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A gradual (or subtle) depressurisation occurs over a long period time and, due to the gradual change in air pressure, it can be difficult to recognise before cabin altitude warning devices activate or oxygen masks fall from the cabin ceiling.
Gradual depressurisation will usually occur due to a leak in the aircraft pressure vessel, a malfunctioning outflow valve or a reduction in the cabin air inflow due to a malfunctioning compressor or other component. In addition to a gradual loss of cabin air pressure, there have been cases of aircraft failing to pressurise after takeoff, either due to equipment malfunction or incorrect manipulation of the pressurisation controls. If this is not detected and corrected by the flight crew before the aircraft reaches cruising altitude, the lack of pressurisation can also be difficult to recognise and may result in incapacitation due to Hypoxia.
- Aircraft Pressurisation Systems
- Explosive Depressurisation
- Rapid Depressurisation
- Decompression Sickness
Accidents and Incidents
B733, en-route, northwest of Athens Greece, 2005 (Helios 522 - On 14 August 2005, a B737-300 aircraft belonging to Helios Airways, crashed near Grammatiko, Greece following the incapacitation of the crew due to Hypoxia)
- See FAA "Lessons Learned from Transport Airplane Accidents": Pressurization / Decompression Failures