Hydraulics & landing gear

021 Aircraft General KnowledgeChapter 2

Hydraulic fundamentals

Formula — Pascal's law

Pressure applied to enclosed fluid acts equally in all directions
Area(A) × distance(A) = Area(B) × distance(B)
Hydraulic power = system pressure × volume flow

Key fact

Most commercial systems operate at 3000 psi. Common fluid: phosphate ester (Skydrol) — purple. Synthetic oil has highest resistance to cavitation. Fluid types must not be mixed (seal damage, internal leaks).

Monitored parameters: pressure, fluid temperature, quantity. Flight deck pressure indication via transducer to indirect gauge.

Fluid type	Colour	Typical use
Mineral based	Red	Military / turboprops
Phosphate ester (Skydrol)	Purple/green/amber	Commercial jets
Synthetic	—	Highest cavitation resistance

System components

Key fact

Variable displacement pump: Adjusts output to demand — fluid moved only when needed; no separate pressure regulator required. Constant delivery pump: Needs ACOV (automatic cut-off valve) or separate regulator.

Accumulator: Stores fluid under pressure; dampens fluctuations; emergency limited supply; thermal expansion; caters for small leaks. Pre-charged ~⅓ system pressure with nitrogen. Fluid in gas side = internal leak.

Key fact

Reservoir: Pressurised (bleed air or piston) to prevent pump inlet cavitation and foaming. Level decreases slightly on power-up; fluctuates with jack displacement and accumulator pressure. Fluid temperature measured at reservoir.

Quill drive: Shears on pump failure to protect gearbox. RAT: Emergency hydraulic power for flight controls.

Valve	Function
Shuttle	Selects higher-pressure source to actuator
Check / NRV	One-direction flow only (like diode)
Selector	Directs pressure to either side of piston
Relief	Limits maximum system pressure
Hydraulic fuse	Prevents total fluid loss downstream of leak
Fire shut-off	Blocks pump output on fire handle

Exam trap

Internal leaks cause fluid temperature rise (pistons/accumulators work harder). External leaks cause reservoir level fall. Hydraulic lock = actuator stationary with no fluid flow — piston cannot move.

Twin-engine jet hydraulic layout

Key fact

Typical three systems (e.g. Green, Blue, Yellow). Separate reservoirs — no fluid transfer between systems. PTU: Transfers power between systems without exchanging fluid. Pump preference: EDP → PTU → electric pump → RAT.

Tandem double-acting actuator: Fed by two systems for critical surfaces (e.g. flight controls).

Landing gear

Key fact

Oleo strut: Nitrogen (spring) + hydraulic oil (damping). Torque links/scissors: Prevent piston rod rotation relative to strut. Over-centre lock: Down-lock uses over-centre geometry.

Ground retraction prevention: Weight-on-wheels switch, anti-retract latch, lever latch, locking pins with flags.

Indication	Meaning
Green	Gear locked down
Amber / red	At least one wheel travelling/unlocked
None (lights out)	Up and locked

Key fact

VLO: Max speed for gear operation. VLE: Max speed with gear extended. Emergency extension: Gravity/free-fall, compressed nitrogen bottle, or auxiliary hydraulics.

Shimmy: Nose wheel oscillation on ground — overcome by accumulator on steering cylinder.

Brakes and anti-skid

Formula — Aquaplaning speed

Speed (kt) ≈ 9 × √(tyre pressure in psi)
Speed (kt) ≈ 34 × √(tyre pressure in bar)
Hydroplaning when friction coefficient = 0

Key fact

Multi-disc brakes: Transport category standard. Carbon brakes: Lighter, absorb more heat; improve with temperature; susceptible to wear when cold or snubbed during taxi.

Anti-skid: Speed sensor, control unit, control valve. Activates when main wheel spin-up detected; decreases brake pressure on slower-spinning wheels. Available with autobrakes. RTO mode: Max braking + anti-skid above set speed when take-off rejected and throttles idle.

Exam trap

Parking brake: Own accumulator and components. Autobrake disconnects when pedals pressed or pilot action after landing. Under-inflated tyres: More wear at shoulders. Tubeless tyres: Lower burst risk, no tube friction, requires branched wheels.

Thermal fuse (hollow bolt in wheel rim): Melts at set temperature to deflate tyre and prevent explosion from prolonged braking.

Key fact

Steering: Mechanical rods to rudder pedals (plus tiller on large aircraft). Auto-brake: Works during thrust reversal above pre-set minimum speed; low setting works without delay; disconnects on pedal input.