Aeroplane Systems - Hydraulic, Pneumatic, Fuel and Ice and Rain Protection
Hydraulic systems and fluids
Mineral fluid MIL-H-5606 (red dye) is the traditional light-aircraft fluid; MIL-H-83282 is a less flammable synthetic; phosphate-ester fluid such as Skydrol (purple) is used on transports and needs EPR/EPDM seals, not natural rubber or nitrile. Never mix fluid types. Light aircraft systems typically run at 1,000-1,500 psi; transports at about 3,000 psi. Every circuit needs a relief valve, filters and often a hydraulic fuse, which closes on excess flow to limit fluid loss after a line rupture. Accumulators are precharged with dry nitrogen only; oxygen plus hydraulic oil is an explosion risk.
Fuel
AVGAS 100LL is dyed blue; Jet A-1 is straw-coloured kerosene. Water is the main contaminant, so sump each tank and watch for microbial growth at the fuel/water interface. Know the vent system, crossfeed and transfer logic, boost pumps, and the use of fuel system icing inhibitor.
Ice and rain
Anti-ice prevents ice forming: bleed-air thermal panels, electrically heated elements, TKS weeping wing. De-ice removes ice already formed: pneumatic boots. Carburettor heat is a preventive anti-ice system. Rain protection includes wipers, repellent and heated windscreens.
Worked example
Gear retraction is slow; pump pressure is 3,000 psi but only 1,800 psi at the actuator. A pressure drop downstream means a restriction or internal leak: a collapsed hose liner, clogged pressure filter or leaking selector, not a failed pump. A hydraulic fuse protects against gross loss, not partial restriction. Check differential pressure indicators first.
Traps
Mixing Skydrol and mineral fluid; oxygen instead of nitrogen; treating boots as anti-ice; skipping fuel sumps; over-tightening flareless fittings.