Fuel is a resource Air Units need to run their engines properly.
Air units consume fuel as they fly. Running out of fuel does not cause an air unit to crash - instead, its speed is greatly reduced, rendering it vulnerable and ineffective. Aircraft will slowly regenerate their fuel when landed. Alternatively, fuel will be instantly replenished upon landing on an Air staging facility, Siren Class, Fatboy, or any Aircraft Carrier.
Transports quickly recharge fuel when landed because they cannot enter Carriers or land on landing pads. They can also be assisted with an engineer to speed the refueling process.
Different air units have different amounts of fuel and, as such, can fly for different lengths of time before running out. Lower tech units typically have lower fuel capacities and higher tech units typically have higher fuel capacities. In Vanilla Supreme Commander, the CZAR cannot refuel because it cannot land, but has enough fuel for nearly 9 hours. In Forged Alliance, none of the air experimentals require fuel.