Airplanes (unfortunately) cannot stay in the air forever. There is a time limit, or maximum time, that an airplane can stay aloft; and the time limit is usually determined by the amount of fuel. When the airplane runs out of fuel, the engine stops.

If an airplane is running low on fuel, the pilots follow a strict, multi-stage protocol to ensure a safe landing. First, if they anticipate landing with less than their "planned reserve" (usually 30–45 minutes of flight time), they will declare "Minimum Fuel" to Air Traffic Control (ATC). This is an advisory that they cannot accept any further delays, but it is not an emergency. If the situation worsens and the fuel level reaches a critical point where any further delay would be dangerous, the pilots declare a "Mayday Fuel" emergency. This grants the aircraft absolute priority over all other traffic, allowing it to take the shortest possible route to the runway. Modern 2026 aircraft like the Boeing 787 have sophisticated "Fuel Management Systems" that alert the crew long before a crisis. In rare cases of complete fuel exhaustion, the aircraft becomes a glider, and the Ram Air Turbine (RAT)—a small wind turbine that drops out of the fuselage—deploys to provide essential electrical and hydraulic power to the flight controls until the plane can be safely landed.