Date:11 Mar 99 03:54:09From:"K Hall" <kevin.hall@cwcom.net>Organization:Pavilion Internet USENET ServerReferences:1Followups:1

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Negative pressure altitudes do exist. The pressure law can be extended below sea level to give an equivalent altitude. Most mechanical altimeters will read down to negative 10,000 feet. Negative pressure altitude represents a static pressure in excess of 1013 millibars (ISA sea level pressure) and therefore a height below mean sea level. Assuming the cabin differential of 8.65 psi you quote, at 31,000 feet the cabin pressure would have been 12.82 psi. This represents a cabin altitude of 3,730 feet. By the time the aircraft had reached 5,000 feet the cabin pressure would have increased to 20.88 psi. This is equivalent to approximately 10,000 feet below mean sea level. The high pressure would not in itself cause ear damage, but the sudden pressure reduction resulting from opening the outflow valves at 5,000 feet might. In summary, if you have difficulty in expressing negative cabin altitudes, try expressing them in terms of cabin pressure instead. The cabin altitude is only an equivalent altitude after all, not an actual one. Hope this helps Kevin Hall