Nusa Flying International

AIRCRAFT STRUCTURES
(CPL and Instrument Rating)

Table of Contents

1. Landing Gear – Undercarriages

▪ General description
▪ Powered nose-wheel steering
▪ Retractable undercarriages
▪ Emergency extension systems

2. Landing Gear – Wheels, Tyres and Brakes

Page 1 of 34
 1. LANDING GEAR/UNDERCARRIAGES

INTRODUCTION (JAA Training, chapter 7)

From the first flight of the Wright Brothers’ Kitty Hawk Flyer, which took off from a small
trackmounted trolley and landed on two skids, it became apparent that for practical purposes aircraft
required wheels for ground operations. In these early days, bicycle wheels were used. As the Wright
Brothers’ engines improved, these wheels allowed the aircraft to take off without the need of the fixed
rail and drop weight. Another benefit was in landing.

Since those first days, aircraft have been adapted to take off and land from the following surfaces:
▪ Water
▪ Snow and ice
▪ Unprepared hard surfaces
▪ Prepared hard surfaces

This allows aircraft to be categorised into three groups:

▪ Land planes
▪ Seaplanes/Floatplanes
▪ Amphibians

Land planes can take off from prepared or unprepared surfaces including snow and ice. Normally, land
planes are fitted with wheeled landing gear. However, for operations on snow and ice, either skis or
bear paws replace the wheels. In some cases, the skis or bear paws are retractable to allow the
aircraft to operate from both snow and a hard surface.

Seaplanes are those where the fuselage is designed to be a floating hull. They only land and take off
from water. Floatplanes are land planes where two floats, which allow the aircraft to take off and land
on water, replace the wheeled undercarriage.

Amphibians are aircraft that can land or take off from water or prepared runways. Amphibious aircraft
can either have a fuselage hull with retractable landing gear, or floats that incorporate retractable
landing gear.

Originally, all land planes used fixed undercarriages. In the very early days, this amounted to a cross
axle with two bicycle wheels attached to a V frame by bungee cords, which was in turn attached to
the fuselage. The distance between the two wheels, the undercarriage track, was narrow, which led
to poor landing and take-off stability. While the cross axle can still be found on some light fixed-gear
aircraft, it is not in common use since, apart from the narrow track, the axle can create sufficient drag
in long grass to tip a landing aircraft onto its nose.

With increases in aircraft weight, engine power, and aircraft performance, the designers replaced the
V frames and bicycle wheels with wheels and tyres attached to undercarriage legs in an attempt to
reduce the shock load when landing. In the quest for increased airspeed and operational efficiency,
these gears were refined by fairing the landing gear legs and wheels to reduce the drag created at
higher airspeeds. The ultimate drag reduction for landing gear is achieved by fully retracting it when
not in use.

Page 2 of 34
REQUIREMENTS FOR A MODERN UNDERCARRIAGE

The progress in the design of landing gear over the last century has led to the following specific
requirements for air transport aircraft undercarriages that all designs must meet:
▪ Absorb the landing load and damp vibration
▪ Withstand side loads when landing and taxiing
▪ Support the aircraft on the ground when it is manoeuvring
▪ Provide minimum friction between the aircraft and the ground
▪ Possess a low coefficient of drag
▪ Withstand the flight air loads

DRAG AND UNDERCARRIAGES

Diagram 7.1 Undercarriage Drag

As an aircraft’s speed increases, its profile drag increases. To reduce drag and save fuel, the aircraft
should be as streamlined as possible. For fixed landing gear, the method used to streamline the gear
is to fit aerofoil cross-section fairings to the legs and more bulbous spats around the wheels, which
conform where possible to the fineness ratio of 4:1.

Retracting the gear into wheel bays and fairing the apertures with doors removes the gearcreated
element of the profile drag. Diagram 7.1 represents the same aircraft fitted with a fixed gear without
any fairing, then with the gear fully faired, and finally with a retractable gear raised.

LANDING

When an aircraft lands, there are three factors that come into play. These are mass, vertical velocity
at touchdown (sink rate), and forward velocity at touchdown.

As mass times velocity equals force at touchdown, the aircraft exerts a downward force via its landing
gear to the Earth. Obviously, the greater the landing mass or the greater the sink rate, the greater the
force created. The Earth pushes back against the aircraft, as every force has an equal and opposite
reaction. Unless controlled, this causes the aircraft to recoil or bounce back into the air before
touching down again. Thus, a landing can continue in a series of kangaroo hops down the runway, or
until the aircraft is pushed skyward but is lower than its stalling speed and crashes.

Page 3 of 34
This is due to the fact that at the initial touchdown, the vertical component of the landing force is
absorbed, leaving the mass. As the reaction is now greater than the mass alone, the aircraft bounces
back into the air to restart the process.

SUSPENSION/SHOCK ABSORBER SYSTEMS FOR LIGHT AIRCRAFT

The original Wright Flyer had no shock absorbing system as its landing gear took the form of a
wooden sledge. Attachment of pneumatic bicycle wheels to later versions gave very little suspension,
for like all pneumatic tyres, the mass of the vehicle to which the tyres are fitted is supported by the
gas pressure acting against the tyre cover.

To improve the suspension, the following systems were introduced:

▪ Bungee cords/bungee blocks
▪ Spring steel legs
▪ Torsion bar

Bungee Cords/Bungee Blocks

Diagram 7.2 Bungee Cords/Bungee Blocks

Diagram 7.2 illustrates bungee cords/bungee blocks, a system used on early aircraft and still in use on
many aircraft. It consists of a series of bungee cord rings or rubber blocks (bungee blocks) mounted
inside a tubular housing, which is connected to the aircraft’s structure. The lower section of the leg
with the wheel attached presses upward against the bungee blocks.

As the aircraft lands, the action of the upper and lower leg is to compress the bungee blocks. This act
of compression absorbs energy from the touchdown and slows the sink rate. It also reduces the
spine-jarring thump that early rigid systems subjected their pilots to, as part of the landing load has
been absorbed into the bungees prior to it being fed into the aircraft’s structure. As this system is un-
damped, the reaction force from the touchdown and the recoil from the compressed bungees push
the aircraft upward. However, under normal conditions, the aircraft bounces up and down without
recoiling into the air. During taxiing, each bump causes the aircraft to sway and bounce.

Spring Steel Leg

Page 4 of 34
This system is frequently used on very light G.A. aircraft. It consists of several strips of spring grade
steel clamped together to form a bar. One end is attached to the fuselage structure, the other to a
stub axle, as shown in diagram 7.3. On landing, the force of the aircraft sinking against the resistance
of the runway causes the spring steel legs to start bending and absorbing the landing energy. As with
the bungee system, energy absorbed by the spring steel is dissipated back into the fuselage structure
at a controlled rate. Again, the aircraft sways and bounces as it taxis over uneven ground.

Diagram 7.3 Action of a Spring Steel Leg

Torsion Bar

Diagram 7.4 Torsion Bar

The torsion bar, made of an elastic material that can withstand torsional loading, is attached to the
fuselage structure at one end. A bearing supports the torsion bar where it passes through the
fuselage skin. It is attached to a radius arm, so named as in its action it scribes an arc of a circle, as
diagram 7.4 shows.

Page 5 of 34
In flight, the torsion bar is unloaded. On touchdown, the radius leg moves backward and upward,
applying a torque load to the torsion bar. As with the bungee system and spring leg system, the
landing load energy absorbed by the torsion bar is dissipated back into the fuselage structure at a
controlled rate. Again, the aircraft sways and bounces as it taxis over uneven ground.

As can be seen in diagram 7.4, these systems do not absorb the landing load other than controlling
the rate at which the load is dissipated into the aircraft’s structure. To control the landing load, absorb
the shock, and control the rate of recoil, oleo-pneumatic shock absorbers are used.

NOSE WHEELS

Diagram 7.16 Tricycle Undercarriage

Nose wheels, or in JAA terms, auxiliary wheels, allow aircraft to be almost horizontal when parked or
ground manoeuvring. This gives the pilot the advantage of a clear view ahead without having to
weave the aircraft. It also allows the aircraft to be steered so that the fuselage follows the nose,
rather than the tail having to be swung in the opposite direction to the intended course. A tricycle
arrangement is such that the nose wheel forms the apex of the landing gear triangle.

Fully Castoring Nose Wheel

For some light aircraft, the nose wheel’s axle is located behind the vertical leg to simplify the steering.
This arrangement allows the wheel to pivot like the front wheels of a shopping trolley. To steer the
aircraft, the pilot uses differential braking, whereby one main wheel is braked more than the other.
This results in the aircraft pivoting around the braked wheel. As the nose wheel is behind the leg, it
pivots to follow the fuselage.

While this system is simple, it has the disadvantage of suffering from shimmy. To overcome this, a
specialist tyre called a Marstrand tyre is used. Both shimmy and Marstrand tyres are covered later in
these notes. As this form of steering is not precise, most modern tricycle undercarriage light aircraft
use a direct steering system.

STEERABLE NOSE WHEELS — LIGHT AIRCRAFT WITH FIXED NDERCARRIAGES

Steerable nose wheels are directly linked to the pilot’s rudder pedals. For most light aircraft, the pilot
is able to steer the nose wheel through an arc 37° each side of the aircraft’s longitudinal centreline. A
bungee spring is incorporated into both steering rods. This prevents excessive force created by
sudden large rudder pedal movements from being transmitted to the nose wheel steering system
when the wheel is on the ground.

STEERABLE NOSE WHEELS — LIGHT AIRCRAFT WITH RETRACTABLE UNDERCARRIAGES

Page 6 of 34
On all aircraft with retractable gear, the nose wheel bays are very narrow in relation to the nose gear
itself. This requires the nose wheel to be in a fore and aft condition before it is raised. Otherwise, the
wheel impacts against the bay’s structure, damaging it and possibly jamming, thus preventing it from
being fully retracted or lowered. During flight, the nose wheel’s steering must be disconnected from
the rudder pedals to prevent any input turning the wheel while it is retracted.

Diagram 7.17 Steering for a Retractable Nose

Referring to diagram 7.17, the steering head of the retractable leg engages into a pivoting link
(blocked in black), which is connected to the steering rods. On retraction, the steering head moves
out of the pivoting link and follows a guide, which centralises the wheel.

SHIMMY DAMPERS

Diagram 7.18 Light Aircraft Shimmy Damper

Where the nose wheel is steerable, it is possible for the tyre to oscillate either side of the aircraft’s
track due to play in the linkages. To overcome this, a shimmy damper is fitted. On light aircraft, this
consists of a housing attached to the upper leg, which contains a piston and two compression springs,
as shown in diagram 7.18. The piston is connected to the steerable lower leg. When the wheel is fore
and aft, the compression in the springs is equal. If the wheel starts to turn, it increases the

Page 7 of 34
compression, resisting its movement, which acts to return it to the fore and aft position. Worn shimmy
dampers and torque links are the most common cause of nose wheel shimmy.

Hydraulic Shimmy Damper

For larger aircraft, hydraulic damper struts are used similar to the one shown in diagram 7.19. This
system uses a balanced actuator with metered orifices drilled through the piston. As the lower leg
starts to oscillate, the piston is moved through the fluid. As the orifices limit the rate of transfer from
one side of the piston to the other, damping occurs, and the pressure build-up against the piston
resists the turning force of the oscillation. Attaching the shimmy damper to the steering ring allows
the nose wheel to be turned and still maintain shimmy damping.

Diagram 7.19 Hydraulic Shimmy Damper

STEERABLE NOSE WHEELS FOR MEDIUM AND LARGE AIRCRAFT

To overcome the greater forces acting on the nose wheel, these aircraft use hydraulic power systems.
The pilots have two steering controls. For alignment of the aircraft to the runway during landing and
take-off, the pilots’ rudder pedals move the nose wheel through 7° of arc either side of the centreline.

Page 8 of 34
When the aircraft’s speed falls below 60 kt, the pilot’s control wheel or tiller bar takes effect. On some
older aircraft, this was a small hand wheel attached to the control column with a reference datum to
show when the nose wheels were fore and aft. Modern aircraft mount the control wheel on the side
panels. These are biased to neutral, and both captain and co-pilot have this facility. In diagram 7.20,
the steering motor in the photograph is a rectangular box attached to the upper leg casting with the
torque links below it. Depending on design, the lower leg can be rotated using two hydraulic linear
actuators or a rotary actuator.

Diagram 7.21 Steering Cylinders

Where linear actuators are used, when the pilot makes an input to turn right, fluid from the aircraft’s
hydraulic power system is allowed to enter the first chamber of the right hand actuator and the
second chamber of the left hand actuator, as diagram 7.21 illustrates. This turns the steering ring,
and the lower leg is turned via the torque link.

When the wheels are in alignment with the centreline of the aircraft, hydraulic pressure can be
brought into a central feed to all four chambers that are diagonally interlinked. As movement of the
wheel in either direction results in expelling fluid from the first chamber of one actuator to the second
chamber of the other actuator, the pressure resists this movement and damps the oscillations.

Page 9 of 34
 Diagram 7.22 Nose Wheel Centralised

As the pilot rotates the aircraft and the nose leg extends, an input is given to the steering to centralise
the nose wheel in the fore and aft position. Refer to diagram 7.22 for an illustration. When
undercarriage retraction is selected, an input hydraulically locks it in the fore and aft position.

Nose Wheel Rumble

As nose wheels are not braked, there is a possibility that the nose wheel will continue to rotate for a
period of time after retraction, which creates a rumble and a gyroscopic mass. To prevent this
rotation, a sprung steel strip that is fitted in the nose bay contacts the tyre and stops the wheel from
turning. Diagram 7.20 shows this.

BODY GEAR STEERING

Diagram 7.23 Body Gear

For large air transport aircraft such as the Airbus A340, Boeing 747, Lockheed Tri Star, and McDonnell
Douglas DC-10, the aircraft is supported on both wing gear and body gear. On these aircraft, the
body gear is linked to the pilot’s control wheel to allow the aircraft to turn. When the aircraft has
slowed down below 60 kt, steering inputs to the nose wheel activate a proportional steering signal in
the opposite direction for the body gear. This reduces the aircraft’s turning radius. In diagram 7.23,
the body gear can be seen between the wing gears.

Turning Circle

Page 10 of 34
Diagram 7.24 shows a medium aircraft’s turning circle. The nose wheel can be steered to 75° either
side of the centreline, but the effective angle of steering is 70 °. The aircraft manufacturer’s handbook
always quotes the aircraft’s minimum turning radius. This should not be reduced, as it leads to
excessive stresses being created in the inside leg. This smallest turning circle is achieved using nose
wheel steering, inner wheel braking, and body gear steering where fitted.

Aircraft should always be turned using the largest turning radius practicable to reduce the stresses on
the main undercarriage gear.

Nose Wheel Steering Disconnect

To allow the nose wheel of aircraft fitted with powered steering to be turned when they are being
towed or pushed, a steering disconnect mechanism is fitted. In many designs, it is not possible to
attach the tow bar until the steering disconnect cover cap is removed and the steering disconnected.

As a safeguard, it is physically impossible to refit the cover cap without reconnecting the steering. The
pilot is also informed by a caution caption if the steering is inoperative. Both of these measures
ensure that the pilot has control of the steering prior to a take-off run.

RETRACTABLE MAIN UNDERCARRIAGE FOR AIR TRANSPORT AIRCRAFT

To support the lower leg against the forces of friction, a fore stay or drag strut is fitted, as in the left
picture of diagram 7.29. To prevent the undercarriage from collapsing sideways, a side stay is fitted.
As the gear has to retract sideways into the wheel bay, the side stay is articulated. In the left
undercarriage of diagram 7.29, the side stay is held locked up and down by hook locks. This is not

Page 11 of 34
always the case, as in the right undercarriage of the diagram, where a geometric locking mechanism
is used for the side stay.

A smaller cross-bracing geometric lock, actuated by a small hydraulic jack, can also lock the side stay.
Using overcentre locks ensures that the gear remains down when hydraulic power has been removed.
To retract the gear, hydraulic sequencing is required to operate the locking actuator first before the
retraction actuator can retract the gear.

Diagram 7.29 Main Undercarriage Bracing

OVERCENTRE OR GEOMETRIC LOCKS

Page 12 of 34
 Diagram 7.30 Overcentre or Geometric Locks

To ensure that all the components of the side stay form a solid rod when extended, it is manufactured
as an overcentre or geometric lock, as in diagram 7.30. The two parts of the lock are manufactured so
that when they butt together at the hinge (as shown in 1), their hinge point is below a line drawn
from the centre of each rod end. Any force applied from the direction of arrow A, locks the strut more
firmly, as does compression force applied to each end.

To unlock the joint, a force in the direction of arrow B must overcome the compression force applied
to the ends of the strut and be able to push the hinge up through the centreline (as shown in 2). The
compression forces are a proportion of the mass of the aircraft. When the wheels have left the
ground, the compression load is removed. A small hydraulic actuator can exert sufficient force in the
direction of arrow B to push or pull the strut through the centreline, breaking the lock (as shown in 3).

Note: Overcentre locks are not to be used as up locks, only as down locks.

UP LOCKS

Diagram 7.31 Hook Locks

Page 13 of 34
In an air transport aircraft, the undercarriage is held in the up position using mechanical hook locks.
This ensures that the gear remains up in normal flight in the event of loss of hydraulic power, as the
cam roller makes a physical lock preventing the hook from rotating open, as shown in the left of
diagram 7.31.

The hook lock can take the form of a single hook (as shown in diagram 7.31) or a pincer. The hook
lock has a pivot point and a profiled outline. A small hydraulic actuator operates a lever with a roller,
which follows the cam profile on the hook. When the pilot selects undercarriage down, hydraulic
pressure is sequenced into the up lock actuator, forcing the ram outward. The roller is forced to ride
over the cam profile of the hook and moves into the lower profile cut out, where it pushes against the
hook and with the action of the spring rolls the hook open.

On the “up” selection, the undercarriage roller strikes the hook and forces it to rotate back to the
closed position, as the force produced by the retraction actuator is greater than the force produced by
the up lock actuator.

EMERGENCY LOWERING

There is a DGCA requirement that in the event of hydraulic failure, there must be a separate means of
lowering the undercarriage. In some aircraft, this takes the form of compressed nitrogen or air. This is
a one-shot system. If the pilot is unable to achieve satisfactory gear down indications, the
undercarriage selector is left in the down position, and a guarded emergency selector is operated.
Operating this selector allows compressed gas into the aircraft’s undercarriage down line. To ease the
operation, fluid from the undercarriage up lines is jettisoned overboard. This ensures that any back
pressure created by the fluid is minimal, and the gear can snap to the locked down, safe condition.

To comply with the DGCA requirement, g force release force is an acceptable method. The pilot puts
the aircraft through a series of manoeuvres, first creating negative g, then positive g (manoeuvres laid
down in the aircraft flight manual). The positive g effectively increases the weight of the gear acting
downward on the up lock hooks. As the pivot point of the hook is offset from the centre point of the
undercarriage roller, a large turning moment is made. This forces the hook round and at the same
time, due to the gear lever being in the down selected position, the cam roller is forced to turn. As the
hook starts to rotate, the spring ensures the hook snaps clear, allowing the gear to drop. For the main
gears, the mass times acceleration by gravity snaps the gear into the locked condition. Airflow aids
extension of nose gears that retract forward.

RETRACTABLE UNDERCARRIAGE FOR LIGHT AIRCRAFT

Page 14 of 34
 Diagram 7.36 Retractable Main Leg of a Light Aircraft

Diagram 7.36 shows a light aircraft’s retractable main undercarriage leg showing the down lock/side
stay which forms an overcentred lock, the springs that ensure that the side stay snaps locked, and the
extension/retraction actuator. As these aircraft are light, the oleo leg does not require the additional
bracing of drag stays. The hydraulic power source for the gear’s extension and retraction is described
below.

Many light aircraft retractable undercarriages use self-contained hydraulic power packs. Refer to the
system schematic diagram 7.37. In this system, a spur gear pump, operated by a reversible electric
motor, provides hydraulic power to raise and lower the aircraft's undercarriage. For protection in the
event of a power failure, a freefall system is fitted. In this system a hydraulic lock holds the gear in
the up position, and a mechanical lock holds it down.

A warning horn sounds to prevent inadvertent wheels up landings if the gear is not selected down
when the flaps are set to landing, or the throttles retarded to flight idle. Selecting the gear down,
raising the flaps, or advancing the throttles removes the warning.

OPERATION OF THE POWER PACK

A down selection causes the pump to rotate in an anti-clockwise direction. This draws unfiltered fluid
from the reservoir and supplies it under pressure to the left-hand side of the unit. The fluid pressure
closes off the return line, via the filter to the reservoir, and forces the gear up check valve piston to
the right. This unseats the check valve, which breaks the hydraulic lock that held the gear up, and
allows the fluid in the up line to enter the pump and be transferred to the down line side of the unit.
The build up of pressure forces the shuttle valve to the left, blocking the return to the reservoir and
supplying fluid to the down line.

As the gear is falling with gravity, and there are no mechanical locks to open, the down line functions
at a low pressure between 400-800 psi. To control this pressure, a relief valve, termed a low pressure
control valve, allows excess pressure to return to the reservoir.

Page 15 of 34
 Diagram 7.37 Power Pack for a Light Aircraft

Restrictors in both the down line and the up line of the nose gear control the fluid supply to the three
gears’ actuators. These restrictions to flow allow the main gears to move first before the nose gear
starts to move. This ensures that the drag created by lowering the gear is controlled when the main
gears have locked down. Likewise, the nose gear lowered into wind is stopped from moving rearward
too quickly by the restrictors. When all three gears are down and locked, the red gear unsafe light
extinguishes, and the electric power to the pump is removed. As the pressure in the down line decays,
the shuttle valve moves to the right by spring pressure.

On up selection, the pump rotates in a clockwise direction drawing fluid from the reservoir via the
filter, the pressure closes off the return to the reservoir, and forces the gear up check valve piston to
the left. Fluid pressure unseats the check valve, and fluid enters the up line. As the gears have to be

Page 16 of 34
raised against gravity, the piston areas are reduced by the rams, and the nose gear has to retract
against the air flow, the operating pressure of the high pressure control valve is set at a nominal 1800
psi.

The initial action of the retraction actuators is to unlock the down lock/side stays mechanical hooks,
and then pull the down locks through the overcentred condition, before starting to retract the gear.
The main gear moves first due to the restrictors. When the gear is up and cannot travel any further,
the pressure in the up line increases to a value of 1800 +/- 100 psi. At this point, the pressure switch
in the up line removes the electrical power from the pump and cancels the red light. When pumping
stops, the gear up check valve reseats, forming the hydraulic lock holding the gear up. If the pressure
drops by a preset value, the pressure switch re-energises the pump motor, increasing the system
pressure. Fluid from the down line is returned to the reservoir.

In the event that the pump fails to operate, the pilot selects down on the normal selector, isolates the
pump electrically by pulling the circuit breakers, and operates the guarded emergency lowering
selector. This opens the free fall valve breaking the hydraulic lock, which allows gravity and air flow to
lower the gears. The down lock springs ensure that the gears snap locked.

2. WHEELS, TYRES AND BRAKES

INTRODUCTION TO BRAKES (JAA Training, chapter 8)

Page 17 of 34
Heavier aircraft flying at higher airspeeds led to increased landing speeds, requiring a wheel braking
system to be fitted. The original wheel brake system applied a braking force to both wheels equally at
the same time. By the 1930s, differential-braking systems had become standard equipment for larger
aircraft.

All aircraft wheel brake systems are a means of converting the aircraft’s forward motion (kinetic
energy) into heat energy via friction. A braking system’s efficiency is determined by how rapidly
kinetic energy can be converted into heat and the heat dissipated to other structures and the
atmosphere.

TYPES OF WHEEL BRAKES

▪ Mechanical drum brakes — very old aircraft and cars only

▪ Hydraulic expander brakes — older aircraft only
▪ Pneumatic bag brakes — older aircraft
▪ Single disc — light GA aircraft
▪ Multi-disc — air transport aircraft

BASIC BRAKE MASTER CYLINDER

Diagram 8.2 Basic Brake Master Cylinders

Referring to diagram 8.2, when the brakes are not applied, the main spring extends the master
cylinder. The valve is prevented from being closed by the upper spring by stops in the lower cylinder.
This keeps the valve open and allows the reservoir to replenish the cylinder.

As the toe pedal is depressed, the cylinder shortens, compressing the main spring. The upper spring
acts to close the valve, and the fluid displaces into the wheel brake system where it acts on the slave
cylinder.

The fluid displaced into the brake pipes raises the pressure, which acts against the larger surface area
of the piston, creating the force that is exerted against the disc. As the brake material and the disc
progressively wear down, the amount of fluid returned to the reservoir reduces so the reservoir
replenishes the cylinder when the brake is at rest.

Simple Hydraulic Braking System

Page 18 of 34
 Diagram 8.3 Basic Brake Circuit

To achieve differential braking, each wheel has its own separate hydraulic circuit. Diagram 8.3 shows
one wheel’s circuit. Refer to the following text for explanations of the function and operation of both
fixed and sliding discs.

Light Aircraft Fixed Disc

Diagram 8.4 Fixed Disc System for Light Aircraft

For light aircraft where the disc attaches to the wheel, a sliding calliper (similar to a car’s) is used, as
shown in diagram 8.4. Pressure exerts force equally in all directions, in this case, against the piston
and the brake housing.

The displaced fluid moves the piston, pushing the puck against the disc. The brake housing slides to
pull the fixed pad against the opposite side of the disc, thus creating friction and generating heat.

Page 19 of 34
Springs fitted between the calliper and its mounting points push the brake housing away as the brake
pedal is released, returning the fluid to the reservoir.

To apply the parking brake, both toe brakes are depressed, and the parking brake lever is pulled and
locked (refer to diagram 8.3). This creates a hydraulic lock in both systems simultaneously and traps
pressure at the wheel brakes. Note that all wheel brake systems lose fluid/pressure back to their
reservoirs over time and, therefore, must be reapplied regularly.

POWER BRAKES

Diagram 8.7 Basic Power Brake System Components

Diagram 8.7 shows the basic components used on an older air transport aircraft where the landing
masses are greater than the 5700 kg, but the landing speeds are low. System pressure passes
through a non-return valve to the brake control valve and charges the wheel brake accumulator. A
direct reading gauge indicates the supply pressure available.

The brake control valve operates via servo pressure from the pilot’s toe pedals. It directs hydraulic
pressure to the appropriate wheel brakes proportionate to depression of the toe pedal. The servo lines
are filled with hydraulic fluid, but this fluid does not mix with the system fluid, so that in the event of
a servo line leaking in the flight deck, the pilot is not sprayed with highpressure fluid.

A sensing line from each wheel brake allows the pilot to see the application of brake pressure to each
wheel brake. Prior to take-off, the pilot fully depresses both toe pedals to ensure that equal pressure
is supplied to the brake units. If the pressures are not even, the take-off must be cancelled and the
system corrected.

As with the supply pressure gauge, where direct reading gauges are fitted, pressure relays are used.
These ensure that any failure of the gauge does not result in high-pressure fluid spraying around the
flight deck.

For power brake systems, the pilot receives artificial feel via the toe pedals to indicate the amount of
braking applied. This can be achieved through backpressure in the servo lines acting on the master
cylinder or compression springs.

In some older low-pressure braking systems, the aircraft hydraulic system pressure must be
decreased. This is termed deboosting. A debooster valve, normally located on the main legs, reduces
the pressure and increases the flow rate to the brakes.

Page 20 of 34
ANTI-SKID SYSTEMS
There are three broad categories of anti-skid:

▪ On/Off
These are mechanical systems.
▪ Semi Modulating
These are first generation electronic systems.
▪ Fully Modulating
These are the modern electronic systems fitted to air transport aircraft.

Mechanical ON/OFF Anti-Skid System

Diagram 8.8 Mechanical Anti-Skid System

On/Off systems are the simplest of the three types of anti-skid systems. In diagram 8.8, a mechanical
unit called a maxaret provides the on/off anti-skid protection.

Page 21 of 34
Fluid at supply pressure, nominally 3000 psi, is supplied to a wheel brake accumulator. This
accumulator acts as an emergency reserve in the event of loss of hydraulic power, allowing the pilot
six full applications of the brakes before it is exhausted.

The brake control valve directs the system pressure, proportionate to the toe pedal deflection, to the
wheel brake units via modulating valves (see modulating valves below) and maxaret units. The control
valve regulates the brake pressure to a max value of approximately 1750 psi.

For these systems, fully metered brake pressure as commanded by the pilot is applied until wheel
locking is sensed. While the brakes are still being applied, the maxaret unit allows the wheel to spin
back up by releasing pressure. When the system senses that the wheel is accelerating back to
synchronous speed (i.e. groundspeed), full-metered pressure is again applied. The cycle of full
pressure application/complete pressure release is repeated throughout the stop, or until the wheel
ceases to skid with brake pressure applied.

In this system, the parking brake is applied when the wheel brakes have been released. The excess
fluid in the brake housing is expelled by the return springs via the brake control valve. Operating the
parking brake lever allows system fluid pressure to move the shuttle valves and apply the brake. In an
emergency situation, if the power supply fails and the brake accumulator is exhausted, non-
differential, non anti-skid braking is available using the parking brake lever.

ELECTRONIC ANTI-SKID SYSTEM

Mounted in each wheel axle and driven by the wheel is a small electrical tacho-generator. As the
wheel turns faster, the signal produced by the tacho-generator increases and vice versa. This signal
feeds to an anti-skid controller unit. When the anti-skid controller detects a skid condition, pressure
from the affected brake unit is bled off to return. See semi-modulating and fully modulating anti-skid
systems for their function and operation.

Semi Modulating System

Semi modulating, or quasi-modulating systems attempt to continuously regulate brake pressure as a
function of wheel speed. Typically, brake pressure releases when the wheel deceleration rate exceeds
a pre-selected value. Brake pressure is re-applied at a lower level after a length of time appropriate to
the depth of skid. Brake pressure then gradually increases until another incipient skid condition is
sensed. In general, the corrective actions taken by these systems to exit the skid condition are based
on a pre-programmed sequence rather than the wheel speed time history.

Fully Modulating System

Fully modulating systems are a further refinement of the semi or quasi-modulating systems. The
major difference between these two types of anti-skid systems is in the implementation of the skid
control logic. During a skid, corrective action is based on the sensed wheel speed signal rather than a
pre-programmed response. Specifically, the amount of pressure reduction or reapplication is based on
the rate at which the wheel is going into or recovering from a skid. Also, higher fidelity transducers
and upgraded control systems are used, which respond more quickly.

Page 22 of 34
 Diagram 8.10 Electronic Anti-Skid System Schematic

Touchdown Protection and Braking Efficiency

Whichever electronic system is used, on touchdown the brakes are prevented from being applied until
the wheels speed up to between 15 and 21 mph. If the pilot lands with the toe pedals fully depressed
and anti-skid selected on, there is no braking action until the wheels have achieved this speed. As a
precaution to prevent the nose leg from being slammed onto the runway, large aircraft braking
systems are also prevented from applying full pressure until the air/ground logic system senses that
the nose wheel is in contact with the ground.
Page 23 of 34
To achieve maximum braking efficiency, the pilot also needs to apply the wheel brakes as early as
possible, and increase the friction between the tyres and runway surface, by lowering the nose wheel
onto the runway as soon as possible to initiate the braking action. Deploying the speed brakes and
spoilers to disrupt the airflow over the wings destroys lift, which allows the aircraft’s weight to sink
onto the main wheels, increasing the friction between the tyres and runway surface.

AUTO BRAKE SYSTEM

Diagram 8.11 Graphical Representation of the Effects of Auto Braking

Modern aircraft fitted with auto land systems are also fitted with auto brake systems. In the auto
brake system, the pilot does not touch the toe pedals during the landing run. Normal braking is more
effective at higher speeds and then tails off as the speed decreases and the brake temperature
increases. Aircraft braking performance is shown in diagram 8.11. Anti-skid systems, whilst increasing
the braking effectiveness, still suffer the same drop as manual braking. Auto brake systems, through
electronic programming, maintain a constant rate of retardation g (deceleration). To achieve this and
prevent the wheels from locking up, a serviceable anti-skid system must be set to the on position.

The pilot has a flight deck control panel from which to select the rate of deceleration, as shown in
diagram 8.12. This can be altered throughout the landing run to increase or decrease the rate of
braking. The auto brake system uses aircraft system hydraulic pressure and feeds it directly to the
anti-skid valves and then the wheel brakes. This extra pressure increases the force of the friction pads
on the brake disc and keeps the wheels at a point even closer to the skid than the normal anti-skid
system. Operation of the auto brake system increases the wear and tear on tyre covers and brake
units considerably.

In the event of an auto brake system failing, the braking returns to manual anti-skid. In the event of
the anti-skid system failing, the auto brake system disconnects and the braking system returns to
manual. Fail lights illuminate to warn the pilot of any unserviceable condition.

Page 24 of 34
 Diagram 8.12 Auto Brake Schematic

Brake Temperature

A sensor unit with a telescopic pin (see diagram 8.18) that is in contact with the pressure plate sends
a signal to the flight deck (an analogue gauge showed temperature in older aircraft). This defaults to
the highest wheel brake temperature. Each brake unit has an amber caution light with the legend
OVHT (overheat). By pressing the appropriate caution light, the pilot can read that brake unit’s

Page 25 of 34
temperature. Pressing the test button checks the continuity of the indications by illuminating the
overheat lights and deflecting the gauge needle.

Diagram 8.18 Brake Temperature Indications

For modern aircraft with CRT screens, the brake temperature appears as a number inside a box. As
the temperature increases, the number increases and the colour of the number and the outline box
change. Prior to take-off, the pilot must ensure that the brakes can absorb the energy of a rejected
take-off at that mass. If they cannot, the option is to reduce the mass or await a brake-cooling period.

Recommended Taxi Speed

The DGCA recommends that an aircraft taxis at 21 mph or less as taxiing and turning create more
heat than would normally be expected. Brake units from the late 1960s were manufactured from steel
alloys for the discs and ceramics for the pads. Modern brake units are manufactured from carbon fibre
composites. These units operate more efficiently at the higher temperatures of fast/heavy weight
landings than the older units, but are less tolerant of slow speed, low temperature braking.

Brake Cooling
As the wheel shields modern multi-disc brake units, the heat generated by the brakes can only escape
via conduction and radiation into the surrounding structure and convection through any air passing
across the brake unit. The airflow across the brake unit is restricted to that which enters through the
wheel’s lightening holes. To increase this airflow and cool the brakes more quickly, some large aircraft
fit cooling fans in the wheel units. These cooling fans come on automatically upon reaching a
threshold temperature.

Brake Overheat
If an aircraft suffers a brake overheat, the airport Fire Chief must be informed and the aircraft cannot
be refuelled until the Fire Chief provides permission. It is standard that an aircraft suffering from
overheated brakes on landing must stop when they are clear of the runway to allow for inspection by
the fire service and cooling, prior to taxiing to the stand.

WHEELS AND TYRES

Types of Wheel Hub

Page 26 of 34
There are several designs of wheel hubs used in aviation. These are:
▪ Whell base
▪ Detachable flange
▪ Split hub

Wheel Base

Diagram 8.21 Well Based Wheel Hub

Well-based wheel hubs are similar to car wheels in that they are one piece. This requires the tyre’s
internal diameter to be forced over the wheel rim, which makes them only suitable for use on light GA
aircraft with low-pressure tyres.

Detachable Flange

Diagram 8.22 Detachable Flange

Detachable flange wheels (see diagram 8.22) allows removal of the flange and the tyre to be taken off
the wheel. The pressure in the tyre retains the flange, trying to force it over a locking ring. For
tubeless tyres, a gas seal fits between the flange and the hub.

Split Hub

These wheel hubs consist of two parts, referred to as half hubs, which are cross-bolted together. As
this allows for easy fitment of tyre covers, it has become a common wheel type used on many GA and

Page 27 of 34
air transport aircraft. To make the hub gas tight where tubeless tyres are used, an O-ring seal is fitted
between the two half hubs.

Diagram 8.23 Split Hub

REGIONS OF A TYRE

Diagram 8.25 Regions Of a Tyre

Referring to diagram 8.25, the tyre divides into 4 areas:

Crown
This area has the tyre tread and is designed to withstand the wear of normal operation.

Shoulder
This is a change in profile thickness from the crown and is not designed to take wear.

Sidewall
This is the thinnest and, therefore, weakest section of a tyre and is designed to flex when loads
are applied.

Bead
This is designed to fit against the rim of the wheel, known as the bead seat.

STRUCTURE OF A TYRE

Page 28 of 34
Refer to diagram 8.26 of a cross ply tyre. The steel wire cores located in the bead form the backbone
of the tyre. These cores consist of strands of high tensile steel coated in rubber and formed into a
core. All other parts of the tyre anchor to these. Layers of casing plies provide the strength of the
tyre.

Diagram 8.26 Build Up of a Cross Ply Tyre

Each ply is formed from a series of high modulus cords individually coated with rubber and laid side
by side to form a sheet. Each sheet then attaches to the core wires, and the plies build up in
successive layers.

There are two basic types of tyre lay ups. These are belted radial ply and biased cross ply. In the
cross ply tyre, each casing ply is laid with the cords running at opposite bias angles. The number of
plies and their angle dictate the tyre’s strength and its load capability. During rotation under load, the
bias ply tyre flexes to such an extent that the inter-ply friction creates heat within the tyre’s structure.
Adding reinforcing plies limits this flexing but increases the tyre’s mass.

Radial Belted Tyre

Page 29 of 34
 Diagram 8.27 Build Up of a Radial Ply Tyre

Radial belted tyres have casing plies orientated in the plane of the tyre’s cross-section. These are
frequently made of steel, coated in rubber, and laid up as a sheet. Biased textile cord belts lay over
the casing plies, and above this is a steel protector ply.

When the tyre is loaded, the tread is stabilised by the belt package. This reduces tyre wear due to the
tread scrubbing against the runway. As there is a reduced number of plies, there is a reduction in
inter-ply friction and, therefore, less heat build up within the tyre casing. When comparing two tyres,
one as a bias ply, the other as a radial belted ply, the radial belted ply tyre can have a mass saving of
approximately 20%.

Footprint Area

Diagram 8.28 Cross Ply and Radial Ply Tyres and Tread Footprints

Diagram 8.28 shows two tyres of the same size and their footprint area when equally loaded. The tyre
on the left and the left footprint is of a cross ply tyre. The right tyre and footprint is a radial ply tyre.
Comparing the two tyre footprints shows that the cross ply tyre has more tread in contact with the
runway, therefore exerting the aircraft’s weight over a larger area, which in turn reduces the friction
between the tyre and the runway.

Tubeless Tyres

Page 30 of 34
 Diagram 8.30 Tubeless Tyre

In tubeless tyres, a gas tight rubber compound is manufactured as part of the tyre’s build up. This
saves weight, and these tyres run cooler. The inflation adapter for tubeless tyres fits into the wheel
hub. The tyre’s bead setting against the flange of the wheel forms a gas tight seal. Whilst these tyres
also take time to bed in, tyre creep is not such a problem as it is for tubed tyres.

TYRE MARKINGS

Moulded into the external sidewall of the tyre is a series of markings, as per diagram 8.32. These
include the:

Type of tyre
In this case, the tyre is tubeless type H.

Ply Rating
In this case, the tyre has the strength equal to 22 cotton plies.

Note: The ply rating number does not indicate the physical number of piles. Together with
the load rating, it indicates the strength and corresponding inflation pressures. See AEA No.
below.

Load Rating
In this case, the tyre has maximum static load of 30 100 lb.

Part No.
This is a number specific to the company who manufactured the tyre.

Speed rating
In this case, 245 mph is the maximum groundspeed for which the tyre is tested and approved.

Tyre Pressure
This indicates the tyre pressure at which the tyre is inflated to prior to fitment to the aircraft.

Page 31 of 34
If the tyre has been re-treaded, there is also an identifier as to the date and stage of re-tread.

TYRE PRESSURE CLASSIFICATION

Aircraft tyres are subjected to high impact loads, high rolling speeds, and high temperatures.
Therefore, they are manufactured to high specifications. Tyres are classed as either high pressure or
low pressure. Exceeding the pressures or groundspeeds can cause the tyre to explode. See the Tyre
Classification Table below.

AQUAPLANING

The term given to a condition where the aircraft’s tyres are riding on a liquid film and are not in direct
contact with the runway surface is aquaplaning. The resulting effects are:
▪ Wheel skids, which damage or burst the affected tyre(s), due to the brakes locking the
wheel(s)
▪ Increased landing roll, due to the loss of braking efficiency
▪ Loss of directional control

Aquaplaning, a European term, can also be referred to as hydroplaning, the American term.

There are three forms of aquaplaning:

Reverted rubber
This can occur on touchdown.

Dynamic
This can occur during the landing roll or take-off roll.

Viscous
This can occur during taxiing on taxiways or on stands.

Page 32 of 34
Anti-skid systems aid in controlling aquaplaning conditions by modulating the wheel brakes and
ensuring that brakes do not lock the wheels up, so that they can spin up and start to clear the water
beneath them.

Reverted Rubber Aquaplanning

Reverted rubber aquaplaning is less well known than dynamic aquaplaning. It is caused by the
friction-generated heat that produces superheated steam at high pressure under the tyre’s footprint.
The high temperature causes the rubber to revert to its uncured state and form a seal around the tyre
area that traps the high-pressure steam.

This condition occurs on damp runways or if the touchdown point is on an isolated damp spot of an
otherwise dry runway. The effect is that the wheel does not spin up, which results in a reverted
rubber skid. Refer to wet braking flat spots for the type of tyre damage that this can cause.

Dynamic Aquaplanning

Dynamic aquaplaning occurs when standing water on a wet runway is greater than the tread depth of
the tyre. In this case, the tyre‘s treads cannot displace water from under the tyre fast enough to allow
the tyre to make pavement contact over its total footprint area. If the tread depth of the tyres on an
aircraft is greater than the depth of the water on the runway, then dynamic aquaplaning does not
occur.

As the treads are unable to clear the water, it is pushed as a wedge in front of the tyre. When
dynamic aquaplaning occurs, the tyre rides up onto the wedge of water, as in diagram 8.38. When
the tyre is no longer in contact with the runway surface, the tyre is totally aquaplaning. In this
situation, the centre of pressure in the tyre footprint area can move forward, and the wheel can stop
rotating with the attendant losses of braking and directional stability.

Diagram 8.38 Tyre Aquaplanning

AQUAPLANING SPEED FOR RIBBED TYRES

Page 33 of 34
Aquaplaning speed links to tyre pressure. For the same tyre, the higher the pressure, the smaller the
footprint area in contact with the runway. Therefore, the greater pressure per unit area (square
inches or square centimetres) punches the tyre’s tread through the standing water to contact the
runway surface.

The above description of dynamic aquaplaning refers to standing water. However, this can occur
whenever the runway is contaminated with ice, sleet, slush, or snow. As these are lighter than water,
the aquaplaning formula can be modified to account for their lower Specific Gravity by dividing the
tyre pressure by the given SG of the contaminant first.

The formula for working out the onset of aquaplaning speed appears in the formulas above. The SG
for water is 1. When calculating the aquaplaning speed for a wet or water contaminated runway, the
formula dispenses with the SG value. This is how most people remember it and discuss it.

The Aircraft Performance Notes cover the different depths of allowed contaminant.

Viscous Aquaplanning
Viscous aquaplaning is a low-speed phenomenon and can cause complete loss of braking action on a
wet runway, taxiway, or stand if there is any contamination such as:
▪ A film of oil or grease
▪ A layer of dust
▪ Rubber from previous touchdowns or skids
▪ A smooth runways surface

The contamination combines with the water and creates a more viscous mixture. Note that viscous
aquaplaning can occur with a water depth less than dynamic aquaplaning, and skidding can occur at
lower speeds, like taxiing to the gate during light rain, applying the brakes, and rolling over a patch of
spilt oil.

ACTIONS TO MINIMISE AQUAPLANING ON LANDING

▪ Avoid landing in heavy precipitation. Allow time for the runway to drain.
▪ Know the aquaplaning speed of the main tyres and nose wheels.
▪ Use flaps to land at the lowest practical speed.
▪ Do not perform a long flare or allow the aircraft to drift in the flare.
▪ Touch down firmly to punch the tyres through any moisture and do not allow the aircraft to
bounce, as the distance covered in the bounce and the bounce protection system reduces the
available braking distance.
▪ Keep the aircraft centreline aligned with the runway centreline.
▪ Apply anti-skid braking steadily to full pedal deflection when automatic ground spoilers deploy
and main wheel spin-up occurs.
▪ Deploy ground spoilers manually if automatic deployment does not occur.
▪ Apply maximum reverse thrust as soon as possible after main gear touchdown. This is when it
is most effective.
▪ Get the nose of the aircraft down quickly. Do not attempt to hold the nose off for aerodynamic
braking.
▪ Apply forward column pressure as soon as the nose wheel is on the runway to increase weight
on the nose wheel for improved steering effectiveness.
▪ If the aircraft is in a skid, align the aircraft centreline with the runway centreline.

Page 34 of 34