Module 11B.10–11B.11 — Piston aeroplanes: fuel systems (ATA 28) and hydraulic power (ATA 29)

Scope and levels

Item	A2	B1.2
11.10(a) Fuel system layout, tanks, supply	1	3
11.10(b) Cross-feed, transfer, refuel/defuel	1	3
11.10(c) Indication and warnings	1	3
11.10(d)(e) Dumping, inerting, balance fuel systems	—	—
11.11(a) Hydraulic system description	1	3
11.11(b) Pressure generation (electric/mechanical), control, indication, servicing	1	3
11.11(c) Pneumatic/emergency generation, interfaces	—	—

Part 1 — Fuel systems (ATA 28)

Fuels for piston engines

Grade	Colour	Notes
AVGAS 100LL	Blue	Low lead; the standard grade
AVGAS 100 / 100 (high lead)	Green	Largely superseded
UL91 / unleaded AVGAS	Colourless/undyed (check specification)	Only where the engine/aircraft is approved
MOGAS	Variable	Only with an approval (e.g. STC/TC data); vapour lock and alcohol content issues
Jet fuel (Jet A-1)	Clear to straw	Must never be used in a spark-ignition piston engine — causes detonation and engine failure; filler necks are placarded "AVGAS ONLY" and jet nozzles are wider (J-spout) to prevent mis-fuelling

Diesel (compression-ignition) piston engines are approved to run on Jet A-1 — always check the placard.

Key fact: AVGAS 100LL is dyed blue. Putting jet fuel into a petrol piston engine causes detonation — check fuel grade and placards before every refuel.

Tanks

Type	Examples
Integral (wet wing)	Sealed wing bays — common on low-wing singles and twins
Rigid removable	Welded aluminium tanks in the wing or fuselage
Bladder	Rubberised fabric cells laced into the wing; check for wrinkles that trap water, and for cracking if left dry
Tip / auxiliary tanks	Wing-tip tanks, nacelle tanks on twins

Every tank needs: filler with cap (sealing check — a leaking cap can siphon fuel in flight), vent (check open; often with an anti-icing/anti-siphon design), sump drain at the lowest point, finger screen at the outlet, and baffles to limit sloshing.

Supply systems

• Gravity feed (high-wing singles): tanks above the engine; selector LEFT/RIGHT/BOTH/OFF; no booster pump needed (fuel-injected models may still have an auxiliary pump).
• Pump feed (low-wing aircraft): engine-driven fuel pump plus an electric auxiliary/boost pump — used for starting, take-off, landing and tank switching, and as backup if the engine pump fails; on fuel-injected engines the aux pump may also purge vapour.
• Fuel selector valve: rotary valve; detents and placards; each tank's usable fuel marked. Selector settings and their positions are a classic maintenance and pilot-error item.
• Fuel strainer (gascolator): at the lowest point of the system, before the carburettor/pump — traps water and sediment; drained before flight and cleaned per AMP.
• Primer: small hand or electric primer injecting fuel into the cylinders for starting.

Cross-feed and transfer (twins)

• Cross-feed valves let either engine draw from the opposite wing tanks (single-engine operation, fuel balancing).
• Transfer pumps move fuel from auxiliary/nacelle tanks into main tanks.

Refuelling and defuelling

• Over-wing gravity refuelling through filler caps; bond the aircraft to the fuel truck/pump (and nozzle to the aircraft) before opening caps; ground the aircraft.
• No smoking/naked flames; no refuelling with engines running (unless specifically approved procedures); fire extinguisher at hand; avoid refuelling in electrical storms.
• After refuelling: wait for water to settle, then drain sumps and the strainer and check for water/contamination; verify fuel grade (colour/smell).
• Defuelling: through sump drains or defuel points into approved, bonded containers; observe fire precautions.

Indication and warnings

• Fuel quantity: float-type senders with variable resistors (most common), magnetic float gauges, sight gauges, capacitance systems on some aircraft; dipsticks for verification. Calibration with known quantities after component change.
• Fuel pressure gauge (carburettor engines) or fuel flow (injection — from flow divider pressure or a flow transducer); fuel totalisers.
• Low-fuel warning lights, aux pump on lights.

Part 2 — Hydraulic power (ATA 29)

Where hydraulics are used on piston aeroplanes

Wheel brakes (independent systems with master cylinders — see 11B.13), retractable landing gear, flaps on some twins, cowl flaps, sometimes nose-wheel steering.

Fluid

Usually mineral-based (MIL-PRF-5606, red) with nitrile seals. Phosphate-ester fluid is rarely found on piston aircraft — never mix fluid types.

11.11(a) System components

• Reservoir — often vented to atmosphere or integral with the power pack; filler and level indication.
• Filters — in the pump outlet/return; bypass relief.
• Accumulator — occasionally fitted to hold gear-up pressure or for brakes.
• Selector valves, check valves, relief valves, restrictors, actuators, lines and hoses.

11.11(b) Pressure generation

Method	Description
Hand pump	Older/simple aircraft; also emergency gear extension on some types
Engine-driven gear pump	Constant-displacement; needs an unloading/relief arrangement or open-centre system when not in use
Electric hydraulic power pack	Electric motor + reversible gear pump + reservoir + valves in one unit — very common for retractable gear (e.g. on Piper and Beechcraft singles): running one direction = gear down, the other = gear up

Pressure control

• Pressure switches stop the power-pack motor at the set pressure (closed-centre) and restart it if pressure decays (gear-up "holding" pressure — repeated cycling indicates an internal leak).
• Relief valves protect against over-pressure; thermal relief for trapped fluid.
• Open-centre systems: fluid circulates back to the reservoir at low pressure until a selector is operated.

Indication and warning

• Gear position lights (three greens, in-transit/unsafe light), power-pack running light, low-fluid indications on some types.

Servicing

• Check reservoir level as specified (gear down, system depressurised); top up with the specified fluid only.
• Bleed air from brake and gear systems after component changes.
• Leak checks; hose condition and age; filter replacement.

Key fact: Many light retractable aircraft use an electric hydraulic power pack with a reversible pump; a pump that cycles frequently with the gear up indicates internal leakage.