Fuel Storage, Feed, Transfer and Gauging
Storage and feed
Fuel is carried in wing tanks (integral structure), fuselage tanks, or tip tanks, and in transports often in centre and outer wing tanks. Feed is normally from the tank serving the engine on that side, via boost pumps, with a cross-feed valve for balancing or for feeding both engines from one side. Gravity feed is used on many light aircraft; transports use pressure feed with boost pumps. Know the purpose of vent valves (prevent tank over/under-pressure, allow refuelling and defuelling), check valves (prevent backflow), and drain valves (water and sediment sampling).
Transfer and management
Transfers move fuel between tanks to maintain balance and to keep the centre of gravity within limits; they may be automatic or manual. Gauging uses float, capacitance or ultrasonic sensors; fuel quantity indicating systems must be calibrated and are subject to drip-stick or dip-check verification.
Traps and worked example
Traps: assuming the fuel gauge is accurate at all attitudes (it is not, especially in unusual attitudes or with a failed probe); forgetting that cross-feed does not transfer fuel, it only changes the source; and confusing unusable fuel with reserve fuel.
Worked example: a question gives total fuel, burn rate and distance to destination and asks whether reserves are met. Reasoning: subtract taxi and climb allowances, compute trip fuel, add contingency, alternate and final reserve, then compare with usable fuel. Usable fuel excludes unusable fuel, and the answer must state that the comparison is against usable, not total, fuel.