An emergency landing is a premature landing forced by an imminent threat to the aircraft's safety or a sudden need to terminate the flight, such as a medical emergency, often requiring a diversion to the nearest suitable airfield or an off-airport landing. There are three main types: a forced landing (caused by critical system failure, prioritizing immediate landing), a precautionary landing (due to abnormal conditions or limited information), and a ditching (a forced landing on water). When engine power fails, pilots rely on gliding (for fixed-wing aircraft) or autorotation (for helicopters), and they practice simulated forced landings, with light aircraft capable of landing on fields or roads, while larger airliners require long, prepared runways. Notable real-world examples include the Gimli Glider (an Air Canada Boeing 767 that ran out of fuel and glided to safety in Manitoba on July 23, 1983), British Airways Flight 9 (which lost all four engines after flying through volcanic ash in 1982 before recovering), and Aloha Airlines Flight 243 (which suffered explosive decompression in 1988). Since 2003, researchers have been working on enabling unmanned aerial vehicles to perform forced landings autonomously, and upon declaring an emergency, flights receive priority over all other air traffic.