Descent of Aircraft A320 operated by Lufthansa from Frankfurt, Germany to Lisbon, Portugal.
A descent during air travel is any portion where an aircraft decreases altitude, and is the opposite of an ascent or climb. Descents are part of normal procedures, but also occur during emergencies, such as rapid or explosive decompression, forcing an emergency descent to below 3,000 m (10,000 ft) and preferably below 2,400 m (8,000 ft), respectively the maximum temporary safe altitude for an unpressurized aircraft and the maximum safe altitude for extended duration. An example of explosive decompression is Aloha Airlines Flight 243. Involuntary descent might occur from a decrease in power, decreased lift (wing icing), an increase in drag, or flying in an air mass moving downward, such as a terrain induced downdraft, near a thunderstorm, in a downburst, or microburst.