12.11 and 12.12 Helicopter fuel and hydraulic power systems

Scope

Both sub-modules are A level 1 and B1.3 level 3.

• 12.11 Fuel systems (ATA 28): layout, tanks, supply, dumping/venting/draining, cross-feed and transfer, indications and warnings, refuelling and defuelling.
• 12.12 Hydraulic power (ATA 29): layout, fluids, reservoirs and accumulators, pressure generation (electric, mechanical, pneumatic), emergency pressure, filters, pressure control, distribution, indication and warning, interfaces, servicing.

Fuel system layout

Helicopter fuel tanks are normally located close to the CG (under the cabin floor or behind the cabin) so that fuel burn has little effect on balance. Modern installations use crash-resistant flexible bladder cells with self-sealing breakaway valves in the lines, so that fuel is not released if the airframe deforms in a crash.

Typical elements:

• Tanks / cells with baffles, sumps and drain valves at the lowest point.
• Booster pumps (electrical, in-tank) to supply the engine-driven pump and prevent vapour lock; some light helicopters use gravity feed.
• Fuel shut-off valve (firewall shut-off), linked to the fire handle on turbine types.
• Filters/strainers with impending-bypass indication (pop-up button or caution light).
• Vent system to keep tank pressure near ambient; with flame arrestors and float/check valves to prevent spillage in manoeuvres.
• Cross-feed (twins) lets either tank feed either engine; transfer pumps move fuel between tanks (e.g. auxiliary tank to main tank).
• Fuel dumping (jettison) is fitted on some larger helicopters to reduce weight quickly.

Indications and warnings

Indication	Typical sensor
Fuel quantity	Capacitance probes (measure mass — dielectric changes with fuel height and density) or float-type transmitters
Low fuel warning	Separate thermistor or float switch, independent of the gauging, giving a fixed-time warning (e.g. ~20 min)
Fuel pressure	Pressure transmitter after the boost pump; low-pressure caution
Fuel filter	Differential-pressure switch: "FUEL FILTER" caution (impending bypass)
Fuel flow	Flowmeter (turbine types)