13.16 Landing Gear Flashcards
Apart from supporting the aircraft on the ground, what are the secondary functions of the undercarriage (UC)?
Support the aircraft in a stable attitude
Provide mobility on the ground
Absorb landing loads
Dampen out noise and vibration during taxiing
Steering
Braking
What is a tricycle UC?
Two main UC units and one nose UC unit (standard)
Variations of the above apply
What are the main advantages of tricycle UC?
Pilots view during taxiing, landing and take–off is virtually unobstructed
Layout of the UC units give forward stability
Maximum braking efficiency can be utilised
What are the disadvantages of tricycle UC?
Stability and CofG can be affected during maintenance operations
CofG must remain within area bounded by UC units otherwise structure becomes unbalanced
Where are the ideal strength points in a) the fuselage and b) the wing?
a) adjacent to a frame or juncture of a frame or stringer
b) adjacent to the mainspar or juncture of a rib or spar
What is a structural fuse?
Points between the landing gear and wing spar attachments
Strong enough to handle normal operating loads
Designed to fail under excessive loads before damage to the primary structure can occur
What are the advantages of fixed UC?
Simple
Less weight
Less maintenance
Lower cost
How should a damaged/overheated wheel be approached?
Do not approach until the tyre has cooled to an ambient temperature
Approach at an oblique angle in the direction of the tyre shoulder
What are the two main types of wheel assembly?
Removable flange
Split hub
In the two main types of wheel assembly, how is sealing achieved when used with tubeless tyres?
Use an o–ring seal
What are the 3 methods of heat dissipation?
Wheels are vented and flared
Heat shields may be installed around the brake unit
Axle fans may provide a cooling flow of air through the wheel
What wheel component can prevent explosion due to overheated wheel brakes and how do they work?
Thermal fuse plugs
Melt to release the tyre pressure when their temperature exceeds a set figure
What type of bearings are wheels usually mounted on to?
Tapered roller bearings
What action should be taken if any wheel tie bolt is found to be defective?
The wheel should be removed and the complete set renewed
What is the area of a tyre called where the carcass plies are located?
Sidewall
How are Radial plies laid in relation to the direction of rotation?
90° to the direction of rotation
Each successive layer laid at the same angle
What is the purpose of the chine in a nosewheel tyre?
Deflect water and slush to the side and away from intakes
What are the awl vents and how are they identified?
Identified by green or grey painted spots
Vents which pierce the sidewall and allow nitrogen or air trapped during the tyre’s manufacturing process to percolate through the carcass plies
What should the red spot or triangle on a tyre be aligned with on the wheel?
Tubed tyres – Heavy spot of the tube
Tubeless tyres – Heavy point of the wheel assembly
On both occasions may be the inflation valve
What is tyre creep?
Tendency of the tyre to move relative to the wheel
What allowance is made to the tyre pressure when it is subjected to the weight of the aircraft?
4%
What action should be taken if a cold tyre is found to be 5% below loaded inflation?
Any tyre between 5% and 10% below loaded inflation pressure should be reinflated to the correct pressure
Checked at the next daily check
Rejected if 5% below again
What action should be taken if a tyre on a multi–wheel UC has run flat?
All tyres on that UC should be scrapped
What is the maximum oxygen content of a tyre?
5% by volume
What affect will a rise of 3°C have on tyre pressure?
Approximately 1% change in pressure
Describe the 3 forms of aquaplaning?
Dynamic
Viscous
Reverted rubber
What are the advantages of a multi–wheel UC?
Easier to stow when retracted
Provide greater safety (burst tyre is not so critical)
Easier to handle during maintenance (wheels are smaller)
Increasing brake efficiency (brake units on every wheel)
Weight of aircraft is spread
What is the purpose of the bogie hop damper/tilt jack?
Means of positioning the bogie beam at a suitable angle for retraction and landing
Acts as a damper unit
What component of the bogie maintains the weight force on the rear wheels during braking?
Equaliser rods/compensating rods
What might be the result of lack of lubrication on a bogie UC?
Corrosion
or
Hydrogen embrittlement
What is the purpose of the torque links on a nose UC assembly?
Prevent rotation of the lower strut within the upper strut
Limit extension of the lower strut within the upper strut
How can nosewheel shimmy be prevented on larger aircraft?
Using twin nosewheels, each attached to a ‘live’ axle
Shimmy dampers
Twin actuator steering system restriction of flow
Why must the nose UC be centred?
Must align with the wheel bay once the weight has been removed or damage would occur when the UC is retracted
How are nose UCs usually centred (non-steerable)?
Centring cams are located inside the shock absorber
With weight off, oleo strut is extended by gas pressure within the strut
Sliding cam contacts the fixed cam and slides along it
In so doing, rotates the nose gear into a straight ahead direction
What are the 2 main types of oleo pneumatic shock absorber?
With separator
Without separator
How does the shock absorber with separator achieve rebound restriction?
When landing, oil in the outer cylinder lifts the flutter plate and passes freely into the lower cylinder
When the compressed gas expands, oil flows back to the outer cylinder but spring now closes the flutter plate
Flow of oil is now restricted therefore speed of extension is limited
How is correct extension of an oleo ascertained during daily servicing?
Check daily against the centre of gravity/loading graphs provided in the approved maintenance manual
What is the general order of replenishing an oleo strut?
Dispel gas
Drain old oil
Recharge oil
Recharge gas
How are normal taxying loads cushioned?
Cushioned by gas pressure and dampened by the limited flow of fluid through the orifice
Levered suspension struts are more likely to use what type of oleo?
Small liquid spring shock absorber
What must friction brakes do?
Convert kinetic energy into heat
Provide drag (retarding force)
Disperse heat
How is a working clearance maintained in the brake pack when the brakes are released?
Springs in the adjuster assemblies move the pressure plate back to maintain a working clearance and permit free rotation of the wheel
What condition should the brakes be in to measure the wear pin?
Brakes need to be applied
What effect would air in the brake hydraulic system have?
Degrade the performance of the brakes
Sluggish or less effective due to sponginess
What colour coding relates to brake overheat on an EICAS display?
White
Where can Maxeret mechanical anti–skid units be mounted?
Either rim driven or axle mounted
In the electronic anti–skid system, what outputs do an AC speed sensor and DC speed sensor produce?
AC – DC voltage proportional to the frequency of the current produced
DC – voltage output directly proportional to the rotational speed of its armature
What are the schematic components of an electrical anti–skid system?
Wheel transducer
Control box
Anti–skid control valve
Annunciator
Brake units
In a retractable UC, what braces the shock absorber a) longitudinally and b) laterally?
a) rear or drag bracing strut
b) side bracing strut (may also provide a means of retraction)
What is the purpose of the ‘walking beam’?
Serves to reduce the reaction force going into the aircraft structure from the actuator
On a British aircraft, what would 3 red lights indicate on an UC position indicator?
UC is in transit
How is an up selection of the UC prevented when the aircraft is on the ground?
Safety device
Spring–loaded plunger retains the selector in the down position and is released by operation of a solenoid
Electrical power to the solenoid is controlled by WOW switch on shock absorber strut
Electrical power is therefore only restored on takeoff
From which service does a hydraulic steering system receive its main operating pressure?
Mainly UC down line
(emergency supply from a hydraulic accumulator)
How is a steerable nose leg centred before retraction?
Inner cylinder in each steering jack is connected to the landing gear UP
Supplied with fluid under pressure when the landing gear is selected UP
Steering jacks thus extend equally to centralise the nose wheel
What does the flow of fluid between the 2 steering jacks enable?
Allowing the aircraft to be towed, or the nosewheel to return to the central position after a turn has been initiated with the steering wheel
What slows down the rate of lowering of the UC?
Restrictor valves
What 3 conditions would give a red landing gear position and warning indication?
Doors are ‘not closed’ with the landing gear in the up position
Any engine throttle is retarded to idle position with the landing gear in the up position
During landing gear transit; or when any landing gear is not in the position that corresponds to the position of the landing gear control handle
What condition would give an aural warning?
An unsafe condition
- Landing gear not down and locked, engine is at idle, aircraft below predetermined speed (can be silenced)
- Landing gear not down and locked, flaps extended beyond the approach position for landing (cannot be silenced)
What is the main method of attachment of landing gear?
Pivot point called a trunnion
What are the disadvantages of a retractable UC?
Increased weight of additional moving parts
More complicated design
Additional maintenance
What is the melting point for the following thermal fuse plugs:
Red
Green
Yellow
Red – 155 °C
Green – 177 °C
Yellow – 199 °C
What is a ‘live’ axle with regards to a nose wheel?
When the axle itself rotates within bearings in the nose wheel leg
What are the regions of a tyre?
Crown
Shoulder
Sidewall
Bead
How is bias ply constructed with regards to the centreline or direction of rotation?
Between 30° and 60° angles
Successive plies are laid with the cord angles opposite to each other to provide balanced carcass strength
What does ply rating indicate?
Strength of the tyre
Why is creep a particularly dangerous issue in tubed tyres?
May move the inner tube far enough to tear out the inflation valve, causing sudden deflation of the tyre
How might a creep indication be given without a creep mark?
Red triangle or red spot indicating the tyre light spot would be positioned at the wheel inflation valve and could provide an indication of creep
What should be used when inflating a tyre for pressures above 100 psi?
Screw–on connector
What should be done with a tyre which is more than 10% below loaded inflation pressure at ambient temperature?
Rejected together with the companion tyre on the same axle
What should happen if a tyre has been under load while in a deflated condition?
Tyre removed from the aircraft
What should happen if a tyre has been taxied while in a deflated condition?
The deflated tyre and its axle companion should be scrapped
How might the heavy side of a tube be marked?
With a red or yellow band approximately 10mm wide
Following tyre inflation after tyre and wheel assembly, what should be used and why?
Safety cage
Prevent injury to the operator
What are the three stages of venting of a tubeless tyre after tyre and wheel assembly?
First stage – Fairly rapid but ceases after 20 minutes
Second stage – Slow seepage lasting several hours
Third stage – Continuing process resulting from normal permeation of nitrogen through the inner lining
When must the testing of tubeless tyres be carried out after tyre and wheel assembly?
After the first stage of venting has ceased
When carrying out a duration pressure test, when should the tyre be rejected?
Loss exceeds 10%
When carrying out a duration pressure test, what should happen when the loss of pressure is less than 10%?
Re–inflated to the original pressure and checked in 12 hours
How long should a duration pressure test last in total?
24 hours
How might concussion of a tyre occur?
Heavy impact on, for example, a stone during touchdown
How is concussion detected externally?
Dark stain on the tyre or very slight bulge where the rubber is bruised
How is concussion detected internally?
A diagonal line or a ‘star’, dark in colour, at the point where the impact occurred
What must be ensured when removing tubed tyres from split wheels?
Completely deflated before any attempt is made to remove loose members
What is the expected error of ‘stick’ type gauges?
Up to 4%
What is the expected error of ‘dial–type’ gauges?
1/2% within the range of pressures specified for light aircraft and older types of transport aircraft
How should tyres be stored?
Vertically upright
Two–thirds of the tyre should be stored above the support tubes and one–third below
How should tyre tubes be stored if the original wrapping is not available?
Slightly inflated and stored inside tyres of appropriate size
What is the shelf life of tyres and tubes?
Provided they are kept in ideal storage conditions, 7 years from the date of manufacture
What depth of water could cause dynamic aquaplaning?
1/10 inch”
Explain reverted rubber aquaplaning?
Friction caused by skid increases the temperature of the tyre, causing rubber particles to build up at the rear of the tyre contact area
Forms a ‘seal’ delaying the escape of water
Water boils and is converted to steam which lifts the tyre off the surface of the runway
What other name might the bogie hop damper/tilt jack have?
Bogie levelling strut
How is any unevenness of the runway compensated for?
By the tilt jack allowing the bogie to pivot about the bottom of the main shock absorber
In a shock absorber with separator, what does the lower cylinder piston head contain which normally covers holes drilled through the head?
Spring loaded flutter plate
What can the number of charging points tell you about the type of shock absorber you have?
With separator – separate charging points for the oil and gas
How does the shock absorber without separator achieve rebound restriction?
Using a metering pin
When the gas expands after initial landing and having been compressed
Oil flow is restricted through the metering pin and so slows the speed of extension
What are the conditions for when a tyre must be inflated with nitrogen?
Braked wheels of aircraft over 5700 kg MTWA on retractable UC
Where are the wear pins located on a brake unit and what do they do?
Attached to the pressure plate and protrude through the torque plate
Amount of wear which has taken place in the disc pack
What might the wear indicator pin also be known as?
Retraction pin
What would the emergency hydraulic system not be connected to with regards to brakes?
Anti–skid device
How is the parking brake applied?
Both brake pedals are depressed and the parking brake handle pulled
What is a brake debooster and what is their principle of operation?
Pressure reducing valves
Pressure differential being generated by an area differential
In a Maxeret mechanical rim driven anti–skid unit, by how much will the flywheel overrun the drum when the wheel is beginning to skid?
Up to a maximum of 60° relative to angular movement
In a Maxeret mechanical axle driven anti–skid unit, by how much will the flywheel overrun the drum when the wheel is beginning to skid?
30°
What are the pre–requisites for the operation of an electronic brake anti–skid control system?
Anti–skid is selected ON
Undercarriage is down
Ground sensing in ground mode
Wheels are spinning at a predetermined RPM eg 30 knots and above
What is a DC wheel speed sensor?
Small permanent magnet direct current generator whose voltage output is directly proportional to the rotational speed of its armature
On a British manufactured aircraft, what does ‘no lights’ visible on the UC position indicator show?
UC is locked up
Why is the hydraulic steering system supplied from the UC down line?
Ensures that the steering system is only in operation when the nose UC is down
In a steerable nosewheel, when is castoring allowed?
When the steering wheel is released
Control valve returns to neutral under the action of its centring springs
Nose wheel is free to castor
