System layout and tanks
- Wing tanks (inner and outer, or main tanks) are integral tanks in the wing box (Chapter 5). A centre tank in the centre wing box is common; surge/vent tanks near the tips collect fuel that enters the vent system and drain it back. Long-range types may add additional centre tanks in the cargo hold and a trim tank in the horizontal stabiliser.
- Tank fittings: sealed access panels, sumps and water drain valves at low points, baffle ribs with flapper check valves, fuel quantity probes, temperature sensors, low-level sensors, pump canisters and valves.
- Fuel is Jet A-1 (freezing point −47 °C; Jet A −40 °C), density about 0.78–0.84 kg/litre. Fuel temperature in long cold flights is monitored against its freezing point.
Supply to the engines and APU
- Boost pumps — usually two AC motor-driven centrifugal pumps in each tank, feeding the engine at a positive pressure that prevents vapour lock and cavitation at the engine-driven pump. Each pump sits in a canister with a check valve, so it can often be changed without defuelling.
- Centre-tank (override) pumps deliver a higher pressure than the wing pumps, so the centre tank empties first while the wing fuel relieves wing bending.
- Jet (ejector) pumps use a motive flow of fuel to scavenge fuel and water from low corners and to transfer fuel, with no moving parts.
- Check valves stop fuel flowing backwards through failed pumps; a pump bypass valve allows suction feed — the engine's own pump drawing fuel — if the boost pumps fail, within altitude limits (dissolved air and vapour come out of solution at low pressure).
- The low-pressure (spar) shut-off valve isolates the engine; it is closed by the engine master/fuel control switch and the fire handle.
- The APU feeds from a tank through its own line and a DC pump for starting when AC is unavailable. On some aircraft hot fuel returning from the IDG or engine oil coolers is recirculated to a tank.
From the spar valve onwards the fuel enters the engine's system — LP and HP pumps, fuel/oil heat exchanger, filter and fuel metering unit — covered in the powerplant modules.
Fuel handling
Cross-feed and transfer
- Cross-feed — opening the cross-feed valve connects the left and right feed manifolds so that either side's pumps can feed either or both engines; used after an engine failure or to correct a lateral fuel imbalance (by switching off the pumps on the lighter side).
- Transfer — valves and pumps move fuel between tanks: outer to inner tank, centre to wing, from additional tanks, and to or from a trim tank. Imbalance limits in the flight manual protect handling and structure.
Refuelling and defuelling
- Pressure refuelling — through a single-point coupling under the wing at up to about 50 psi. A refuel panel selects the quantity; refuel valves in each tank close automatically at the preselected quantity or when high-level sensors detect a full tank, so tanks cannot be overfilled into the vents. The refuel gallery distributes fuel to all tanks at once.
- Overwing (gravity) refuelling through filler caps is used on light aircraft and as a back-up on some transports.
- Defuelling — by suction from the refuel coupling, or by pressure using the aircraft's boost pumps through a defuel valve into the refuel gallery.
- Fuel quality — correct grade, water and sediment checks at the refueller and by draining the aircraft sumps, use of water-detection capsules, and filter/water-separator monitors on the hydrant cart.
Bond the refueller to the aircraft before connecting the hose or opening a cap, so no static spark can jump. No smoking, naked flames or unapproved electrical equipment near the vents; no HF or weather-radar transmission; fire-fighting equipment available; stop if lightning is near. With passengers on board, follow the operator's procedures (exits manned, cabin crew informed, no-smoking signs on, fire service notified where required).
Indication and warnings
- Quantity — capacitance probes in each tank, with a compensator (a probe always covered with fuel, measuring its dielectric constant) and a densitometer, so the computer can display mass. Ultrasonic gauging is used on some types.
- Measuring (dip/drip) sticks or magnetic level indicators under the wing allow the quantity to be checked on the ground without power, using tables corrected for aircraft attitude and fuel density.
- Fuel used comes from the engine fuel-flow meters; fuel temperature is shown from a tank sensor.
- Warnings and cautions: low fuel level (thermistor sensors), boost-pump low pressure, fuel imbalance, filter impending bypass, cross-feed and valve disagreement, fuel temperature, and on some types fuel leak detection (comparing fuel used with quantity change).
Special systems
Jettison (dumping)
Aircraft whose maximum take-off mass is well above the maximum landing mass may need a jettison system (CS 25.1001): it must be able to dump enough fuel within 15 minutes to meet the climb requirements. Fuel is pumped through jettison valves to nozzles near the wing tips or trailing edges, well clear of the airframe and engines, and jettison stops automatically at a preset quantity so the landing reserve is protected.
Venting
Draining and contamination
Water enters fuel by condensation and from the supply; it collects in the sumps and must be drained regularly through the drain valves. Water in tanks also supports microbial growth (fungi and bacteria, notably Hormoconis resinae) at the fuel–water interface, which forms dark mats that block filters, damage sealant and cause corrosion. Prevention is regular draining; contamination is treated with approved biocides and tank cleaning.
Fuel tank inerting
After the 1996 centre-tank explosion of a B747 (TWA 800), the rules (FAA and EASA CS 25.981) were changed to reduce fuel-tank flammability, especially for centre tanks heated by air-conditioning packs beneath them. Besides strict control of ignition sources (bonding, wiring separation, pump protection), most airliners now carry a nitrogen generation system, also called OBIGGS or a flammability reduction system, that keeps the oxygen concentration in the tank's empty space (ullage) below about 12 %.
The atmosphere in an inerted tank cannot support life. Tank entry requires full ventilation and gas testing (Chapter 5), and NEA lines must be isolated.
Longitudinal balance fuel systems
Moving fuel fore and aft changes the centre of gravity. Some long-range aircraft (A330, A340, A380) have a trim tank in the horizontal stabiliser: in the cruise the fuel management computer moves fuel aft to place the CG near its aft limit, reducing the stabiliser download (trim drag) and saving fuel, then moves it forward again before landing or if a limit is approached. Concorde transferred fuel between forward and aft trim tanks to follow the rearward movement of the centre of pressure as it accelerated to Mach 2. Lateral balance is handled by cross-feed and transfer.