What It Looks Like

The gas tanks in some aircraft are not tanks at all, but flexible fuel bladders, such as the one pictured here from a Cessna 182.

Fuel bladders, or fuel cells as they are sometimes called, are made of synthetic rubber compounds like nitrile rubber. The advantage of a bladder over a conventional metal tank (found in Cessna 172s, among other types) or a so-called "wet wing" (sealed compartments in the wing structure, such as those on Mooney models), is that a bladder is relatively easy and inexpensive to remove and replace.

Why remove a fuel bladder? The rubber compounds can develop age-related problems like dry rot that make the bladder porous and subject to leaks. If that occurs, the bladder must be repaired or replaced. To find out if the airplane you fly has a bladder-type tank, look in the systems description section of the pilot's operating handbook.

The flexible nature of a fuel bladder has a few operational implications for the pilot. A clogged fuel vent line or running the bladder completely out of fuel can result in its distortion, much like sucking the air out of a plastic bag. Filling the bladder with fuel usually restores its shape, but a few wrinkles may be left on the bottom of the bladder. These wrinkles can trap water that enters the tank through seepage past the fuel cap or from condensation of moisture in the air space in the tank. If the trapped water is not dislodged and drained during the preflight inspection, it could be dislodged as the aircraft jostles and turns during taxi or flight.

To prevent water trapped in a fuel bladder from reaching the engine, shake the wing vigorously before starting the preflight inspection. Then do a thorough job of draining fuel from the sumps to check for water contamination.

Fuel helps to lubricate the bladder material so that it stays flexible. For that reason, manufacturers recommend that a bladder be kept full of fuel when the airplane is not flown. Hangaring the airplane to avoid extremes in ambient temperature also helps to extend the life of bladders. Well-maintained fuel bladders can last 20 years or longer.

Stains on wing skins, especially on the underside of the wing, may indicate pinhole leaks in the bladder. Avgas fumes in the cabin also may signal leaks. Black flakes in fuel samples may indicate a deteriorating bladder. If in doubt, consult a mechanic.