Fuel systems

A high-wing light aircraft can feed fuel by gravity; low-wing and larger aircraft need pumps. An engine-driven pump does the main work, and electric boost pumps (often submerged in the tank) supply pressure for starting, takeoff and landing, when switching tanks, and to stop vapor lock (fuel boiling in the lines on hot days or at altitude).

Tanks

• Integral (wet wing): the sealed wing structure is the tank. Leaks are fixed by resealing the seams with sealant.
• Bladder: a rubberized fuel cell inside a cavity. Kept wet with fuel, or coated with oil when empty for long, so it doesn't dry and crack; check for wrinkles that trap water.
• Rigid removable: a welded or riveted metal tank.

Every tank has a vent: a blocked vent lets a vacuum form as fuel is used, which can starve the engine or even collapse the tank. Sumps and drains at the low points collect water and sediment, which are drained before the first flight and after fueling.

Gauges and controls

Fuel quantity is shown by a float or, in larger aircraft, capacitance probes: fuel and air have different dielectric constants, so the probe's capacitance changes with fuel height, and it can read in pounds. A fuel selector chooses tanks, crossfeed lets one tank feed another side's engine, and some large aircraft can jettison (dump) fuel to get down to landing weight.

Contamination

Water, dirt, the wrong fuel and, in jet fuel, microbial growth (a slime that lives where water meets fuel and corrodes tanks) are the usual problems. Drain water often, use filters and strainers, and use biocide as the manual allows.

Entering a fuel tank is a confined space: purge and ventilate it, test the air, use explosion-proof lights and tools, wear the right respirator and clothing, and keep a trained person outside watching.