In the cabin of an aircraft, pressure is always held at comfortable levels corresponding to about 8 000 ft. Tins allows for proper function of the human biological system. If a hole in the pressure hull of an aircraft develops, not only oxygen must be provided to the passengers, but the aircraft has to descent immediately to an acccptabcl altitude Even when aspirating pure oxygen, it cannot be transferred to the blood at pressure levels comparable to more than about 35 000 ft altitude; and sudden decompression will cause the blood to boil after a short time.
In ease of a pressure loss, therefore, the aircraft has to descend immediately and rapidly. During this steep descent, pressure levels in the cabin must never fall below cabin pressures of 40 (XX) ft; after at last two minutes a pressure altitude of at most 24 000 ft must be reached To descend fastly w ithout destruction of the aircraft, generation of drag is crucial (high altitude of about 50 000 ft must be reduced to 24 000 ft and simultaneously supersonic speed must be reduced to subsonic speed, both parts contributing almost the same part of required energy destruction).
There arc two main reasons for sudden decompression:
– engine burst:
probability of large holes can be reduced by suited positioning of the (most critical inner) enginc(s).
– "20 ft2 hole" (size may be a bit smaller for a narrow fuselage):
size of this hole is pure geometrical. It was introduced after DC-10 accidents (which lost doors) and is mainly a door size. There is no measure to reduce probability or increase safety against this hole. e g. by building double doors or a double hull; because geometry deliniuon is not influenced by it It cannot be seen now how this rule will be replaced by a physically based rule in the future.
At the high cruise levels of an SCT it is impossible lo maintain the requested pressure levels using conventional techniques. Really new technologies and intelligent solutions are needed