Fuel Systems
Storage and venting, boost pumps and crossfeed, capacitance quantity indication and CG management.
Fuel systems, summary notes
- Fuel is stored in INTEGRAL tanks (the sealed wing box) and sometimes centre/tail tanks; each tank is VENTED to atmosphere so pressure stays balanced as fuel is used or the aircraft climbs, and has water drains at its low points.
- BOOST PUMPS deliver fuel under positive pressure to the engines (preventing vapour lock and feeding at altitude); a CROSSFEED system lets any engine draw from any tank to balance the aircraft and manage an engine-out case.
- Fuel QUANTITY is measured by CAPACITANCE probes (the fuel between the plates changes the capacitance with level), not floats; temperature and flow are also indicated.
- Large aircraft manage CG and structural loads with fuel, sequencing tank usage, and some have a trim tank in the tail; jettison/dump systems reduce weight for an overweight landing.
- Refuel/defuel is done through a pressure-refuel panel controlling tank valves; the system is bonded and earthed to dissipate static.
- ⚠ Exam trap: fuel quantity is sensed by CAPACITANCE probes (level changes the dielectric between the plates), not by floats; CROSSFEED lets any tank feed any engine to keep the aircraft laterally balanced, especially engine-out.
Why must each fuel tank be vented, and what could happen if a vent became blocked?
Venting keeps the tank's ullage at ambient pressure as fuel is drawn off (which would otherwise create a vacuum and starve the pumps) and as the aircraft climbs/descends (pressure changes). A blocked vent can cause fuel starvation to the engine on one hand, or, in a descent or during refuel, an over-pressure or collapse of the tank structure, so vents and their float/flame-arrestor protection are critical.
Fuel systems concept map
Fuel Systems
Fuel systems quiz
Fuel Systems, quiz
1. In the fuel system shown, the valve marked X lies in the pipe joining the two tank feed lines. Opening it allows:
Either tank to supply either engineThe tanks to be ventedFuel to be jettisoned2. Fuel quantity in large aircraft is measured by:
Capacitance probesFloat switches onlyA dipstick in flight3. Boost pumps are fitted to:
Deliver fuel under positive pressure and prevent vapour lockCool the cabin by circulating fuel through a heat exchanger in the air conditioning packCharge the battery from the pump generator whenever the engine driven pumps are running4. A crossfeed system allows:
Any engine to draw from any tankOnly the left engine to runFuel to be jettisoned only5. Fuel tanks are vented to:
Keep the ullage at ambient pressureIncrease pressureStore extra fuel6. Integral wing tanks are:
Sealed sections of the wing structure ('wet wing')Separate steel drumsIn the tail cone only7. Water drains at tank low points are used to:
Remove settled water before flightAdd fuelVent air8. A fuel jettison (dump) system is used to:
Reduce weight for an overweight landingIncrease rangeCool the engine