Fuel starvation and fuel exhaustion

In an internal combustion engine, fuel starvation is the failure of the fuel system to supply sufficient fuel to allow the engine to run properly, for example due to blockage, vapor lock, contamination by water, malfunction of the fuel pump or incorrect operation, leading to loss of power or engine stoppage. There is still fuel in the tank(s), but it is not delivered to the engine(s) in sufficient quantity. By contrast, fuel exhaustion (also called fuel depletion) is an occurrence in which the vehicle in question becomes completely devoid of usable fuel, with results similar to those of fuel starvation.

All modes of transport powered by internal combustion engines can be affected by fuel starvation, although the problem is most serious for aircraft in flight. Watercraft are also at risk, as a drifting vessel cannot maneuver to avoid a collision or prevent itself from running aground. On aircraft, fuel starvation is often the result of incorrect fuel management, for example by selecting to feed the engine from an empty tank while fuel is present in another one.

When the fuel level is low, some aircraft are vulnerable to a specific type of fuel starvation known as unporting, which occurs when hard maneuvers, a steep climb, or a departure from coordinated flight causes the fuel to slosh away from the line leading to an engine. Some aircraft have special operating limitations on steep climbs with low fuel due to this concern.

Aircraft with fuel tank filler caps on the surface of the wings or fuselage are also susceptible to fuel exhaustion if a filler cap is left off or not refitted correctly after removal. On an aircraft in flight, the airflow over the wing or fuselage leads to air pressure outside the fuel tank being lower than inside; as a result fuel can be sucked out of the tank. In addition to improper fitment, lack of maintenance of an aircraft fuel tank filler cap is also an issue. A defective seal on a cap may allow water into the tank when an aircraft is washed or parked outdoors in wet weather, leading to fuel starvation.