11.13 Landing Gear Flashcards
Where is the aircrafts centre of gravity found in relation to the landing gears?
Near the main gears
Where can the support gear be located?
On the nose or tail
What are the advantages of having a nose gear over a tail gear?
- allows more forceful application of brakes
- greater decrease in lift after touch down (smaller angle of attack)
- pilot can see more during taxiing and landing
Where the weight of the aircraft is carried on the main wheel, what is this called?
Main wheel loading
What is a “bogie”?
Gears with 4 or more wheels
What is the purpose of body and centreline gears?
Help spread the loading
What are nose gears not designed to do?
Take initial landing loads, can cause damage to the aircraft if it does
Which way does the nose gear retract?
Forwards into the fuselage
If hydraulic power was lost, how can the nose gear be extended?
Air stream can push the gear into position
How can steering be controlled?
Rudder pedals or by a tiller
When using the rudder pedals to steer, what is the maximum amount of movement that can be achieved?
10 degrees
When using the tiller to steer, what is the maximum amount of movement that can be achieved?
80 degrees
On small aircraft, what type of steering do they have?
Direct nose wheel steering wheel with a direct mechanical connection between the rudder pedal and nose gear
What are the benefits of retracting gear doors? (4)
- decrease noise
- decease parasitic drag
- give higher air speeds
- lower fuel consumption
What are the different types of extension and retraction system? (3)
- landing gear control system
- landing gear and door extension and retraction system
- alternative extension system (free fall extension system)
What is the purpose of a free fall system?
Used if the normal system doesn’t operate (eg hydraulic power lost)
What are the main gear components? (7)
- shock strut
- drag brace
- side strut
- an actuator
- torsion links
- down lock mechanism
- up lock mechanism
What is the shock struts function? (2)
- Supports the aircraft on the ground
- protects the aircraft structure by absorbing shock on landing
How does the shock strut absorb shocks?
Using compressed nitrogen and hydraulic fluid
What does the compressed nitrogen and the hydraulic fluid, work like?
- Compressed nitrogen gas = spring to absorb shock
- hydraulic fluid = dampening function
What does the trunnion link do?
Transmit landing gear shock loads from the drag strut to the aircraft structure
What does the trunnion link rotate in?
Spherical bearings
What are spherical bearings designed to do?
Transmit vertical and drag loads into the structure without transferring torque to the structure and the outer cylinder
What can be found at the bottom of the inner cylinder?
Attachment lugs for the lower torsion link
What do torsion links do?
Permit the inner cylinder to move up and down in the outer cylinder but not permitting it to rotate in the outer cylinder
Where are the torsion links connected to the gear?
The inner and outer cylinders of the shock strut
What is the purpose of a torsion link damper?
Designed to counter act wheel shimmy
What is wheel shimmy?
Rapid oscillation, caused by uneven tyre pressure, uneven tyre wear or uneven runway surface
What does the drag strut do?
Stabilises the shock strut in the fore and aft direction during take off, landing and taxiing
What does the side strut do?
Provides lateral support to the shock strut
What 2 parts make up the side strut?
Upper and lower side strut
What ensure both parts of the side strut cannot fold together when the gear is extended?
Gear down lock mechanism
What can you find in most main gear actuators?
Fluid metering devices
What is the purpose of fluid metering devices?
Controls the speed at which actuator cylinder operates,
Restrict flow toward the end of retraction and extension
What is the function of the gear down lock mechanism?
Keep the gear in a down and locked position
What are the main components of the gear down lock mechanism? (3)
- lock strut
- downlock springs
- down lock actuator
What position are the lock strut kept in?
Over centred down and lock position by the down lock springs
What can be found in the lock strut to prevent inadvertent retraction?
Ground lock pins
What does the down lock actuator do?
Unlocks the lock strut against down lock spring pressure and locks the lock strut in position
What does the down lock actuator have which controls the flow during extension and retraction?
A valve housing
What is the function of a gear up lock?
To keep the gear in an up and locked position
What components can you find on an up lock mechanism? (4)
- up lock hook
- up lock roller
- up lock spring
- up lock actuator
How is the up-lock usually operated, and what could you use if that was to fail?
Hydraulics
Alternative electrical or mechanical systems
How is the hook lock kept in an over centred position?
By the up lock spring
What is the function of the up lock roller?
To push the up lock hook out of the release position into the spring engaged position
What is the function of the up lock actuator?
To open the up lock hook
What are the 3 flow control devices?
- laminar flow orifice
- tapered metering pin
- rebound valve
What does the laminator flow orifice do?
Permit laminar flow (flow with little resistance) in one direction, and causes turbulent flow in the other direction (high resistance flow), prevents bouncing after touch down
What type of flow control device is known as progressive dampening?
Tapered metering pin
What happens to the diameter of the tapered metering pin when the strut begins to collapse? What does this do?
The diameter increases
Creates a restriction of fluid flow decreasing the vertical motion of the aircraft
What is a rebound valve also known as?
Restrictor valve or recoil valve
How many bearings does the shock strut have?
2,
Upper and lower bearings
What do the 2 shock strut bearings do?
Upper = keeps inner cylinder aligned with outer cylinder Lower = keeps inner cylinder aligned inside the outer cylinder
What is the function of the wiper ring?
To keep any unwanted material out of the shock strut during compression
What does the main gland seal do?
Keeps nitrogen gas and hydraulic pressure in the shock strut
How would you service the shock struts hydraulic fluid and nitrogen gas?
Released nitrogen gas
Replenish hydraulic fluid
Recharge the gas pressure
Measure shock strut dimension (dimension X)
If there is too much gas in the shock strut, what is this known as?
Soft strut
If there is too much hydraulic fluid in the shock strut, what is this known as?
Hard strut
Why do truck beams tilt?
So that the wing gears can be retracted into the wheel well
What components can be found on the tilt mechanism? (3)
- truck positioning actuator
- bell crank
- positioning link
When the landing gear is retracted, the tilt position must be maintained even when hydraulic pressure is lost. How is this done?
By a hook which engages during gear retraction and hold truck beam tilt angle
Why is body gear steering used?
To protect damage to tyres from scrubbing of the side wall
What does a larger footprint do?
Spreads the aircraft weight over a greater area of the runway and runaway loading is decreased
What does a shortening mechanism do?
To shorten the gear during retraction
What are the main components of a shortening mechanism? (4)
- adjustable link
- bell crank
- a connecting link
- upper and lower links
How does the shortening mechanism operate?
Forces from the wing are transmitted through the adjustable link to the bell crank lever moving itself, the connecting link and turns the upper link. This breaks the over-centre lock with the lower link pulling it up.
This pulls the shock absorber into the main fitting decreasing the length
What is the function of a lock stay in the nose gear?
Gives secondary support for the down and lock position and controls alignment of the fold drag strut
What can limit the movement of the lock stay if it were to fail?
Down lock actuator
If the down lock actuator and lock stay were both to fail, what can keep the gear in a down and locked position?
2 springs
Where are the nose gear proximity switches located?
Attached to the support on the upper link
What are the components of the nose gear? (6)
- drag strut
- lock stay
- nose gear actuator
- down lock actuator
- shock strut
- shock absorber on some aircraft
What does the actuator convert to extend and retract the nose gear?
Hydraulic pressure into mechanical energy
How many types of nose gears are there?
2
In type 1 nose gear, at what angle is the centre of the strut at?
A right angle to the ground
In nose gear type 1, what is the purpose of 2 centring cams?
To ensure that the nose gear is in a straight and forward during retraction
In nose gear type 2, how is the nose gear aligned?
Shock strut inclined forward with the wheel axle on the centre line of the shock strut
In nose gear type 2, how does the steering work?
The steering actuator cylinder transmit hydraulic power through a rack and pinion mechanism
Why are inclined forward nose gears designed that way?
So that the wheels return freely to centre position
What is designed to shear if the towing load is above the limit, to prevent damage to the sliding tube in the nose gear?
Towing lugs
What two methods of steering is there?
- rack and pinion
- 2 jack method
What are the main components of a mechanical nose wheel steering system? (9)
-Steering tillers
-rudder pedals
-rudder pedal steering interconnect mechanism
-the electrical rudder pedal and interconnect actuator
-steering control valve
-steering collar
-steering cables
Steering actuating cylinder
-steering cooler cable compensator
When will the rudder pedal steering interconnect mechanism connect the rudder pedal to the nose steering wheel system?
When the aircraft is on the ground and nose gear shock strut compressed
What is used to transmit a steering input from the rudder pedals to the nose wheel steering system?
Via steering crank, the rudder pedal steering quadrant and cables
What is the function of a cable compensator?
To prevent false inputs from the steering control valves
What do the pivot links on the nose gear allow?
Allows the nose gear to extend and retract without interference to the cable system
When the desired amount of turn is reached, what happens to the tension in the cable and the pistons of the steering control valve?
Tension reduced,
Pistons return to neutral
When using the tiller to steer, what must be held throughout to ensure a turn occurs?
The tiller needs to be held throughout the turn, as centring spring returns system to centre
What is the point called where the pulling actuator of the steering control valve reaches the end of its stoke?
Null point
What is the purpose of the by pass valve in the nose steering wheel system? And when may it operate?
Generally during towing.
The actuator pistons act like pumps during towing.
The piston force fluid out of the steering cylinder, and creates a differential pressure across the by pass valve. This opens to allow flow of hydraulic fluid from one side of the steering actuator pistons, to the other
What prevent movement of the summing lever during towing?
Towing safety pins
What components can you find on an electrical nose steering wheel system? (6)
- hand wheels
- rudder pedal
- brake and steering control unit
- hydraulic block
- position pistons
- steering actuating cylinders
How are steering orders from the hand wheels transmitted to the brake and steering control unit?
Via potentiometers
What are the two main valves on the hydraulic block called?
- Electrically operated selector valve
- servo valve
How is the selector valve operated?
Solenoid operated which receives signal from the brake and steering control unit
What 3 methods are used to overcome damage to tyre due to scrubbing?
- main gear steering using a castored truck beam
- main gear steering using a rotating beam
- single axle for the main gear
What do gear doors include? (3)
- forward doors
- aft doors
- leg doors
How are forward gear Doors aligned?
Symmetrical about the centre line of the aircraft
How are forward gear doors operated?
Hinged to the aircraft and operated hydraulically
What happens to the forward door when it is in the closed position?
Mechanically locked by a door up lock
What is an alternative way to open the forward gear doors when on the ground?
By operating the ground control handle which connects to the door up-lock and door bypass valve via rods
What must be done before the ground control handle can be moved to the open position?
Quick release pin removed and the pusher pressed in
What are the signals of a proximity sensor used for?
Used for position indication and the sequencing of the gear extension and retraction
How are aft nose gear doors aligned in relation to the aircraft?
Symmetrical about the centre line of the aircraft
what position are the aft nose gear doors in when gear extension is complete?
Left open
What position are the main gear doors in when gear extension is complete?
Closed
Other than the main door, what other components can be seen when the gear is extended connected to the shock strut? (2)
- Hinged fairing
- fixed fairing
How are the main doors locked in the closed position?
A hook on the door up lock engages with an up lock roller on the door
What 2 methods of main door up locking is there?
- up lock roller and hook
- mechanical lock within hydraulic actuator
What is the gear lever shaped like?
Gear wheel
What do the indications on the flight deck show?
Show that the gear is safely locked in the up or Down position
In a mechanical and electric lever systems, what are the position of the lever?
Mechanical = 3 positions. Up, down and off Electrical = 2 position. Up and down
What needs to done to move the control level to a new position?
Pull the spring loaded handle
When the lever is moved to a new position, what is the command transmitted to?
Selector valves which direct hydraulic pressure to the landing gear and door actuators
Where does the electrical landing gear lever send the command signals to?
Landing gear control interface unit (LGCIU)
How many LGCIU’s are there installed on the aircraft?
2.
1 is active, 1 is standby
What happens to the LGCIU after each landing gear cycle? Why does it happen?
The back up LGCIU, becomes the active LGCIU
If the new active LGCIU were to fail, the other LGCIU is proven to be functional
What type of valve is the selector valve?
4/3 way valve
What is the difference between a mechanical selector valve and a electrical selector valve?
The electrical selector valve is moved by hydraulic pressure which is controlled by a solenoid
When can’t the gear lever be moved?
When the aircraft is on the ground
What is the gear lever locked by when it’s on the ground, to prevent the lever from moving?
By a de-energised solenoid, which is released only during flight
On a mechanical control system, what can be pressed to override the lock on the gear lever, when on the ground?
Override push button next to the lever
At what airspeed does the electrical system automatically lock the lever position?
Above 250 knots
What light indication is shown on the flight deck for:
A) gear down
B) gear up
C) gear no locked
A) green
B) no light
C) red
On the ECAM, how is the gear indicated for:
A) gear down
B) gear up
C) gear not locked
A) green triangle
B) no triangle visible
C) red triangle
On EICAS, how are the gear doors indicated?
The word “closed” appears for gear door closed
Box shaded when the doors are not closed
What is the purpose of the three sequence valves?
To ensure the doors and gears extend and retract in the correct sequence
What are the main components of wing gear and door extension and retraction system? (5)
- three sequence valves
- an uplock mechanism with an actuator and hook
- down lock actuator
- door actuator
- door safety valve
What does the door safety valve do?
Blocks pressure to the close port of the door actuator and connects the close port with the return line
What controls the sequencing of valves in an electrical extension and retraction system?
LGCIU
What prevents hydraulic supply being sent to the landing gear during high speeds in an electrical extension and retraction system?
A solenoid controller safety valve, automatically closes when a certain airspeed is reached
What are the main components of an electrically operated alternative extension system? (5)
- gear up lock mechanism
- a differential
- door safety valve
- door actuator
- rods and cables
During alternative extension, what happens to the doors?
They stay open
What is the main components of a mechanically operated alternative system? (5)
- gear up locks
- gear door up locks
- free fall cut off valve
- vent valve
- cables and rods
When the alternative handle is turned, what happens to the free fall cut out valve?
It’s repositioned so that it closes normal hydraulic supply line and connects the entire landing gear to return line
How is the lifespan of a tyre calculated?
On the number of take off and landings
How much load can each tyre carry?
Approx 25,000kg
How are the 2 wheel halves held together?
Tie bolts
If a tie bolt was to fail, what happens to the stress distribution?
Changes around the wheel rim, and remaining bolts may become overloaded
What is installed between both halve to prevent pressure leaks when the tyre is inflated?
Packing
How are the rotating brake disks driven?
By steel drive keys
What are installed in the tyre to automatically release tyre press when a certain temperature is reached?
Fusible plugs (thermal relief plugs)
What are the melting points for the fusible plugs for:
A)inboard half
B)outboard half
A) 300 degrees C
B) 180 degrees C
How long does it take for a tyre to deflate via a fusible plug?
Approx 10 minutes, enough time to reach the stand
How are tyres classified? (5)
- speed rating
- load rating
- ply rating
- type
- size
What types of tyres are there? And where are they used? (3)
- type 3 = low pressure, propeller powered with speeds less than 160mph
- type 7 = high pressure, carry large loads, standard tyres for jet aircraft speeds up to 225mph
- type 8 = extra high pressure, speeds of 280mph, military aircraft
What are the 3 main measures of tyre sizes? (3)
- section width
- rim diameter
- outside diameter
What does the letter “H” signify on tyre size designations?
Tyres have a special design in the bead area and will only fit certain wheels
What are aircraft tyres be marked with? (11)
- manufacturers name
- design type
- country of manufacture
- aerial number
- part number
- tyre size
- load rating
- tubeless applicable specification
- ply rating
- skid depth
- speed in mph
What are the main construction of a tyre? (4)
- beads
- carcass
- tread
- side wall
What is the carcass made of?
Layered rubber coated nylon cord fabric
What are beads made of?
High tensile strength steel wires embedded in rubber
Where does the load and braking force terminate on the tyre?
The beads
Where does the side wall extend from?
From the tread to the bead area
What colour dot is the balance mark on the tyre? Where is it usually located?
Red
Lightest part of the tyre
What is also positioned at the red dot of the tyre?
Inflation valve
What are the positives and negatives does a radial tyre constructed tyre have?
Positive
- lighter
- better performance
- more landings
- less rolling resistance
Negative
- expensive
- cannot be retreaded as much
What is hydroplaning?
When the tyres generate water pressure which can partially lift the tyre off the runway
What are the 3 types of hydroplaning?
- viscous hydroplaning
- dynamic hydroplaning
- reverted rubber hydroplaning
What are the definitions for the 3 types of hydroplaning?
Viscous hydroplaning = normal slippery action of a thin film of water
Dynamic hydroplaning = when the tyre lifts off the runway and rides on a wedge of water
Reverted rubber hydroplaning = occurs when the tyres are locked and skid over an icy or very wet runway, generating enough heat from the friction, creating steam pressure lifting the tyre off the runway
What is the deflection percentage of a properly inflated tyre?
32 degrees
What are tyres inflated with?
Nitrogen
What must be done to the tyre before it is removed?
Reduce the pressure
When a tyre with a centre tread, when must it be replaced?
When no middle groove is visible over a length of 10cm or more
With tyre with no centre tread, when must a tyre be replaced?
When one of the 2 treads closest to the centre line is no longer visible over a length of 10cm
What are small chevron shaped cuts in the tread area caused by?
Landing on a grooved runway
What do bulges indicate on a tyre?
Ply or tread separation
If an object was found in a tyre, what mustn’t you do?
Remove the object
When would an abnormal pressure be detected? (2)
- when a tyre falls below or increases above a specific limit
- when the difference of tyre pressure between two tyres on the same axle exceeds a specific limit
What components may be found on the tyre pressure indicating system?(3)
- Pressure transducer (on each wheel)
- transmission unit (on each wheel)
- central computer
How does a tyre pressure indicating system work?
- A transducer inside the wheel transfer the tyre pressure into proportional electrical voltage which is sent to the transmission unit.
- the computer analyses the signal from the tyre and compares it with allowed limits and gives alerts if outside of the limits
What 3 types of brakes are used on aircraft? (3)
- single disc
- dual disk
- multiple disk
What are the 3 main components of a multiple disc brake system?
- piston housing
- torque tubes
- heat pack
In regards to brakes, what must aircraft have for safety reasons?
Normal brake and alternate brake systems
What are the 3 functions that the brake piston fulfils?
- apply brake force
- compensate for wear on the brake discs
- Automatically adjust the brake clearance
What is connected to the pressure plate to compensate for brake wear and responsible for brake clearance?
An adjuster
What does the heat pack consist of?
Alternate rotors and stator made out of carbon or steel
What happens to the stators and rotors during braking?
Squeezed together between the pressure plate and pressure ring
What are the advantages of carbon heat packs over steel heat packs? (3)
- weigh less
- less wear
- withstand high temperatures
- can be recycled (2 discs to make 1 new disc)
What provides a visual indication of wear of the heat pack?
A wear indicator pin
How can you tell when a heat pack is worn by looking at the wear indicator pin?
When the wear pin aligns with the bracket
What do inflight brakes do?
Stops rotation of the wheels during gear retraction
What is know as differential braking?
Independently operating the left and right hand brakes, assist with steering and used if steering faulty
What does a power brake with a mechanical cable transmission operate?
A brake metering valve
What does the brake metering valve do?
Amplifies a mechanical input into a proportional hydraulic output
What are the main components of the electrically controlled brakes? (5)
- Brake Pedals ( with transmitter unit)
- Servo valve
- Brake piston
- Hydraulic supply valve
- Automatic selector
The servo valve receives max voltage in brake release position. What is the reason for this?
If a wire was broken in the electrical transmission you’d still get maximum braking.
What side would the servo valve force into when the brake is in release position?
Into the right side by a spring
What does the auto selector do to the alternate supply pressure and normal supply pressure?
Closes alternate and opens normal supply.
Where does the right and left orifices connect to?
Left jet and right jet respectively.
What is the the left jet in the left orifice in the brake system closed by and what does it assist with?
It is closed by the electrically operated flapper and assists the spring to move the piston to the right side.
What functions does the Brake and Steering Control Unit (B.S.C.U) perform? (5)
- Normal braking
- Auto braking
- in flight braking
- Nose wheel steering
- Temp. Monitoring
What does the brake pressure transducer in a BSCU system limit in proportion to brake pedal travel?
Brake pressure
What would in flight braking be used for?
Stop rotation of wheels during gear retraction. High gyro forces can cause damage to wheel well.
When is the Alt. Brake system used?
When normal brake system fails or has no hydraulic pressure available
The anti skid function is available when using the alternate brakes. Which system would not be available in this situation?
Auto Brake
What are the main components in the Alt. Braking system and what kind of pressure does it operate at? (5)
-Low pressure
- Pedals
- Brake metering valve
- Reservoir
- Two master cylinders (left and right)
- Brake metering valve
When should the parking brake system be applied for parking?
When the system is pressurised
If the brake systems are depressurised what can be used to hold the parking brakes?
Brake accumulator
What is used to pressurise the electrically operated park brake system?
Yellow high pressure manifold
What two functions does the electrical control valve serve in a park brake system?
- Sends pilot pressure to to parking brake operated valve
- Sends reduced parking pressure to brakes via shuttle valve
In a mechanical parking brake, what move towards the lock pin to lock the brakes in position?
A pawl
In a mechanical parking brake system, what does the micro switch control?
A parking brake valve closes all the anti-skid return lines to maintain brake pressure
What does the anti-skid system do?
Designed to give optimum braking effectiveness whilst preventing skidding
What and where does the anti-skid control circuit receive?
- wheel speed signal
- from the wheel speed transducer
What drives the wheel speed transducers?
The wheel hub cap
What do aircraft have to release the brakes during approach and touchdown?
Touchdown protection
How does touchdown protection work?
Negative signal in regulator, the anti skid system interprets as a skid, releasing brakes
At what speed does the anti skid system automatically switch off? What does this avoid?
- 10 knots
- allows the pilot to apply full brake pressure to stop the aircraft at the gate
How does the anti skid system regulate brake pressure?
Via an anti skid module, each containing a valve for each brake on the axle
How quickly does the auto brake system react after touchdown?
Less than 1 second
What automatically extend when the aircraft touches down?
Speed brakes
How is auto brake divides into 4 stages?
- brake application
- on ramp
- auto braking
- off ramp
What does “on ramp” and “off ramp” mean?
On ramp = smooth build up of brake pressure until the preselected deceleration rate is reached
Off ramp = smooth decrease in brake pressure
If reverse thrust is selected during auto braking, what happens to the brake pressure?
Brake pressure decreases so that the total deceleration rate (generated by the brakes and reverse thrust) is the same as the preselected rate
On a electrical auto brake system, what 3 selections can be made in regards to delectation rate?
- low
- medium
- maximum
What does a high and low temperature indication of the brakes tell you?
High = dragging brakes Low = brake not working
What type of sensor are the temperature sensor for brakes?
Thermocouple type generating a DC voltage proportional to the temperature
What is not permitted if the aircraft has hot brakes from a previous landing?
Take off
What can be used to cool the brakes!
Electrically driven cooling fans
What does protects the brake cooling fan from damage?
Debris guard
What can be used to indicate the pilot that the aircrafts weight is transferred or removed from the landing gear?
Weight on wheels sensors
What can be used in front of a proximity sensor to place the aircraft into “air mode”?
Dummy target
What are used instead of micro switches where there is high risk of contact corrosion due to dirt and moisture?
Proximity sensors
Where may proximity sensors be used? (3)
- flight controls
- landing gear
- doors
