Date: 22 Feb 99 03:30:16 From: Puranjai Dass <PJ@CAAF.ORG.FJ> Followups: 1 2 3 4
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This question is directed towards experts in Aircraft Performance & Airframe Pressurization (sorry for sending such a long email):- I work for a regulatory authority and I am helping senior investigators who are writing a report on a depressurization incident involving a B737-500. They requested me to graph the variation of both the actual aircraft altitude and the cabin pressure altitude with time by extracting data from the ATC transcript. I need some help on this graph. The details of the incident are as follows:- " The aircraft was cleared by Approach Control to descend from FL 370 to FL 350 then re-cleared to descend to 10,000 ft. At FL 360 the F/O called "one to go". Just below FL 360 the Master Warning sounded and the Left Bleed Trip-Off light came on. The F/O reached for the QRH. 15 to 20 seconds later the Right Bleed Trip-Off Light illuminated. The F/O operated the Trip Reset button, then turned to the Captain who was putting on his oxygen mask. The F/O donned his mask. The cabin altitude warning (an intermittent horn) sounded at 10,000 cabin altitude and this was silenced by the F/O operating the horn cutout. He then armed the passenger oxygen switch. The aircraft cabin altitude continued to climb. Approaching a cabin altitude of 14,000 ft the Captain ordered manual deployment of the passenger oxygen masks. At about FL340 in the descent both bleed lights extinguished and the duct pressures returned to a normal reading. An emergency descent was initiated. The F/O selected manual mode of the pressure controller and manually operated the outflow valves to the closed position. A duct overheat illuminated at about the halfway point during the descent. This was extinguished after reference to the checklist. The aircraft was leveled out at 5,000 ft. The descent from cruise level to this altitude lasted approximately 15 minutes. The cabin outflow valve was opened at 5,000 ft. The aircraft was now 100 nm from the destination and continued to descend for a visual approach and landing." As a result of this incident a passenger suffered permanent ear damage. The senior investigators believe that the pressure differential was maintained at 8.65 psi (by the safety relief valves) some time after the F/O manually closed the outflow valve (passing 34,000 ft) until the a/c reached the 5,000 ft altitude when the outflow valve was re-opened. If we assume the pressure differential reached 8.65 psi when the a/c was at say 31,000 ft then the cabin altitude would be 4,000 ft (using standard atmosphere calculations). My problem is - how do you express negative cabin pressure altitudes since the cabin pressure was definitely below sea level when the a/c reached 5,000 ft. As far as I know all the pressure equations are based on altitudes of sea level and above. Negative altitudes are not a reality of life, are they? I would be very grateful if the experts could help me out.