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landing gear > landing gear > Flashcards

landing gear Flashcards

1
Q

functions

A
  1. to absorb kinetic energy of landing and reduce vibration
  2. provide means of controlling landing/descend and braking
  3. maneouvre on ground
  4. support the aircraft at a convenient height that gives clearance to propellor and flaps
  5. facilitate loading

Once airbone, landing gear is dead weight and unuseful.

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2
Q

Fixed landing gear

A

Found on:
light & slow and simple aircrafts/some larger ones
Fixed to wings or fuselage

Fixed means:
non-rectractable

Pros:
light weight
simple, low maintenance, low initial cost

cons
Lowered performance due to drag

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3
Q

3 Fixed landing gear types

A

Found on:
light & slow and simple aircrafts/some larger ones
Fixed to wings or fuselage

Fixed means:
non-rectractable

Pros:
light weight
simple, low maintenance, low initial cost

cons
Lowered performance due to drag

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4
Q

types of fixed landing gear

A

Cantiliver Spring Steel Leg
* Leg is a tube/strip of steel
* Employed at the mainundercarriage
* the upper end attached by bolts to the fuselage
* lower end terminating in an axle on which the wheel and brake are assembled.

Rubber Bungee / Cord
* absorbs shock and directs the shock to the rubber
* the undercarriage is usually in the form of tubular struts
* designed and installed so that the landing force is directed against a number of turns of rubber in the form of a grommet or loop.

Oleo-pneumatic strut
- Uses combination of gas and hydraulic fluid to absorb shock
- Found in advanced light aircrafts
- in some fixed main undercarriage
- in all nose undercarriage
- in all rectractable gear
- Struts come in differnent forms
- fitted with Spats

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5
Q

what is a spat

A

found in:
Oleo pneumatic struts

it’s an aerodynamic fairing required to minimize drag

con:
they capture mud from grass when landing/taking off = adds to the weight of the aircraft and affect it’s landing/take off peroformance

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6
Q

how often must a spat be cleaned

A

before every take off

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7
Q

Construction of the Oleo penumatic strut

A
  1. consist of one outer cylinder attatched to the airframe
  2. It houses the Piston assembly
  3. It houses the inner cyclinder
  4. Inner cyclinder is free to rotate & move up and down within the outher cyclinder
  5. the movement is limited/controlled by Torque link (which connects by cyclinders together on the outside)
  6. Gas and Fluid are spearated by free-flowing separator piston
  7. Above the separator= Hydraullic fluid
  8. Below the separator = Filled with compressed gas ( nitrogen gas or air)
  9. inner cylinder is attached to the axil
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8
Q

Torsion load on pneumatic struct

A

the torsion load is absorbed by the Torque link when maneuvering on ground

the smaller the turn the aircraft does, the more load is felt on the torque links

turns should be made as wide as operationally possible

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9
Q

role of compressed gas in the strut

A

supports weight of the aircraft on ground

cushions pumps during taxxing

absorbs shocks on landing

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10
Q

role of the fluid in the strut

A

to dampen oscillations

control rate of compression and extension of the cylinders

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11
Q

the piston connected to lower cyclinder

A

has holes in it to restrict the flow of fluid through the piston

therefore this dampens the movement between the 2 cyclinders

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12
Q

Oleo pneumati strut when static

A

When static:
* the weight of the aircraft is balanced by the strut gas pressure
* inner cylinder takes up a position approximately midway up its stroke.

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13
Q

Oleo pneumatic strut when taxing (moving on bumps)

A

Bumps are cushioned by the gas pressure in lower cyclinder

and dampened by the limited flow of fluid in the orfice

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14
Q

oleo pneumatic strut after take off

A

the gas pressure will cause the inner cyclinder to extend to its fullest extent

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15
Q

oleo pneumatic strut after when landing

A

The strut is shortened

fluid pushed thru the flutter valve (in outer cyclinder)

this restriction limites the speed at which the strut compresses

As the internal volume of the cylinders decrease, air pressure rises till it dampens the force of the landing aircraft

as the upward force decreases, the gas pressure acts as a spring and extends the inner cylinder. The speed of extension is limited by the restricted flow of fluid through the orifice

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16
Q

how to identify gas pressure leakage in the strut?

A

the strut will not expend to its fullest extent

uneven amounts of **fescalized metal **will show on each gear (shiny material forms)

the shiny part will indicate the portion that has the gas leak

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17
Q

Rectractable landing gear

A

in majority of modern aircrafts

in some light aircraft

Improve performance as they reduce/eliminate drag in high speeds

  • use hyraulics for retraction and extension

*heavy weight due to mechanism and stowage

Types:
Nose gear
Main gear
Bogie gear (multiple wheels set up)

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18
Q

Retraction and extension

A

Rectraction:
usually hydraullic
But sometimes pneumatic or electrical system

Extension:
In some cases affected by gravity or slipstream (while rectraction is powered)

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19
Q

mechanical locks & gear devices

A

Locks:
* ensure rectractable gear is secure under each undercarriage
* Undercarriage wells are normally sealed by doors for aerodynamic reasons.

devices:
* indicate to the crew the position of each undercarriage;
* means by which the landing gear can be extended in the event of failure of the power source.
* means provided to prevent retraction with the aircraft on the ground
* means to guard against landing with the landing gear retracted.

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20
Q

Tricycle layout of the gear

A

Nose gear at front
- allows aircraft to stay level
- can be stirred for maneuvering on ground
- prevents aircraft from tipping over if there is a strong tailwind
- prevents from ground looping
- gives better visibility forward for the pilot for manoevering

the 2 main undercarriages
* are just ‘after centre of gravity’
* this supports 90% of aircraft’s weight and absorbs landing shock

in some, a tail might be used/or tail skid
to help with skidding or to prevent tipping

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21
Q

Tail dragger

A

Used on older planes to give more clearance to the propeller

helps aircraft not tip on its tail

tail gear at back

the 2 main undercarriages are just ‘after centre of gravity

needs extra training to land/take off

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22
Q

factors affection the layout of rectractable gears

A

Size of aircraft.

Weight of aircraft.

Role of aircraft.

High or low wing.

Performance.

Construction of aircraft and associated stowage problems.

cost

ability to be ‘multi-role’

height off the ground ( to ease loading of passengers and planes, rectractable is better, otherwise the aircraft doors are too high to reach and geears would be too long to be fixed)

23
Q

Under the wing landing gear have:

A
  • The leg (pin jointed to the aircraft)
  • the wheel
  • a means to absorb landing shock
  • a means to control deaccleration
  • a means to withstand turning and break stresses

in boeing, the main gear assist with steering during tight turns by reducing the turning radius. When the nose wheels are turned the main wheels turn in the opposite direction

24
Q

Fuselage Mounted Landing Gear have

A

*same as wing landing gear….but..

1) there is no geometric lock, so provision is required to lock the undercarriage up and down

2)depending on the **wheels, they may need shock absorbers **or steering motors

3)can access the fuselage undercarriage to **lower the gear manually **in case of emergency.

25
Q

types of load on landing gear

A

Torsional: ground movement

forward: during pushback

compression: landing or static state

side: during sidewind, taxing and take off

rearward bending

26
Q

Nose undercarriage traits

A

Lighter than main unit

only withstands compression

carries less weight

has towing attachments

undergoes shear load (from towing)

27
Q

Design aspects of nose gear

A

1. Castoring:
* Wheels must castor (move) freeliy through subjected compression and shear loads or else it creates bearing issues.
* Catoring is Ability for nose wheel to turn to either side in response to braking, or aerodynamic forces on the rudder.

Self centring
* Automatic self-centring so that it can align itself to be stowed properly in the stowage, otherwise the space won’t fit and will cause damage to the aircraft structure as the hydraulic system forces the gear upwards
* Need spring loaded cam or hydraullic dashpot

Shimmy
A vibration induced to the nose wheels because of:
* flexible tyre walls
* Worn or broken torque link
* wear in the wheel bearings
* uneven tyre pressures

To reduce shimmy:
* maintain good tyre pressure
* double nose wheel
* twin contact wheel
* Provision of a hydraulic lock across the steering jack piston.
* Fitting a hydraulic damper.
* Fitting heavy self-centring springs.

Withstanding sheer loads

Nose wheel steering

28
Q

Nose wheel steering

A

Simple steering
* for light aircraft
* the nose wheel is mechanically linked to the rudder pedals

Powered steering
* Uses ‘powered steering’ - on large aircrafts
* Power steering uses a hydraullic system

29
Q

Power Nose wheel steering

A

Nose wheel rotated by electric, pneumatic or hydraullic system (most common)

  • Allowing the engines to be set at the minimum thrust for taxiing,
  • saving fuel,
  • reduces tyre and brake wear
  • reduces noise pollution.

Steering is controlled, depending on the type of aircraft, by:

  • A separate steering wheel.
  • Operation of rudder pedals.

Incorporated in the steering system are:
* Self-centring jack.
* Shimmy damper.

30
Q

power steering design

A

In hydraullic system:

  • a cockpit steering wheel or tiller
  • a control valve
  • steering cylinders to turn the nose gear
  • a mechanical feedback device to hold the steering at the selected angle
  • a power source, normally the aircraft hydraulic supply fed from the engine driven pumps
31
Q

Undercarriage considerations

A

the size of aircraft and the ALL UP WEIGHT lead to the increase in wheel loading

This is the load on each wheel at take off weight

high wheel loading can affect the runway surface (ie it can damage a low strength runway)

solution - to spread the load:
replace large single wheel with high pressure tyres with small wheels with lower pressure

32
Q

configuration of undercarriage relies on

A

the stowage when retracted

the load spreading consideration

33
Q

Muti wheel benefit

A

Weight: small wheels weigh less

Ease of service

safety factor if one bursts, there are others to carry load

easy of stowage

34
Q

Multi wheel disadvantage

A

small tight turns can cause:

  1. Larger foot print causing crabbing while turning
  2. tyre wear due to scrubbing (treads become worn off)
35
Q

runway contamination affecting gears

A

Might be cause from slush which is deposited during take off, and freezing during climb making it difficult to retract or extend the gear.

to solve:
* cycle the gear after take off
* select gear up , down and up
* the shocks inflicted on the gear during this cycle should be sufficient to remove any deposits from it.

36
Q

Hydraullic gear retraction system

A
  1. Hydraulling system takes its power from engine powered pumps + alterntive system

on small aircraft, there is a self contained ‘power pack’ which houses:
* a reservoir and selector valves for the landing gear and flap systems;
* an electrically driven pump may also be included,
* or the system may be powered by engine driven pumps.

  • This type of system normally provides for powered retraction of the landing gear, extension being by ‘free-fall’, with the assistance of spring struts.
37
Q

restrictor valves

A

Benefitial for main gears:
helps limit the speed (rate) of lowering of the main undercarriage units, which are influenced in this direction by gravity.

Not beneficial for the nose gear
The nose undercarriage often lowers against the slipstream and does not need the protection of a restrictor valve.

38
Q

Pneumatic retraction system

A

Operation of a pneumatic retraction system, is similar to that of a hydraulic system

, except that pressure in the return lines is exhausted to atmosphere through the selector valve.

39
Q

electrical gear retraction system

A

light aircraft

do not require the use of a high pressure fluid system.

The main and nose undercarriage units push and pull forces on the retraction mechanism are obtained by an electric motor and suitable gearing

40
Q

Gear position indication

A

1)An electrical indicating system used to provide a positive indication to the crew of the operation of the locks and of the position of the landing gear.

The system usually consists of microswitches on the uplocks and downlocks,

there is also

2) magnetic indication

3)light indication

41
Q

electrical indication colours

A

Green: locked down
Red: In transit
No colour: Locked In

*bulbs are duplicated to avoid confusion or false indications in case of bulb failure

in some aircraft:
Green: lock down
Amber: Locked up

Some aircrafts also show ‘Gear Door” position

42
Q

Gear safety features

A

Nose wheel centring
* always be aligned in a fore (front) and aft (rear) direction during retraction
* hydraulic nose wheel centring on aircraft with powered steering.

Gear selector lock
device that prevents movement of the selector lever to not accidentally retract whilst gear is on the ground

Ground locks
pins or metal sleeves that interfer with unpowered aircraft
intended to prevent collapse of the gear when the aircraft is unpowered on the ground.
They are fitted with warning flags which should prevent the crew from getting airborne with them still in position on the gear.

warning devices
* a horn connected to a throttle operated switch.
* If one or more throttle levers are less than approximately one third open, as would be the case during approach to land, the horn sounds if the landing gear is in any position other than down and locked.

A horn isolation switch
*often provided to allow certain flight exercises and ground servicing operations to be carried out without hindrance, but
*can be used and forgotton by accident

  • an airspeed switch
  • is a definite advantage, since unlike an isolation switch, it cannot be first used, and then forgotten, with perhaps disastrous consequences.
  • An airspeed switch can also be used to prevent the horn sounding during initial descent from high altitude.

**GPWS - ground proximity warning system
**will be inhibted belelow 500ft and flaps are in landing position

43
Q

Emergency lowering system

A

Is having another means of extending the landing gear and locking it in case the main system fails.

two types:

Uplocks are released mechanically or electrically by manual selection.
* **The landing gear ‘free falls’ **under its own weight (gravity) and the downlocks are engaged mechanically.

  • If the main source of power to the gear has failed, there is no power to retract them after they have been released,
  • the doors will remain open.
  • The doors may contact the ground upon touchdown
  • Some aircraft have doors fitted with a frangible portion at their lowest extent so that replacement problems are minimized.

aircraft the landing gear is extended by an emergency pressure system
Pressure for the emergency system may be supplied by a hydraulic accumulator, a hand pump, a pneumatic storage cylinder, or an electrically powered pump.

A Mechanical Indicator will be provided to indicate gear locked down.

44
Q

Landing Gear operating speeds

A

VLO: Max Velocity for Landing Gear Operation

VLE: Max Velocity for Landing Gear Extended

When the **landing gear is fully retracted or fully extended **it is locked into position and is **more resistant to damage from high airspeeds. **

When the landing gear is in the process of extending or retracting (operating) there is no locking mechanism, and the only thing resisting the airflow is the extension/retraction mechanisms.

Additionally, on some aircraft, the landing gear may swing or swivel in odd directions in order to tuck into their recesses, this can cause odd aerodynamic behaviour in the rest of the aircraft if done at high speeds.

Once the landing gear is extended, it is rare that a pilot would then exceed V LO . Most of the time the landing gear is lowered shortly before landing and the pilot is doing everything he can to slow the aircraft further.

If aircraft had to be flown a long distance with the landing gear extended (such as a ferry flight to a repair facility) the pilot would go ahead and fly V LE .

45
Q

Air/Ground logic system

A

microswitches
position will be changed when the weight of the aircraft compresses the oleo, or alternatively, on take-off, when the weight of the wheel and bogie assembly extends the oleo.

On more modern aircraft,- proximity sensing devices
deduce the extension or retraction of the oleo by capacitive or inductive sensing equipment fitted to the oleo.

Whichever system is used, a controlling signal will be sent to a relay or bank of relays, which in themselves are capable of switching the affected circuits on or off as required.

Some aircraft use sensors on just one main landing gear oleo, but it is common to find the sensors duplicated on both main oleos to provide a degree of redundancy in the system.

46
Q

Shock absorbers

A

Main gear absorbs shock and transfers it to fuselage/airframe via the tyres but mainly by the gear’s shock absorbers

47
Q

kicking off

A

main gear handles vertical load but it can also handle, but not well, some side-load created by crosswind

great care need to minimize side load by
- turning at slow speed
- removing/kicking off any drift before touchdown

48
Q

nose gear doesn’t absorb?

A

doesn’t absorb landing shock..

since it’s not in the centre of gravity, you **cannot land on it **as it will experience great bending load

49
Q

oleo strut system

A

Outer cylinder
- contains hydraulic fluid
- attached to airframe
- actis as a dampner to spring action

between the two:
-Orfice allow flow of fluid between the two cylinders
- Rate of strut can compress dependso on the speed of flow of fluid throug the orific

**Inner cyclinder: **
- contains nitrogen
- attached to wheel axis
- acts as a spring to absorb load

50
Q

what’s the gas pessure like when aircraft is stationary in the Oleo

A

gas pressure = weight force of the aircraft

any additional force on aircraft, puts pressure on the gas

51
Q

oleo extention is adjusted by:

A

gas pressure in the leg

52
Q

excessively compressed oleo indicates:

A

low gas pressure + fluid leak

check for fescalized metal (shiny metal) on the strut.
Shiny part indicates low pressure

53
Q

Landing gear locks

A

Up lock:
-operaterd by hydraulic pressure/actuator
locks up the gear

Down lock
Pins fitting to Prevents gear from retracting when on ground accidentally

geometric
locks gear down.
doesn’t need hydraullic power - its mechanical asssted by weight of gear
Note: to break this, you need hydraulic actuator

54
Q

Landing gear operation

A

1) Gear lever:
when gear is downlocked and aircraft has its weight on the ground or there are sensors that determine that it’s close to the ground - then there are** safety latches/selenoid switches/Weight on Wheels’ that prevent the level to be pulled up**

it can only be overwritten by pulling a safety latch in certain situations

2)Sounding:
when aircraft is configured for landing, there are warning horns that ‘sound’ ** if gear is not down **

gear doors may show light signal on the dahsboard

3)Lights
Red - gear is unlocked & traveling
Green - locked down
No lights - gear up/locked
RED + GREEN. = if emergency is carried out

DOOR UNLOCKED (light after

landing gear (1 decks)

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