Fuel Systems, Module 13, Certify66

Notes

Main ideas

Fuel is stored in INTEGRAL tanks (the sealed wing box) and sometimes centre/tail tanks; each tank is VENTED 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; a CROSSFEED lets any engine draw from any tank to keep the aircraft laterally balanced and manage an engine-out case.
Fuel QUANTITY is measured by CAPACITANCE probes (fuel level changes the dielectric between the plates), not floats; temperature and flow are also indicated, the indication and its computer are core B2 items.
Large aircraft manage CG and structural loads with fuel, sequencing tank usage and using a tail trim tank; a jettison/dump system reduces weight for an overweight landing.
Refuel/defuel is via a pressure panel controlling tank valves; the system is bonded and earthed to dissipate static.
⚠ Exam trap: fuel quantity is sensed by CAPACITANCE probes (not floats); CROSSFEED lets any tank feed any engine to keep the aircraft balanced, especially engine-out.

Solved examples

How does a capacitance fuel-quantity system work, and why is it preferred to a float gauge?
A capacitance probe is a tube capacitor in the tank; fuel between its walls has a higher dielectric constant than air, so as the fuel level rises the capacitance rises. A gauging computer converts the total capacitance of the probes into a mass of fuel, compensating for temperature/density. It has no moving float to stick, measures the whole tank, and gives fuel MASS (what matters for performance) rather than just level.

Fuel Systems