
ing the left toe brake will slow the left main-
wheel down and pressing the right toe brake
will slow the right mainwheel down. Used Brake
pads
separately, they provide differential brak- Master
cylinder
ing, which is useful for maneuvering on the
Brake
ground. Used together, they provide normal line
Slave
straight-line braking. cylinder
A typical system consists of a separate To right
main wheel
Brake
master cylinder containing hydraulic fluid disc brake assembly
for each brake. As an individual toe brake
is pressed, this toe pressure is hydraulically Figure 7-19
transmitted via the master cylinder to a slave Typical simple hydraulic braking system.
cylinder which closes the brake friction pads

208 Access to Flight The Airplane

(like calipers) onto the brake disc. The brake disc, which is part of the wheel assembly,
then has its rotation slowed down.
Most airplanes have a parking brake (usually hand-operated, sometimes in conjunc-
tion with the toe-brakes) that will hold the pressure on the wheel brakes and can be
used when the airplane is parked.
During the preflight external inspection, you should check the brakes to ensure
that they will function when you need them, ensuring that:
• there are no leaks of hydraulic brake fluid from the brake lines;
• the brake discs are not corroded or pitted;
• the brake pads are not worn-out; and
• the brake assembly is firmly attached.
A severely corroded or pitted disc will cause rapid wear of the brake pads, as well
as reducing their effectiveness, and, in an extreme case, the disc may even fail struc-
turally. Fluid leaks from the brake lines or cylinders indicate a faulty system that may
provide no braking at all when it is needed. Any brake problems should be rectified
prior to flight.
Following a satisfactory external inspection, you should still test the brakes imme-
diately after the airplane first moves, by closing the throttle and gently applying toe
brake pressure. Brake wear can be minimized by judicious use of the brakes during
ground operations.

Engine and Propeller

The engine is usually mounted on the front of the airplane, and separated from the cock-
pit by a firewall. In most training airplanes, the engine drives a fixed-pitch propeller, although
more advanced airplanes will have a constant-speed propeller with blades whose pitch can
vary.The engine and its attachments are considered in detail in the next few chapters.

Figure 7-20 Variable pitch propeller.

Chapter 7 Airframe 209

 Review 7
Airframe

1. What is the main structural component of 13. What type of nosewheel is free to turn but is
the wing? not connected to the cockpit by any control
2. Name the four major components of the rods or cables for turning?
empennage. 14. Nosewheel steering in light airplanes is
3. What is the airfoil shape of the wing surface usually operated by:
formed by? a. control rods or cables operated by the
4. What sort of airplanes are designed with rudder pedals.
only one pair of wings? b. a steering wheel.
5. What is the most usual form of fuselage c. the brakes.
construction in training airplanes, in which 15. A castoring nosewheel can be made to turn:
the skin covers a light structure and carries a. by a steering wheel.
much of the stress? b. with differential braking.
6. Does a cracked or severely corroded land- 16. If a tire has moved so that the creep marks
ing gear strut found during your preflight are out of alignment, then:
inspection need to be inspected by a quali- a. the tire is serviceable.
fied maintenance technician before the b. the tire should be inspected and possibly
airplane flies? reinstalled or replaced.
7. What is the agent used to dampen the c. tire pressure should be checked.
rebound action in the oleo strut following 17. Does a tire that has some shallow cuts in
a shock? the sidewalls and a number of small stones
8. True or false? The oleo strut will only extend embedded in its tread need to be rejected for
the same in flight as on the ground. further flight?
9. Why should mud or dirt noticed in a preflight 18. Does a tire that has a deep cut that exposes
inspection be cleaned off the polished section the casing cords or a large bulge in the side-
of an oleo strut prior to taxiing? wall need to be rejected for further flight?
10. What is the nosewheel held in alignment by? 19. Most light airplane braking systems are:
11. What are nosewheel oscillations either side a. operated by cables.
of center damped by? b. operated pneumatically.
12. What is the relative movement between a c. operated hydraulically.
tire and a wheel flange called?

Answers are given on page 769.

210 Access to Flight The Airplane