Fuel systems

021 Aircraft General KnowledgeChapter 5

Fuel types and properties

Key fact

Flash point: Lowest temperature at which vapour ignites on contact with flame and extinguishes immediately.

AVGAS (piston): 100LL blue, 100 green, 80 red. MOGAS: Higher carb icing risk. Always use equal or higher octane grade — never lower.

Fuel	Freezing point	Flash point	Density (kg/l at 15°C)
Jet A	−40°C	38°C	0.78–0.83
Jet A-1	−47°C	38°C	0.78–0.83
Jet B	−60°C	−20°C	0.75–0.80

Key fact

Octane rating: Anti-knock / detonation resistance — higher CR requires higher octane. Cetane number (diesel/AVTUR): Ease of combustion. Jet fuels are combustible (high flash point); AVGAS/Jet B are flammable (low flash point).

Fuelling with AVTAG/Jet B not permitted with passengers on board (low flash point).

Tanks and quantities

Key fact

Usable fuel: Amount deliverable to engine(s). Unusable fuel minimised by sump pads. Keep tanks full when not in use to reduce moisture condensation. Ice traps in delivery line; water settles at bottom of strainer (heavier than fuel).

Integral/wet wing tanks: Built into structure — saves weight and space. Baffles: Restrict fuel movement in manoeuvres/sideslip. Vent space: ~2% of tank volume; pressure kept ambient.

Tank type	Feature
Rigid	Small aircraft wing root
Flexible/bladder	Self-sealing; helicopters
Integral/wet	Wing structure; ribs as baffles

Key fact

Float gauge: Measures volume — affected by attitude/temperature. Capacitance gauge: Measures mass — AC powered; compensates for attitude. Magnetic drip stick / drip stick: Manual ullage measurement under wing.

Pumping and engine supply

Key fact

LP booster pumps: Electric centrifugal, 20–100 psi — prevent vapour lock/cavitation of HP pump; direct positive-pressure feed. HP pump: Engine driven from AGB. DC fuel pump: Can feed APU only.

Crossfeed valve: Ensures all fuel available to any engine; balances distribution after engine failure. Fuel heater: At engine — prevents ice from water in fuel at low temperatures (often oil/fuel heat exchanger).

Exam trap

Fuel pressure measured: Line between booster pump and engine, or outlet of HP filter. Pressurising & dump valves: Admit fuel to nozzles only when pressure sufficient for atomisation; drain manifold after shutdown.

Fuel dumping: Required when MTOW significantly exceeds MLW — jettison to predetermined safe mass within time limit; remaining fuel must allow take-off, climb to 10 000 ft, plus 45 min reserve.

Large jet fuel system operation

Key fact

Typical sequence: inner tanks first (to ~5000 kg) → trim tank to inner → outer to inner via jet ejector pump (LP booster creates venturi suction). Surge/vent tank: Catches overflow from manoeuvres/overfuelling; returns by gravity.

Special consumption schedules: Minimise wing G-loads; preserve main tanks for landing. Wing bending relief: Fuselage/centre fuel used before wing fuel where possible.

Refuelling and contamination

Key fact

Bonding/earthing before filler cap removed — aircraft, nozzle, fuel truck (triple point). Automatic shut-off when predetermined level/mass reached. Pressure refuelling: Under-wing centre; gravity: Over-wing (small aircraft).

Water check: Cloudy sample clearing slowly from top = water in fuel — check before first flight of day. FCII (fuel system icing inhibitor): Reduces water freezing, fungus; lubricates HP pump.

Key fact

Refuelling zone: At least 6 m radius from fill/vent points — no smoking, GPU far away, no refuelling within 30 m of radar in test, within 20 NM of TS, with overheated brakes, or within 6 m of flash equipment.

Vapour lock and low temperature

Key fact

Vapour lock: heat produces vapour bubbles blocking fuel line — high fuel temp, low fuel quantity, low ambient pressure, high AoA. Prevented by fuel under pressure (booster pumps, tank pressurisation).

Low fuel temperature: increased viscosity, wax/paraffin crystals — may require altitude change. AVTUR more viscous than AVGAS — water suspension more likely; water detectors and drains essential.