Landing Gear And Brake Systems Flashcards
Requirements of an undercarriage/landing system
- Uphold the aircraft on the ground at max all up weight (MAUW)
- Absorbs loads produced by landing and provides shock absorption when taxiing across ground undulations
- Withstand side loading due to crosswind take-offs and landings
- Create minimum friction on takeoff roll
- Produce minimum drag when airborne
- Provide maximum traction on the landing roll for efficient braking
- If retractable, provide against accidentally retraction on the ground
- Provide maximum strength for minimal weight
Tricycle
- Two main wheels and a third wheel under the nose
Tailwheel
Two main wheels, the third is underneath the tail
Advantages of a tricycle undercarriage
- Better forward vis on the ground
- Better and safer braking ability
- Better stability on the ground for takeoff, landing and crosswinds
Features of typical undercarriage construction
- Elastic bungee cord type
- Flat leaf spring strut type
- Oleo-pneumatic shock strut type
Elastic bungee cord type
Uses elastic bungee cords to absorb the shock of landing and provides suspension. When the plane lands, the cord stretch, softening the impact.
- After landing they bounce back to their original shape.
- Mainly used in small GA aircraft due to its simplicity and low cost.
Flat leaf spring strut type
- Shape of spring provides natural shock absorption.
- When aircraft lands, the spring flexes and absorbs the landing forces.
- Requires less maintenance compared to more complex landing gear systems
Oleo-pneumatic shock strut type
- Absorbs load by using compressed air and hydraulic oil and is used on both light GA aircraft and larger commercial aircraft.
- Strut has two cylinders that slide together.
- Outer splinter is filled with compressed air air.
- Inner cylinder hold hydraulic oil
- Cylinders are connected by a torque link that let them slide but stops them from spinning.
Shock absorption in an oleo-pneumatic strut is provided by compressing and expanding the air not by the hydraulic oil.
Functions of the hydraulic in an oleo-pneumatic strut.
- Damping
- Sealing
- Lubrication
- Heat absorption for compressed air
Preflight checks
- Nose oleo strut - 7cm
- If oleo are fitted to the main undercarriage -10cm
- Check for fluid, cracks and leaks
- Check the wheel is correctly aligned
Aircraft wheel construction
Light aircraft - one wheel per undercarriage leg
Large aircraft - two or multiple wheels per undercarriage leg
Aircraft type construction
- Inner and outer tube or a tubeless construction.
- Tyres inflated by a tube can blow out it hey creep too much and the inflation valve rubs the rim.
- Tread - usually made from durable rubber compounds
Preflight checks for tyres and operating precautions
- Checking the tyre is properly inflated and no damage/cuts are visible
- Checking there is no bold (or flat) spots on the tyre
- Checking there is sufficient tread
Avoid excessive braking and turning too sharply when on the ground.
Retractable landing gear
- Reduces drag
- Increases fuel efficiency
- Operated hydraulically - but can be pneumatically or electrically operated.
Essential features of a retractable undercarriage.
- Mechanical uplocks/latches - holds the gear in the retracted position and is released when lowering the gear. Hydraulically operated.
- Overcentring cams/struts - ensures gear is locked in the down position and prevents the hear from retracting on the ground.
- Safety lock on the cockpit undercarriage lever - prevents the undercarriage retracting when the UP position is selected while the aircraft is on the ground.
- A indication of gear position in the cockpit - electrically operated and can be in forms on a light or a flag-type indicator
- Cockpit warning system - light buzzer or horn warning you that the gear is not down
- Emergency lowering system - it is used when the lowering system has failed.
Speed limitation of a retractable undercarriage
- VLE - Maximum landing gear extended speed - highest speed at which you can safely fly with the landing gear extended
- VLO - maximum landing gear operating speed. The max speed at which you can safely extend or retract the landing gear. Different for extending and retracting
Emergency gear operation systems
- Hand pump system - provides hydraulic pressure to operate when hydraulic pressure is lost to operate the undercarriage.
- Gravity extension system - landing gear can be released to drop down under gravity if the normal extension system fails
- Manual crank system - pilot can use to lower the landing gear if the primary system fails
- Backup hydraulic system - used to extend the landing gear if the primary hydraulic system fails
- Pneumatic System - compressed air is used to force the landing gear down if the hydraulic system fails
- Electrical motor backup - a secondary electric motor can be used to extend the gear in case of primary system failure.
Gear indication system
- Indicates each landing gear position including the nose and each side of the main landing gear of the aircraft.
- Electrically operated and commonly displayed as light (or flags)
Three indications of gear indication system:
- Locked up - the undercarriage is retracted/up - no lights displayed in the cockpit
- In transit/unlocked - undercarriage is moving from from up to down or vice versa. Can also happen when the gear is up or down but not locked - red lights
- Locked down - the undercarriage is down/extended and locked - green lights
Gear warning system
- Flashing red lights, buzzer, horn, or audible warning
Ground retraction protection system
- Prevents the gear from retracting on the ground by using a ground retraction protection system.
- One method uses overcentrering cams
- Another method uses squat switches or weight-on-wheels (WOW) switches.
These switches sense when the aircraft’s weight is on the landing gear and avoid retraction by opening the switch in the electrical circuit to the motor that powers retraction.
What do wheel brakes generally operate by means of?
Hydraulics
Drum brakes
- Use a steel drum on the rotating part and is older than disc brakes.
Two types of non-rotating assembly - located inside of the steel drum:
- Expander tube type
- Brake shoe type
Expander tube type
- When hydraulic fluid inflates the rubber expander tube, pushing the brake pad against the drum to slow the wheel.
Brake shoe type
- Hydraulic pressure rotates the cams forcing brake shoes to contact the drum, slowing it down.
Upon releasing the brake, springs pull the shoes back, releasing fluid to the cylinder
Disc brakes
- Operated hydraulically
- Pushes brake pads against the rotating discs to slow down
- Non-moving part, the calliper, uses two brake pads to clamp the rotating discs when brakes are applied.
- Lighter GA aircraft will have a single-disc brakes unit, whereas larger commercial aircraft will require multi-disc system.
Brake operating systems
- Independent brake system
- Boosted brakes
- Power brakes
Independent brake system
- Self-contained hydraulic system
- Pressing the top of the rudder pedals activates a piston in a master cylinder, providing hydraulic pressure.
- Brake pressure is proportional to the pedal pressure applied.
Boosted brakes
- Needed by larger aircraft
- Pilot applied brake pressure, the main system’s hydraulic pressure is also introduced into the back of the master cylinder piston via a valve
- Helps achieve greater brake pressure.
Power brakes
- Use main hydraulics or pneumatic system to operate brakes.
- Brake pedals, connected to the brake control valve, regulate pressure to the wheel brakes proportional to the pilot’s force on the brake pedals.
- If the hydraulic system fails, a check valve and accumulator maintain brake system pressure.
Operation of anti-skid brake system
- Best braking occur just prior to the wheels “locking up” and skidding.
- On larger aircraft - “anti-skid braking system” AKA ‘ABS’ is used to prevent the wheels from locking up and reducing brake performance.
- ABS senses the deceleration of each wheel and consists of three parts:
- Wheel sensor
- Control unit
- Control valve
Transducer either AC or DC converts rotation into an electrical signal and the output is proportional to wheel speed. Then sent to the control unit which controls the hydraulic valve for each wheel.
