As an aside of the general public not knowing about pressurization: There was a flight on an Australian airline not that long ago where a pair of new born babies (twins) stopped breathing. The flight was diverted to Adelaide I believe. The babies were some weeks premature and there was insufficient oxygen density even at the cabin pressurization height to keep them alive.
Fortunately there was a doctor on board and the cabin staff had portable oxygen systems. I remember being surprised at the time that the airline hadn't warned the mum. From my days in the Air Force and defence families traveling on military aircraft, the air movement staff were always warning the parents with young babies about the risks with pressurization (eg unable to equalize the ear pressure resulting in damage to ear drums and all the baby is doing is screaming because of the pain and the parents don't know why). Anthony -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mike Cleaver Sent: Monday, 30 August 2004 10:36 PM To: Discussion of issues relating to Soaring in Australia. Subject: Re: [Aus-soaring] Oxygen and airline aircraft pressurisation At 11:12 PM 27/08/2004 +0930, Terry N wrote: >I've been wondering about one other aspect of this fascinating >discussion..... We may have talked about this before. > >Does anyone know to what altitude the cabin of commercial jets are >pressurized when at normal cruising levels ? I did hear once (in the >B-727 era) that it was somewhere around 6,000 to 8,000 feet. > >Is there a manufacturer's recommendation? > >Is there a company policy? > >Are there regulations on the subject? > >The easy answer would be to take an altimeter in one's carry on luggage at >some time in the future. Unfortunately that would probably be viewed with >considerable suspicion, even if it passed the security check. Terry With the exception of the Lockheed Hercules (both Civil and Military versions) which can sustain sea-level pressure to somewhere over 10,000 ft, (hence useful for transporting divers with the bends to the closest hyperbaric chamber for recovery and treatment), most airline aircraft are pressurised to some pressure above ambient but less than sea level. Usually this is operated as a differential which is programmed into the aircraft systems, and reaches the maximum somewhere approaching the service ceiling of the aircraft. Incidentally, this is often defined by a corner of the flight envelope bounded by stall speed approaching Vne limited by flutter considerations, or with power available limiting speed as drag rises with angle of attack. Early model aircraft tended to have a maximum differential of less than 5psi (250 mmHg or 330hPa) which gives an 8000 ft cabin altitude at around 25000 ft. The accidents to early aircraft were often caused by metal fatigue die to the number of pressurisation cycles being underestimated - the DH Comet being the best-known. As designers have sought to fly higher to use more efficient engines and airfoils, they have needed to build structures that withstand greater pressure differentials - directly opposed by the fact that a small radius of curvature is better at containing pressure than a large one. The newer offerings from Boeing and Airbus use around 8.5 psi differential to cruise at up to FL 450, whilst Concorde needed more like 10 psi to cruise at up to FL 570. There are persistent rumours floating around that crews used to set a higher cabin altitude on overnight flights and with football teams on board, to encourage the passengers to sleep - but I have never been able to confirm any such rumour. I used to wear an altimeter watch, and could tell the cruise level in a B747-400 by its reading - flights to the USA usually cruise at FL 280 or 290 initially, rising to FL 410 as they got lighter from burning fuel (average fuel load out of Sydney for LAX is about 150 tonnes) so able to create enough lift at the most efficient angle of attack to cope with reducing air density. It would go from about 6500 ft at FL290 to 8850 ft at FL 410, with the differential they use. The design policies are set by a trade-off between fatigue, structural considerations, and design cruise ceiling - and also passenger safety. Above about 53,000 feet oxygen on of no assistance to unprotected humans, as at that height the ambient pressure is so low that their body fluids (blood, saliva, tears etc) boil at body temperature - which is why Steve Fossett will need a pressure suit to make much inroads into Bob Harris's absolute altitude record. Most regulatory authorities protect their travelling publics by requiring oxygen when the cabin altitude is above 13000 or 14000 feet, and requiring them to carry emergency oxygen for all occupants for 20 to 30 minutes only - so if you suffer depressurisation (like Hawaiian Airlines 737 or the United 747 out of Auckland some years ago, you have to descent to around 13000 ft and divert to an alternate because there is rarely enough fuel on board to fly a jet this low and have the endurance to reach your destination. Hope this is of interest. Wombat _______________________________________________ Aus-soaring mailing list [EMAIL PROTECTED] To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring _______________________________________________ Aus-soaring mailing list [EMAIL PROTECTED] To check or change subscription details, visit: http://lists.internode.on.net/mailman/listinfo/aus-soaring
