
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.
Aircraft operators and aviation regulatory authorities have developed various procedures that should be used during a fuel exhaustion situation in flight so that the aircraft maximizes its chances of completing a safe and successful landing.
During flight, if the pilot of their aircraft determines that they are able to follow cleared instructions and successfully complete an approach and landing, but that any further undue delay may result in the aircraft encountering a fuel emergency or landing with less than their calculated final reserve fuel, the pilot shall inform air traffic control by declaring MINIMUM FUEL. This will inform ATC that any further delay might place the aircraft in a fuel emergency situation, however this is only an advisory declaration and gives ATC no obligation or requirements to prioritize that aircraft for an expedited approach or immediate landing.
If during flight, the pilot of their aircraft calculates that they will be landing with less than their final reserve fuel, or that they will encounter a fuel emergency situation, they must inform ATC by declaring MAYDAY MAYDAY MAYDAY, FUEL. As with any other mayday call in aviation, this will be treated as a full emergency and the pilots will be provided all available aid to execute a safe and successful landing. This may include immediate prioritization above all other aircraft traffic, permission to land on any usable runway and preparation of airport emergency services at the airfield.
Many incidents have happened on aircraft where fuel exhaustion or starvation played a role. A partial list of these incidents follows:

A number of aircraft have been abandoned by their crew (both intentionally and sometimes accidentally) when the aircraft has continued on its own until fuel exhaustion caused it to crash: