Fuel Systems

Storage and venting, boost pumps and crossfeed, capacitance quantity indication and CG management.

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Main ideas

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.

Solved examples

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.

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