An emergency is any situation that threatens the safety of an aircraft, the people on board it, or people on the ground.
An emergency is when an aircraft and/or its crew and passengers are in immediate danger, or when there is immediate danger to people on the ground.
Emergencies have many possible causes, including ones outside the aircraft itself. For example (this is not a complete list):
- Structural damage or failure
- Systems failures
- Flight into severe weather conditions
- Fire on board
- Pilot incapacitation
- Deliberate hijacking or sabotage
- Fuel shortage
Most aircraft have checklists and procedures for some specific emergencies such as engine failure, in-flight fires, and loss of pressurization. However, it is obviously impossible to plan for every possible scenario.
When an emergency occurs, the pilot may make a general mayday radio call or inform Air Traffic Control that he is "declaring an emergency". An aircraft in distress has priority over all other air traffic and the pilot is generally allowed to take any action necessary to ensure the safety of the flight, even if that action might not be allowed in normal circumstances.
If an aircraft cannot reach its original destination as planned because of an emergency, the pilot may choose to land at a different airport or make a forced landing if there is no other alternative.