Fuel Systems, Tanks and Fuel Management
Tank types and venting
Fuel is carried in integral tanks (the wing structure sealed to form a tank), bladder tanks or rigid tanks. Integral tanks save weight but require sealing and inspection. Tanks are vented to atmosphere so that fuel can flow and the tank does not collapse during descent; vent lines also allow expansion. Check valves prevent cross-flow between tanks when not selected. Fuel is fed by engine-driven pumps with electric booster pumps in the tanks; gravity feed is used on some light aircraft.
Management
Fuel management includes balancing fuel between tanks to control centre of gravity and lateral balance, using the fuel quantity indicating system and totaliser. Fuel is measured by capacitance probes (dielectric changes with fuel) or float sensors; a common trap is that capacitance gauges read mass only if fuel density is correctly set, and they can be affected by water or contamination. Crossfeed allows either engine to feed from either tank; it is used for balancing and for an engine failure.
Contamination and safety
Water is the most common contaminant; it collects at sump drains and is removed by pre-flight sampling. Microbial growth can block filters. Fuel jettison (dump) systems are fitted on some transports to reduce landing weight; dump masts are located to avoid fuel reaching engines or igniting.
Worked example
If a tank holds 2000 kg and the gauge is calibrated for 0.8 kg/L, a fuel at 0.75 kg/L reads a mass error of about 6%. Questions on fuel quantity often hide a density trap.
Traps
Assuming fuel flow indication equals fuel remaining (it does not; use the totaliser), and forgetting that unusable fuel is included in some limits but not in usable capacity. Also, crossfeed valve position matters for engine start and for balancing.
Rule of thumb
Balance first, then crossfeed only as required; keep tank quantities within the manufacturer's imbalance limit.