Aircraft systems: hydraulics, fuel and landing gear

In short

Aircraft hydraulic, fuel and landing gear systems are the "muscle, blood and legs" of an aeroplane: hydraulics transmit power to move flight controls, gear, brakes and flaps; the fuel system stores and delivers fuel to the engines at the right pressure and in an order that keeps the centre of gravity (CG) in limits; the landing gear supports the aircraft on ground and absorbs landing energy. These three are mechanical-system subjects, so they are examined through fluid power, thermodynamics of fuel, energy absorption and structural load paths, and they dominate aircraft-systems papers in B.E./B.Tech aerospace and aeronautical programmes as well as AME (DGCA) Paper on Aircraft Systems. In interviews at HAL, Boeing India, Airbus India, Collins Aerospace, Safran, Liebherr Aerospace, Tata Advanced Systems, Air India Engineering Services, AIESL/IndiGo/Air India maintenance and Indian Air Force technical branch, candidates are asked why hydraulics instead of electrics, what an oleo strut does, and how fuel is sequenced. Design, airworthiness certification (DGCA CAR Series/CAR-21, CAR-145, and FAR/CS-25 as adopted) and line maintenance roles all use this knowledge daily.

Explained

Hydraulic system fundamentals

• Hydraulic power = transmission of force through an almost incompressible liquid. Based on Pascal's law: pressure applied to a confined fluid acts equally in all directions, so a small force on a small piston produces a large force on a large piston.
• Why hydraulics on aircraft: very high force-to-weight ratio, stiff (near-incompressible so no spongy control), self-lubricating, infinitely variable speed, and easy to route as tubing through thin wings. Limitations: leaks, fire risk with some fluids, weight of fluid and lines, contamination sensitivity, temperature-dependent viscosity.
• Typical system pressures: 3,000 psi (≈207 bar) on most transport aircraft; 5,000 psi (≈345 bar) on A380, B787 and many modern military types to reduce actuator and pipe size; small aircraft brake-only systems may run a few hundred psi.
• Components, in flow order: reservoir (pressurised by bleed air or by a piston to prevent pump cavitation at altitude) → pump → filter → pressure-relief/priority valves → accumulator → selector/control valves → actuators → return line → cooler → filter → reservoir.
• Pumps: engine-driven pump (EDP, usually variable-displacement axial piston, pressure-compensated so delivery falls to near zero at set pressure), electric motor-driven pump (EMDP/ACMP) for ground use and backup, air-driven pump (ADP) using bleed air, hand pump for ground servicing, and the power transfer unit (PTU) which uses pressure from one system to drive a pump in another without transferring fluid (so a leak in one system cannot drain the other).
• Accumulator: gas-charged (nitrogen) vessel — piston, diaphragm or bladder type. Functions: stores energy, damps pressure pulsations and surges, supplies peak demand (e.g. gear retraction), and provides emergency brake pressure after pump loss. Pre-charge is typically about one-third to one-half of system pressure; it must be charged with nitrogen, never oxygen or air (fire/explosion risk with oil).
• Actuators: linear (single-acting, double-acting balanced/unbalanced) and rotary (hydraulic motor, rotary vane); servo actuators with electrohydraulic servo valves (EHSV) are the output stage of fly-by-wire. EHA (electro-hydrostatic actuator) and EBHA carry their own local pump and are used as dissimilar backup on A380/A350.
• Control and protection devices: selector valve (directs flow), check valve, sequence valve (orders gear-door-then-gear motion), restrictor/orifice check valve (slows one direction), pressure relief valve, thermal relief valve, priority valve (starves low-priority users first), shuttle valve (switches to emergency source), fuses and hydraulic circuit breakers (isolate a burst line), flow equaliser/synchroniser.
• Hydraulic fluids: mineral based MIL-H-5606 (red, flammable, old light aircraft), synthetic hydrocarbon MIL-PRF-83282/87257 (red, fire-resistant), and phosphate-ester (Skydrol/Hyjet, BMS 3-11) — purple/green, fire-resistant, used on almost all transport aircraft; it attacks paint and skin, so gloves and goggles are mandatory. Fluids must never be mixed; seals differ (nitrile/Buna-N for mineral, butyl/EPM for phosphate ester).
• Redundancy architecture: transport aircraft carry three independent systems (B737: A, B, standby; B747/777: 1,2,3,4 or L/C/R; A320: Green, Blue, Yellow). Independence of power source, routing and fluid is required so no single failure or rotor burst disables all. Final fallback: RAT (ram air turbine) driving a pump/generator, and mechanical/manual reversion or free-fall gear extension.

Aircraft fuel systems

• Functions: store fuel, deliver it to engines and APU at required pressure and flow in all attitudes, allow refuel/defuel and transfer, measure quantity, vent the tanks, keep CG inside limits, protect against fire, ice and water, and often use fuel as a heat sink for oil and hydraulics.
• Tank types: (1) integral tanks — the wing box itself sealed with fuel-tank sealant; lightest, used on virtually all transport aircraft ("wet wing"); (2) rigid removable tanks — metal tanks in a bay, small aircraft; (3) bladder/flexible cells — rubberised fabric, crash-resistant, used in helicopters and fighters; (4) auxiliary/centre/trim tanks and external drop tanks.
• Typical layout (transport): left and right wing main tanks feeding their own engine, a centre tank used first, surge tanks at the wingtips, vent system to outboard NACA vent scoop, and sometimes a horizontal stabiliser trim tank (A340/A380) to shift CG aft and cut trim drag.
• Fuel feed: gravity feed in high-wing light aircraft; pressure feed using booster (centrifugal) pumps in airliners to prevent vapour lock and cavitation at altitude, with engine-driven HP pump (gear or piston type) raising pressure to the fuel control unit and nozzles. Jet pumps/ejectors use HP fuel motive flow to scavenge tank sumps with no moving parts.
• Crossfeed and transfer: crossfeed valve lets either engine draw from either side (asymmetric fuel burn, engine-out); transfer pumps move fuel inboard-outboard for wing bending relief (outboard fuel reduces wing root bending moment, so it is burnt last) and for CG control.
• Fuel used in India: Jet A-1 (kerosene, flash point ≥38 °C, freezing point ≤ −47 °C, density about 0.775–0.84 kg/L, taken as 0.80 kg/L nominal; net calorific value about 43.0–43.5 MJ/kg), supplied by IOCL/BPCL/HPCL as ATF; AVGAS 100LL for piston aircraft. Density must be entered or sensed because tanks measure volume while performance needs mass.
• Quantity measurement: capacitance type probes (fuel and air have different dielectric constants; the probe is a coaxial capacitor whose capacitance varies with fuel level), plus densitometers/compensators and a fuel quantity indicating system (FQIS) computer; dripsticks/magnetic level indicators for manual check; float-type and electrical resistance gauges on small aircraft. Flow is measured by vane or mass flowmeter in kg/h.
• Contamination and ice: water (condensation and dissolved water) settles at sump drains and freezes into ice crystals that block filters; controlled by sump draining at every pre-flight, FSII (fuel system icing inhibitor, DiEGME) in some fuels, fuel heaters/fuel-oil heat exchangers, and biocide against microbial growth. Filter bypass is indicated by a differential pressure warning.
• Safety and venting: vent system keeps ullage near ambient so tank skins are not over/under-pressured during climb and descent; inerting (NGS/OBIGGS) floods ullage with nitrogen-enriched air to keep oxygen below about 12%, mandated for transport-category centre tanks after the TWA 800 accident; bonding and static grounding during refuelling; lightning protection of tank structure and fasteners.
• Refuelling methods: over-wing gravity (light aircraft, some turboprops) and under-wing single-point pressure refuelling at about 50 psi (3.4 bar) with automatic level control; defuelling and in-flight refuelling (probe-and-drogue on Su-30MKI/Mirage with IAF IL-78 tankers; boom type on some types).
• Fuel jettison (dump): fitted where maximum take-off weight greatly exceeds maximum landing weight, to reach landing weight quickly; dumped above a minimum altitude.

Landing gear: types and construction

• Functions: support the aircraft on ground, absorb and dissipate landing impact energy, allow taxi/steer/brake, and transmit loads into the structure; and contribute minimum drag in flight (hence retraction).
• Arrangements: tricycle (nose wheel) — the standard; stable, good ground visibility, prevents ground looping, CG ahead of main gear. Tailwheel (conventional) — lighter, better for rough fields, but unstable in a swerve (ground loop) and poor forward view. Tandem/bicycle — main wheels in line with outrigger wheels (Harrier, U-2); the B-52's four main units are the related quadricycle arrangement. Multi-bogie — four- or six-wheel bogies to spread load (B747, A380, An-124) keeping ACN/PCN pavement loading acceptable. Skids (helicopters), floats, skis.
• Main structural types: telescopic (cantilever) oleo strut — most common; articulated/trailing link (good for rough fields and carriers); levered suspension; semi-articulated. Supporting members: drag strut, side strut/side stay, torque links (scissor links) to stop the piston rotating, shimmy damper on nose gear, down-lock and up-lock mechanisms, over-centre linkage and lock stays.
• Shock absorber — oleo-pneumatic strut: a cylinder containing oil and nitrogen. On impact the piston moves up, oil is forced through a metering orifice and metering pin — this damps (converts energy to heat, which is dissipated); the gas is compressed — this stores energy as a spring and returns it slowly through a recoil/snubber orifice. Typical efficiency about 75–90%, far better than steel coil (≈50%) or rubber/bungee (≈60%) springs. Correct strut extension depends on both gas pressure and oil level, so servicing requires both.
• Retraction: normally hydraulic (actuators plus sequence valves, doors, up-locks and down-locks); electric screwjacks on some small aircraft; pneumatic on a few. Emergency/alternate extension is by free fall with gravity plus a release of up-locks, a nitrogen blow-down bottle, or a hand crank; a mechanical down-lock plus a "three greens" indication confirms locked-down. Ground safety: ground lock pins/down-lock pins fitted on ground, and a landing gear squat/air-ground (proximity) switch that prevents retraction on ground and inhibits/enables other systems.
• Wheels, tyres and brakes: split (two-piece) forged aluminium wheels with fusible (thermal) plugs that release tyre pressure if brakes overheat instead of letting the tyre burst; tubeless radial or bias tyres inflated with nitrogen to high pressure (about 200 psi on narrowbody mains, higher on widebodies — check the type's maintenance manual, as values are type-specific); multi-disc carbon or steel brakes actuated hydraulically, with brake accumulator and parking brake valve.
• Anti-skid: wheel-speed sensors detect incipient skid (wheel deceleration/slip ratio above set value) and modulate brake pressure to hold the tyre near peak braking friction (slip ratio ≈ 10–15%), giving shorter stopping distance, protection against tyre burst, locked-wheel and touchdown protection; autobrake selects a deceleration rate.
• Steering: nose wheel steering by hydraulic steering actuators/collar controlled by tiller and rudder pedals, with a centring cam and a by-pass/towing mode; differential braking on small aircraft; body gear steering on A380/B777.
• Loads: design cases include landing impact (limit load factor on gear typically 2–3, with ultimate = 1.5 × limit), spin-up and spring-back, braked roll, turning, towing and jacking cases, plus the sink rate requirement — CS/FAR 25.473 uses a descent velocity of 10 ft/s (3.05 m/s) at design landing weight (6 ft/s at maximum take-off weight).

Integration, failures and maintenance practice

• Interfaces: hydraulic pressure extends the gear and powers brakes and steering; fuel tanks sit in the wing box that also carries gear attachment and hydraulic lines; fuel acts as coolant for integrated drive generator oil and hydraulic fluid; electrical system powers pumps, FQIS and anti-skid.
• Common hydraulic defects: external leaks at seals/unions, internal leakage (actuator creep), aeration/foaming (milky fluid), cavitation (noisy pump, low reservoir), overheating (clogged cooler or stuck pump), pressure pulsation (low accumulator pre-charge), contamination (particulate, water, wrong fluid, chlorinated solvents). Cleanliness is measured by NAS 1638 / ISO 4406 class from a fluid sample.
• Common fuel defects: water in sumps, microbial sludge, leaking tank sealant (seepage/stain classification: stain, seep, heavy seep, running leak), false quantity indication from a wet probe, boost-pump failure, blocked vent (tank distortion).
• Common gear defects: low strut extension (gas or oil), leaking gland seals, shimmy, worn bushings and pins (brinelling), unsafe gear indication (proximity sensor rigging), brake drag, excessive tyre wear or cuts, exceeded tyre pressure loss limit (typically about 5% per day), unserviceable fusible plug.
• Maintenance tasks: gear retraction tests on jacks, functional and leak checks, hydraulic system bleeding/flushing, accumulator pre-charge check, nitrogen servicing, brake wear-pin measurement, tyre pressure/tread checks, NDT (dye penetrant, eddy current, ultrasonic, magnetic particle) on gear forgings, and overhaul at specified landings/calendar life for gear and periodic hydraulic fluid sampling.
• Indian regulation and practice (detailed separately for licensing, so only the system angle here): Indian civil airworthiness derives from the Aircraft Act, 1934 and the Aircraft Rules, 1937 (with the Bharatiya Vayuyan Adhiniyam, 2024 enacted to replace the 1934 Act — the transition and subordinate rules are being implemented in stages, so confirm current status before quoting it), DGCA CAR Series requirements and CAR-21/CAR-145/CAR-M for design, maintenance organisations and continuing airworthiness. Design certification of transport types follows FAR/CS-25 (Part 23 for small aeroplanes), which contain the specific fuel-system (25.951–25.1001), hydraulic (25.1435) and landing-gear (25.471–25.511, 25.721–25.735) requirements. Maintenance is performed to the manufacturer's AMM/CMM/SRM and an approved maintenance programme; defects are deferred only under the MEL/CDL.

Key points

• Pascal's law - pressure in a confined fluid acts equally in all directions; force multiplication = area ratio, with displacement traded off inversely.
• Common system pressure - 3,000 psi (207 bar) standard; 5,000 psi (345 bar) on A380/B787 to cut component size and weight.
• Variable-displacement pressure-compensated EDP - reduces stroke so delivery drops to near zero when system pressure is reached, saving power and heat.
• Accumulator - nitrogen-charged energy store; damps surges, meets peak demand, gives emergency brake pressure; pre-charge about one-third to one-half of system pressure.
• PTU - transfers *power*, not fluid, between hydraulic systems, preserving fluid isolation.
• RAT - ram air turbine deployed in total engine/power loss to drive an emergency pump or generator.
• Skydrol (phosphate ester) - fire-resistant transport-aircraft fluid; never mixed with mineral MIL-H-5606; needs butyl/EPM seals and skin protection.
• Integral "wet wing" tank - sealed wing box; lightest and most common on transport aircraft; bladder cells where crashworthiness matters.
• Boost pump - centrifugal tank pump supplying positive pressure to the engine HP pump to prevent cavitation and vapour lock at altitude.
• Fuel burn sequence - centre tank first, outboard wing fuel last, for wing bending relief and CG control; trim tank shifts CG aft to reduce trim drag.
• Capacitance FQIS - measures fuel level through dielectric difference; needs density compensation because mass, not volume, matters.
• Tank inerting (NGS) - nitrogen-enriched air keeps ullage oxygen below about 12% to prevent flammable vapour.
• Tricycle gear - CG ahead of the mains gives directional stability on the ground and avoids ground loops.
• Oleo-pneumatic strut - oil through an orifice damps, compressed nitrogen springs; efficiency about 75–90%; servicing needs both correct oil level and gas pressure.
• Sequence valves and locks - enforce door-then-gear order; down-locks plus three green lights confirm safe gear; squat switch prevents retraction on ground.
• Anti-skid - modulates brake pressure near the 10–15% slip ratio of peak friction; shortens stopping distance and protects tyres.
• Fusible plugs - melt and deflate the tyre safely if brake heat is excessive, preventing an explosive burst.
• Design sink rate - CS/FAR 25.473 landing case at 10 ft/s (3.05 m/s) at design landing weight; gear ultimate load = 1.5 × limit load.

Formulas and examples

Example 1 — Actuator sizing for landing gear retraction. A gear actuator must supply 60 kN to retract the leg. System pressure is 3,000 psi.

• 3,000 psi = 3,000 × 6,895 = 2.0685 × 10⁷ Pa.
• Required piston area A = F/P = 60,000 / 2.0685 × 10⁷ = 2.9006 × 10⁻³ m² = 29.0 cm².
• Bore D = √(4A/π) = √(4 × 2.9006 × 10⁻³ / 3.1416) = √(3.6936 × 10⁻³) = 0.0608 m ≈ 61 mm.
• At 5,000 psi (3.4475 × 10⁷ Pa): A = 60,000/3.4475 × 10⁷ = 1.7404 × 10⁻³ m² = 17.4 cm², D = √(2.2158 × 10⁻³) = 0.0471 m ≈ 47 mm — a 23% smaller bore, which is exactly why high-pressure systems are chosen.
• If the actuator must extend 0.55 m in 8 s, flow Q = A × stroke/time = 2.9006 × 10⁻³ × 0.55/8 = 1.994 × 10⁻⁴ m³/s = 12.0 L/min, and hydraulic power = 2.0685 × 10⁷ × 1.994 × 10⁻⁴ = 4.12 kW.

Example 2 — Accumulator usable volume for emergency braking. A 4-litre piston accumulator is pre-charged with nitrogen to 1,500 psi; system pressure is 3,000 psi. Brakes remain usable down to 1,800 psi. Assume isothermal behaviour.

• Pre-charge: gas fills the whole shell, V₁ = 4 L at P₁ = 1,500 psi.
• At 3,000 psi: V₂ = P₁V₁/P₂ = 1,500 × 4/3,000 = 2.00 L gas, so fluid stored = 4 − 2.00 = 2.00 L.
• At 1,800 psi: V₃ = 1,500 × 4/1,800 = 3.333 L gas, so fluid remaining = 4 − 3.333 = 0.667 L.
• Usable fluid between 3,000 and 1,800 psi = 2.00 − 0.667 = 1.33 L.
• If each brake application consumes 0.15 L, the accumulator gives 1.33/0.15 ≈ 8 applications. Note: a real fast discharge is nearer adiabatic, which gives slightly less usable fluid than this isothermal estimate.

Example 3 — Fuel mass, endurance and tankering. An A320-class aircraft is uplifted 14,500 litres of Jet A-1 at a measured density of 0.795 kg/L. Cruise fuel flow is 2,400 kg/h total.

• Fuel mass = 14,500 × 0.795 = 11,527.5 kg ≈ 11,528 kg.
• If the flight plan assumed the nominal 0.80 kg/L, it would expect 11,600 kg — an overstatement of 72.5 kg, which is why actual density is used.
• Endurance on 11,528 kg at 2,400 kg/h = 4.803 h = 4 h 48 min; holding back 1,500 kg final reserve and alternate fuel leaves 10,028 kg ⇒ 4.178 h = 4 h 11 min of trip fuel.
• Energy content = 11,527.5 × 43.2 = 498,000 MJ ≈ 498 GJ.
• At an ATF price of ₹95,000 per kL (indicative; ATF prices are revised twice a month by oil marketing companies and vary by airport), the uplift costs 14.5 kL × ₹95,000 = ₹13,77,500.

Example 4 — Oleo strut stroke and peak load. An aircraft of 22,000 kg lands at a sink rate of 3.05 m/s (10 ft/s). Two main gears absorb the energy, strut efficiency 0.85, and wing lift equals the weight during the impact (so the (W − L)·s term is zero).

• Total kinetic energy = ½ × 22,000 × 3.05² = ½ × 22,000 × 9.3025 = 102,327.5 J.
• Energy per main strut = 102,327.5/2 = 51,163.75 J.
• With stroke s = 0.30 m: F_max = E/(η·s) = 51,163.75/(0.85 × 0.30) = 51,163.75/0.255 = 200,642 N ≈ 200.6 kN.
• Load factor per strut relative to its static share (static per strut ≈ 0.47 × 22,000 × 9.81/2 ≈ 50.7 kN) gives n ≈ 200.6/50.7 ≈ 3.96 — hence gear design cases around n = 2–3 with a 1.5 ultimate factor.
• Doubling the stroke to 0.60 m halves F_max to 100.3 kN: a longer stroke means a gentler landing but a heavier, longer leg and more stowage volume.

Example 5 — Static gear reactions and wheel load. Aircraft weight 78,000 kg; wheelbase (nose to main axle) 12.6 m; CG 11.5 m aft of the nose wheel. Four main wheels, two nose wheels.

• W = 78,000 × 9.81 = 765,180 N.
• R_main = W × 11.5/12.6 = 765,180 × 0.91270 = 698,474 N (≈ 71,200 kgf, 91.3% of weight).
• R_nose = 765,180 − 698,474 = 66,706 N (≈ 6,800 kgf, 8.7% of weight).
• Load per main wheel = 698,474/4 = 174,618 N ≈ 17,800 kgf.
• If tyre inflation pressure is 200 psi = 1.379 × 10⁶ Pa, contact patch area ≈ 174,618/1.379 × 10⁶ = 0.1266 m² = 1,266 cm² — this contact pressure is what sets the pavement PCN requirement at an airport.

Where it is used

• Design and OEM engineering: HAL (Tejas, HTT-40, LUH, ALH Dhruv gear and hydraulics), Aeronautical Development Agency, DRDO/ADE, Tata Advanced Systems, Safran Landing Systems, Liebherr Aerospace, Parker, Eaton and Collins Aerospace engineering centres in Bengaluru, Hyderabad and Pune designing actuators, valves, fuel pumps and gear legs.
• Airline line and base maintenance: AME/B1-licensed engineers at AIESL, Air India Engineering, IndiGo, Akasa, SpiceJet and GMR/Air Works/Indamer MRO doing gear retraction tests, hydraulic leak rectification, tyre and brake changes, fuel-tank entry and sealant repair.
• Landing gear and hydraulics overhaul shops: HAL Accessories Division, overhaul of gear forgings with NDT, re-chroming of pistons, actuator and pump overhaul to CMM, hydraulic fluid cleanliness testing.
• Airport fuelling and operations: IOCL/BPCL/HPCL aviation fuel stations, bowser and hydrant refuelling, density and water checks, fuel quality control roles; flight dispatch and performance teams converting litres to kilograms and planning tankering.
• Defence and flight test: IAF and Indian Navy technical officers and artificers on Su-30MKI, Rafale, Tejas, P-8I; in-flight refuelling operations; ASTE/CEMILAC certification of gear drop tests and hydraulic qualification.
• New-technology work: electro-hydrostatic and electromechanical actuation for more-electric aircraft, 5,000 psi systems, hydrogen and SAF (sustainable aviation fuel) compatibility of tanks, seals and pumps, and UAV landing gear.

Interview questions

• Q: Why are hydraulics preferred over pneumatics for flight controls and gear? A: Hydraulic oil is practically incompressible, so actuators are stiff and hold position precisely, and liquids give a far higher force-to-weight ratio than air at the same pressure.
• Q: What does an accumulator do, and why is it charged with nitrogen? A: It stores hydraulic energy, absorbs pressure surges, meets peak demand and gives emergency brake pressure; nitrogen is inert, so unlike oxygen or air it cannot support combustion with the oil.
• Q: What is a PTU and why does it not transfer fluid? A: A power transfer unit couples a hydraulic motor in one system to a pump in another so power crosses over; keeping the fluids separate means a leak or contamination in one system cannot drain or spoil the other.
• Q: Why does the A320 have Green, Blue and Yellow systems instead of one big system? A: For independence — three separate fluids, pumps and routings so no single engine failure, leak or rotor burst can disable all flight controls, with the RAT driving the Blue system as the last resort.
• Q: How does an oleo-pneumatic strut absorb a landing? A: Oil forced through a metering orifice converts impact energy into heat (damping) while compressed nitrogen acts as the spring and returns energy slowly on recoil, giving 75–90% efficiency.
• Q: A strut is sitting low. What could be wrong? A: Either low nitrogen pre-charge pressure or low oil level (or a leaking gland seal); both must be checked, because correct extension depends on both and topping only gas hides a low oil quantity.
• Q: Why is outboard wing fuel burnt last? A: Fuel in the outer wing produces a download that relieves wing-root bending moment in flight, so keeping it there reduces structural loads; it also helps flutter margin and CG control.
• Q: How is fuel quantity measured on an airliner, and why is density needed? A: Capacitance probes sense level through the dielectric difference between fuel and air; the system measures volume, so measured or assumed density converts it to the mass that engine performance and weight-and-balance actually depend on.
• Q: What is an anti-skid system doing physically? A: It senses wheel deceleration or slip ratio and modulates brake pressure to keep the tyre near its peak friction slip (about 10–15%), which shortens stopping distance and prevents locked wheels and burst tyres.
• Q: What are fusible plugs in a wheel? A: Low-melting-point plugs that melt after heavy braking and deflate the tyre in a controlled way, preventing an explosive burst from heat and pressure.
• Q: How is the gear extended if hydraulics fail? A: By alternate/emergency extension — releasing the up-locks so the gear free-falls under gravity, assisted by a nitrogen blow-down bottle or a hand crank, with down-locks and three green lights confirming it is locked down.
• Q: Why must MIL-H-5606 and Skydrol never be mixed? A: They are chemically different (mineral oil versus phosphate ester) and require different seal materials; mixing swells or destroys seals, degrades fire resistance and contaminates the whole system, requiring a full flush.
• Q: What does the squat switch (air-ground sensor) do? A: It tells systems whether the aircraft is on the ground — inhibiting gear retraction, enabling ground spoilers and thrust reverse, and switching logic in pressurisation, anti-skid and warning systems.

Common mistakes

• Treating hydraulic fluid as compressible or, conversely, forgetting that entrained air makes a system spongy and causes erratic actuator motion.
• In accumulator problems, using the pre-charge volume as the gas volume at system pressure, or mixing psi with bar and Pa mid-calculation.
• Forgetting that a double-acting unbalanced actuator gives less force on retraction because the rod area is subtracted.
• Converting fuel litres to kilograms with a hardcoded 0.8 kg/L when the actual measured density is given — this is a classic marked error in dispatch and exam problems.
• Saying the gas spring in an oleo strut provides the damping; the oil through the orifice damps, the gas springs.
• Omitting the wing lift term, or the number of struts sharing the energy, in landing-energy problems, and leaving the answer as energy instead of solving for stroke or peak load.
• Taking CG position from the nose gear and the wheelbase from the main gear inconsistently in reaction calculations, or forgetting to divide by the number of wheels per bogie.
• Assuming the landing gear is retracted purely to look clean — it is a drag reduction trade against weight, volume and complexity, and some aircraft keep it fixed deliberately.
• Quoting one tyre pressure or one accumulator pre-charge as universal; these are type-specific values from the aircraft maintenance manual.

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