AVAM 2230 LANDING GEAR

What type of gear?

1
 LANDING GEAR TYPES

• Tricycle is now the most

popular configuration.

The tailwheel dominated for 40 years. It is still used on older planes. Conventional
gear is now rare. Two main wheels ahead of the aircraft CoG. With one small tail
wheel steered through rudder pedals.

Although tail wheel aircraft are less maneuverable than tricycle aircraft on paved
landing strips, they offer superior maneuverability on dirt and grass runways.

Tricycle is now the most popular configuration.

2
 TAILWHEEL-TYPE LANDING GEAR
DC-3

One of the largest tailwheel aircraft built. The conventional configuration does work on a larger scale,
it’s just not nearly as good as tricycle gear.

3
 TAILWHEEL LANDING GEAR ISSUES

Video: The Ground Loop Monster also available in D2L

The main problem with the tailwheel-type of landing gear is its tendency to cause
the airplane to ground-loop.

The pilot must be careful to keep the airplane rolling straight, or the centre of
gravity will swing around ahead of the wheels, causing the airplane to spin around
on the ground.

It is also possible to over-center a tailwheel plane by applying the brakes too

sharply. The weight of the aircraft will simply pivot over the main wheels and the
nose will be driven into the ground.

4
 TRICYCLE-TYPE LANDING GEAR
• All current aircraft use tricycle configuration.
• Two main wheels behind aircraft CoG
• Steering is provided by the nose wheel

Tricycle-type Landing Gear Tailwheel-type Landing Gear

Almost all current production airplanes use the tricycle landing gear configuration.
The two main wheels are located behind the airplane's center of gravity
and the nose of the airplane is supported by the auxiliary wheel.

Control on the ground for small airplanes is provided by steering the nose wheel through
connections to the rudder pedals, but large airplanes have hydraulic steering cylinders
to control the direction of the nose wheel.

The purple line in this illustration shows us the centre of gravity for the aircraft.
Tricycle on the left and conventional gear on the right.

5
P-38 LIGHTING

• One of the first mass-produced

aircraft to use tricycle gear.

• Tricycle configuration quickly became

the go-to for engineers.

Up until World War II almost all airplanes used the tailwheel-type landing gear, but during these years
such airplanes as the Lockheed Lightning, the Consolidated Liberator, and the Boeing Superfortress, as
well as the commercial Douglas DC-4 used the tricycle gear.

This gear configuration has proved to be superior in ground handling ease and has become by far the
most popular arrangement since this time.

Became popular in WW2

Superior in ground handling on pavement.

6
FIXED OR RETRACTABLE GEAR
• 2 types of aerodynamic drag:
• Parasite drag
• Induced drag

7
 DRAG

In this graph, the line we care about is the red, parasite drag line. Since landing gear causes parasite
drag, the faster we want the airplane to go, the more the gear slows us down.

Notice that as speed increases, the drag experienced increases exponentially.

Slower aircraft lose little efficiency by using the lighter weight fixed landing gear, but faster aircraft
retract the landing gear into the structure and thus gain efficiency even at the cost of slightly more
weight.

When aircraft speeds increased during the 1930s, particularly as aircraft began to reach speeds of 200
miles per hour, the increased weight of retractable gear became less important than reducing drag, and
retractable landing gear became commonplace to most aircraft.

8
 FIXED OR RETRACTABLE GEAR

• Fixed landing gear may use wheel pants

Wheel Pants

Fixed landing gear may have its parasitic drag deceased markedly by
enclosing the wheels in streamlined fairings, called wheel pants.

Many of the light airplanes in the Cessna line have fixed landing gear that use spring
or tubular steel landing gear legs having a very small frontal area that produces a minimum of drag.

9
 WHEEL ALIGNMENT

•Proper alignment:
• Minimizes tire wear
• Decreases shimmy

It is important to keep the wheels properly aligned to minimize tire wear and decrease shimmy.

10
WHEEL ALIGNMENT
CAMBER
• Amount the wheel leans, as
viewed from straight ahead.

Camber and Toe concepts are the exact same as for wheel alignment on automobiles.

Camber is the amount the wheel leans, as viewed from straight ahead.

If the top of the wheel is vertically straight, it has neutral or zero camber.

If the top of the wheel leans inboard, it’s a negative camber. If the top of the wheel leans outboard,
it’s a positive camber.

What camber does the aircraft wheel pictured have?

11
WHEEL ALIGNMENT

TOE

TOE-IN TOE-OUT

Toe in or out describes whether or not the front of the tire is pointed
straight ahead, towards the centerline, or away from the centerline of the aircraft.

Between Toe and Camber, the toe is much more critical. Anything outside of zero
toe will increase tire wear very quickly.
Camber can be up to 2.5° off of neutral with negligible effect on the wear.

Also, many manufacturers will actually have a slightly positive or negative camber
desired for a specific machine. Always read your AMM to learn what the desired camber
is for your AC.

12
 WHEEL ALIGNMENT

In order to measure toe-in, hold a carpenter's square against a straightedge

placed across the front of the main wheels.

First check the alignment of the landing gear with the longitudinal axis of the aircraft,
and if this is correct, then measure the distance between the blade of the carpenter's square
and the front and rear flange of the wheel.

A spring-steel landing gear moves so much, as the weight of the aircraft is placed on the gear,
that an alignment check is difficult unless special procedures are used.

The recommended method of checking for toe-in or toe-out is to roll each wheel onto a pair
of aluminium plates with grease between them.

If you rock the aircraft back and forth a bit before the measurement is taken, the greased
plates will allow the wheels to assume their true position of alignment.

13
 WHEEL ALIGNMENT

The shims are tapered, you can use differently

tapered shims to adjust either the camber or the
toe.

The shims are tapered. You can use differently tapered shims to adjust either the camber or the toe.

14
 WHEEL ALIGNMENT

On landing gear using an oleo-type shock absorber, toe-in is adjusted by adding

or removing washers from between the torque links

On landing gear using an oleo-type shock absorber, toe-in is adjusted by adding or removing
washers from between the torque links.

I will demonstrate this in the shop.

15
 SUPPORT
• Landing gear is supported by primary structure
• Wing spar, primary bulkheads, etc.

• Fixed gear is bolted directly to structure

The wing spars, along with additional structural members, support and attach the
main landing gear to the wings on larger aircraft.

Non-retractable landing gear is generally attached to the aircraft structure by

bolting the landing gear struts to the structure directly.

16
 TRUNNIONS

Retractable landing gear systems must provide for the landing gear to move,
so the upper shock strut is attached to the airframe using trunnion fittings.

Simply stated, these are extensions or shafts attached to the shock strut that fit into
fittings bolted to the airframe.

17
 TRUNNIONS

The word “trunnion” comes from cannon trunnions – the pivot

point for cannons.

A Trunnion is a Pivot Point.

18
 MULTIPLE WHEELS (BOGEY)

When multiple wheels are placed on the same gear unit, they are attached together on a structural
device called a bogey.

The heavier the aircraft becomes the more wheels are typically added to the bogey to spread the
plane's weight more evenly across the runway pavement.

Bogey (bogie)– The landing gear of an aircraft that uses two or more tandem wheels connected by a
central strut. Note: This word is taken from railway cars.

Shown: 6-wheel bogey

19
TRUNNIONS

Trunnions

Bogey

20
GREASING

21
 GREASE FITTINGS

Different types of fittings require different tools to service.

Watch the video in D2L, How to properly load a grease gun.

Content / Landing Gear / How to Properly Load a Grease Gun

22
SMALL AIRCRAFT GEAR RETRACTION

• Huge variety of different designs for retraction.

• Hydraulic
• Mechanical
• Pneumatic

When the design speed of an aircraft becomes high enough that the parasite drag of the
landing gear hanging out in the airstream is greater than the induced drag caused by
the added weight of the retracting system, retractable landing gear becomes practical.

Some of the smaller aircraft use a simple mechanical retraction system, incorporating a roller
chain and sprockets, operated by a hand crank, and one of the older but popular four-place,
high-performance airplanes uses a simple hand-operated lever to raise and lower the wheels.

Many aircraft use electric motors to drive the landing gear retracting mechanism,
and some European-built aircraft use pneumatic systems to do this job.

23
 LARGE AIRCRAFT GEAR SYSTEMS

• Same concept, more complex

• Have wheel-well doors which are normally
closed.

The actual system for retracting and extending the landing gear on large aircraft Is similar to
that we have just described, but there are several additional features and components used
because of the size and complexity of the system.

Normally, large aircraft have wheel-well doors that are closed at all times the landing gear is
not actually moving up or down.

Why are the doors there?

24
 LARGE AIRCRAFT GEAR SYSTEMS

• Sequence valves open doors before landing gear extends

• Mechanical locks hold the landing gear in its UP and DOWN positions
• Locks must release before actuators are pressurized

Sequence valves are used in the system to be sure the doors are opened before the
landing gear is actuated.

Almost all large aircraft use mechanical locks to hold the landing gear in its
UP as well as in its DOWN position.

There must be a provision in these systems for the hydraulic pressure to release
the locks before fluid is directed into the actuating cylinders.

25
 LARGE AIRCRAFT GEAR SYSTEMS

Spring

Over Center Lock

The gear is held in the ‘up’ or ‘down’ position by over-center locks that are held locked by springs.
These locks can be ‘unlocked’ by moving the landing gear lever and overpowering the spring with
hydraulic pressure (Down-lock Actuator).
Springs hold the lock. Lock actuators break the lock.

Watch the Landing Gear Up Lock and Down Lock video in D2l.

Content / Landing Gear / Landing Gear Up Lock and Down Lock

26
LARGE AIRCRAFT GEAR SYSTEMS

• After take-off the landing gear handle will be

moved to the ‘off’ position.
• The gear is held in the up position by an
over-center device.
• When landing, the Over-center is overcome
by a hydraulic actuator.

Shortly after an airliner completes a take-off roll, a flight crew member will place the landing
gear handle in the ‘off’ position.

The aircraft gear are then mechanically held in the up position by spring tension locking an
over center device.

When landing, the spring tension is overcome by a Up-lock actuator.

27
LARGE AIRCRAFT GEAR SYSTEMS

Up Lock
Hook

28
 LARGE AIRCRAFT GEAR SYSTEMS

• Brakes are automatically applied when GEAR UP is selected

• 2 Reasons:
• Gyroscopic Effect
• Noise

Some provision is normally made to apply the brakes when the landing-gear selector
is placed in the GEAR UP position.

This prevents 2 things:

Gyroscopic effect is ability (tendency) of the rotating body to maintain a steady
direction of its axis of rotation. In practice this effect would be fighting the
gear retraction and make the hydraulic system work harder.

Wheels spinning in the fuselage would also make an undesirable noise,

which is prevented by the auto-brake.

29
 LARGE AIRCRAFT GEAR SYSTEMS

• Heavy landing gear free falling could damage the structure, so return
flow from the actuator is restricted
• Orifice check valve in the fluid lines to the actuators
• Unrestricted flow, is allowed when the gear is being retracted.

Most of the large aircraft landing-gear systems, use some form of orifice check valve in the
fluid lines to the actuators.

The weight of the landing gear dropping out of the wheel wells could cause it to fall
so fast that there could be damage to the structure, so the return flow from the actuator
is restricted. Unrestricted flow, however, is allowed into and out of the actuator
when the gear is being retracted.

30
 EMERGENCY EXTENSION SYSTEMS

• Retractable landing gear systems must have

• Emergency extension systems generally use a
variety of methods to lower the gear:
• Mechanical
• alternate hydraulic
• compressed air
• free-fall

Retractable landing gear systems must have a means of lowering the landing gear if the
primary method of lowering the gear fails.

Emergency extension systems generally use a variety of methods to lower the gear.

Some of the methods can include mechanical, alternate hydraulic, compressed air,
or free-fall techniques to lower the gear.

31
 GEAR UP LOCK SOLENOID
• Prevents retraction of the landing gear while the aircraft is on the ground
(weight on wheels)
• The selection handle is physically locked by a solenoid
• Power to solenoid is cut when weight is off the wheels

Most aircraft that have a retractable landing gear system are equipped with a means of
preventing the retraction of the landing gear while the aircraft is on the ground.

If the aircraft's hydraulic system is powered and the gear handle is moved to the up position,
the landing gear would retract.

To prevent this from happening, a switch with a lever attached prevents the gear control
handle from being placed in the up position when there is weight on the aircraft wheels.

If the aircraft is in the air, the switch pulls the lever away from the gear handle so it can
be placed in the up position.

32
 GEAR UP LOCK SOLENOID

• A switch in the landing gear senses whether the aircraft is on the ground or in
the air.
• This switch is referred to as the:
• safety switch
• air/ground sensor (Boeing)
• weight on wheels switch
• squat switch

33
PROXIMITY SWITCHES

• Advantages:
• Will not arc at altitude
• No internal moving parts

34
 PROXIMITY SWITCHES

• A solid state switch.

• Coil and metal slug.

The coil can sense a metal

object entering its field.

35
 PROXIMITY SWITCHES

Advantages:
• Will not arc at altitude
• No internal moving parts

Target is just a piece of metal. When the target is near the sensor, voltage drops and the logic card
receives a logic 0 condition, which it interprets as switch closed. Opposite when target is far from the
sensor.

Proximity switches are the preferred technology as there are no moving parts in the switch and they are
much more reliable and have a longer life span than microswitches. Micro-switches wear out and
become dirty which affects performance.

It is possible to fool these systems into thinking the gear is up.

36
 GROUND LOCKS

• Safety the gear in the down position.

Ground locks are used to safety the landing gear in the down position.

The locks generally consist of a pin inserted into the retraction mechanism
in such a manner as to block the retraction of the landing gear.

37
 GEAR INDICATORS
(Position indicators)
Located near handle

Position indicators, generally located close to the landing gear lever, include
three green gear down and locked lights, a gear door open light, and a gear disagreement light.

Smaller airplanes do not use gear door open lights.

Generally when the gear is up and locked, all the lights will go out
signalling that the gear is up and locked.

A warning system is used to inform the pilots if the gear is not down during a landing attempt.

As the throttles are retarded toward closed, a switch is closed, if the gear is not down,
a warning horn will sound until the throttles are opened or the gear is lowered.

Although there are some variations from system to system, most are based on this
basic idea of warning the pilots the gear is not in the down and locked position
while in a landing configuration.

38
NOSE WHEEL STEERING

39
NOSE WHEEL STEERING

Large aircraft are steered on the ground by directing hydraulic pressure into
the cylinders of a dual actuator.

A control wheel operated by the pilot directs fluid under pressure into one or
the other of the steering cylinders.

Fluid from the opposite side of the piston in these cylinders is directed back to the system
reservoir through a pressure relief valve that holds a constant pressure on the system
to snub any shimmying.

(Note: Large aircraft use the steering actuators to dampen shimmy)

40
NOSE WHEEL CENTERING
• Nose wheel has centering cams in the shock strut
• These center the nose wheel when the strut is extended

The nose wheel is equipped with centering cams located in the nose wheel shock strut.

These centering cams center the nose wheel when the strut is extended after take-off.

The nose gear will remain centered until the weight of the aircraft, upon landing,
contacts the strut moving the centering cams away from their slots.

This allows the wheel to turn as commanded by the steering tiller or the rudder pedals.

41
42

42
 JET BLUE INCIDENT

https://www.youtube.com/watch?v=epKrA8KjYvg

The teardown revealed the upper centering cam had been rotated 20 - 30 degrees
when it was installed in the inner cylinder. This resulted in the anti-rotation lugs at
the upper end of the strut not being properly engaged in the back plate slots.

43
SHIMMY DAMPERS

The geometry of the nose wheel makes it possible for it to shimmy, or oscillate back
and forth, at certain speeds, sometimes violently.
To prevent this highly undesirable condition, almost all nose wheels are equipped
with some form of hydraulic shimmy damper between the piston and cylinder of the
nose wheel oleo strut.

Shimmy dampers are normally small hydraulic cylinders with a controlled bleed of
fluid between the two sides of the piston.
The restricted flow prevents rapid movement of the piston, but has no effect on
normal steering.

(Note: Large aircraft use the steering actuators to dampen shimmy)

44
SHIMMY DAMPERS

45
 SHIMMY DAMPERS

Two types of shimmy dampers are:

• Vane type
• Piston type

46
B737 NOSE GEAR

47
B737 NOSE GEAR

48
B737 NOSE GEAR

49
B737 NOSE GEAR

Note: Down lock has

become the up lock.

50
B737 NOSE GEAR DOORS

51
 B737 SHOCK STRUT

Centering Cams

Upper Bearing

52
B737 STEERING ACTUATORS

Where are these attached?

53
GRAVEL DEFLECTOR

Installed on nose wheel to ensure gravel does not deflect into engine intakes.

54
 B737 MLG
MLG is locked in down position by the over
center down lock and locked in the up position by
the up lock.

Both locks are mechanical locks, held in place by

springs and unlocked by hydraulic actuators.

Walking Beam: Inboard and outboard forces combine to raise the gear.
This tends to rotate the actuator about the gear retraction axis.
The walking beam causes a much-reduced force reaching the aircraft structure.

Advantages:

• The actuator does not require a strong structural attach point on airframe.
• The actuator takes up little space and does not have a long extension.

55
B737 MLG OVER CENTER LOCK

Over Center Lock

56
B737 MLG OVER CENTER LOCK

57
 MANUAL EXTENSION

All three landing gears can be extended from flight

deck by pulling any of three handles connected to
cables which mechanically break the up locks.

All three landing gears can be extended from flight deck by pulling any
of three handles connected to cables which mechanically break the up locks.

58
 B737 CONTROLS

nose gear steering wheel landing gear control

59
 B737 CONTROLS

Landing gear lights.

• Up and locked – Off

• Transition – Red
• Down and locked – Green

Landing gear control lever.

60
 LANDING GEAR VIEWERS
• A viewer or observation window is located in the flight deck for the nose landing gear and the cabin for the
main landing gears.
• These viewers allow the flight crew to visibly confirm the landing gear are down and locked.

61
NOSE GEAR VIEWER

red arrows

flight deck view

62
MAIN GEAR VIEWER

red stripe

cabin view

63
FREIGHTERS

64
 FREIGHTERS

Second set of landing gear lights on overhead panel since the

cabin viewer cannot be accessed with freight on board.

65
Illustration from 737 MM

Air safety sensor = Squat switch = WoW switch (Weight on wheels)

As long as the squat switch is open(weight on wheels), the safety relay & lock
solenoid will not be energized. If the lock solenoid is not energized, the lever latch
is preventing the gear selection handle from being selected up.
In other words, the gear cannot be selected up as long as the aircraft is on the
ground, provided that the squat switch is working and rigged properly.

66
 TROUBLESHOOTING
• Describe the likely cause of the snag in the video and how to fix it.

https://www.youtube.com/watch?v=-
eCvwzOhwYo&list=PLB39EB0B31F13DB08&index=35

67
LANDING GEAR STYLES

68
69 LEAF SPRING

69
70 TUBULAR SPRING

70
 RIGID

Certain older types of aircraft used rigid landing gear that transmitted all the loads of landing
touchdown directly to the airframe's structure.

This type of landing gear system was not only hard on the aircraft's occupants,
but could cause structural failure during a hard landing.
Some aircraft, such as helicopters, that normally land very softly also have rigid landing gear.

71
BUNGEE CORD

• Some aircraft use rubber to cushion the shock of

landing.

72

72
BUNGEE CORD
• Bungee cord has a shelf life.
• Bungee cord is colour coded.
• Bungee cord absorbs the shock of landing and taxiing.

73
BONDED RUBBER SUPPORT STRUT

Mooney

Not very effective or common. Not used very often on newer Aircraft.

AKA: Rubber Doughnut

74
 OLEO SHOCK STRUTS

By far the most widely used shock absorber for aircraft is the air-oil shock absorber, more commonly
known as an oleo strut.

The cylinder of this strut is attached to the aircraft structure, and a close fitting piston is free to move
up and down inside the cylinder.

It is kept in alignment and prevented from coming out of the cylinder (on small aircraft) by torsion
links, or scissors.

The upper link is hinged to the cylinder and the lower link to the piston.

The wheel is mounted on the axle, which is a part of the piston.

Watch the Fixed Gear & Shock Absorption - Landing Gear - Airframes & Aircraft System video
in D2L

Content / Videos / Landing Gear / Fixed Gear & Shock Absorption - Landing Gear - Airframes &
Aircraft System

75
TORQUE LINKS

AKA Scissor Links

76
77 OLEO SHOCK STRUTS

Shimmy
Damper

77
The cylinder is divided into two compartments by a piston tube and
the piston itself fits into the cylinder around the tube.

A tapered metering pin, which is a part of the piston, sticks through

the hole in the bottom of the piston tube.

To fill the strut, the piston is pushed all of the way into the cylinder,
which is filled with hydraulic fluid to the level of the charging valve.

The cylinder is divided into two compartments by a piston tube and the piston itself
fits into the cylinder around the tube.

A tapered metering pin, which is a part of the piston, sticks through the hole in the
bottom of the piston tube.

To fill the strut, the piston is pushed all of the way into the cylinder, which is filled
with hydraulic fluid to the level of the charging valve.

78
 SHOCK STRUT OPERATION
• The taper of the metering pin provides a
graduated amount of opposition to the
flow and smoothly absorbs the shock.

• Taxi shocks are taken up by the cushion

of compressed air above the oil.

Dirt and foreign particles form around the filler plugs / air valves of the landing gear
struts, therefore, before attempting to remove these plugs, or service the strut, they
should be cleaned.

79
SHOCK STRUT OPERATION

Then with the weight of the aircraft on the wheel, enough compressed air or
nitrogen is pumped through the charging valve to raise the airplane until the piston
sticks out of the cylinder for a specified distance.

When the weight is removed from the landing gear, the piston extends the full
amount allowed by the torsion links (or gland nut on large aircraft) and the
fluid drains past the metering pin into the fluid compartment in the piston.

Then, when the wheels contact the ground on landing, the piston is forced up into
the cylinder.

The metering pin restricts the flow of fluid into the cylinder, and much of the energy
of the impact is absorbed by forcing the fluid through this restricted orifice.

80
 FILLING THE
AIR/OIL STRUT

Oleo Strut Servicing:

1. Jack aircraft.
2. Release nitrogen.
3. Remove valve.
4. Compress strut.
5. Fill strut with fluid.
6. Install valve.
7. Fill with nitrogen to spec. PSI.
8. Remove jacks.
9. Measure strut length. And service as required.

81
 SERVICING SHOCK STRUTS
• The air-oil type oleo strut should be maintained at proper strut tube exposures for best oleo action.
• Both the nose and main gear struts will have a specific length of piston tube exposed.
• These measurements should be taken with the airplane sitting on a level surface under normal fuel loading
conditions.

82
SERVICING SHOCK STRUTS

The air-oil type oleo strut should be maintained at proper strut tube exposures for best oleo action.

Both the nose and main gear struts will have a specific length of piston tube exposed.

These measurements should be taken with the airplane sitting on a level surface under normal fuel
loading conditions.

83
SERVICING SHOCK STRUTS

84
SERVICING SHOCK STRUTS

• Information regarding the servicing of shock struts may be found in:

– Manufacturer’s Maintenance Service Manual.
– Information decal or placard located on strut.
– Manufacturer’s Overhaul Manual.

85