A query about cabin pressurisation....
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A query about cabin pressurisation....
One thing struck my attention while on an airline flight recently.
I know that the cabin pressure during flight is reduced to the equivalent of about 8000 feet. Therefore a passenger should feel a change in cabin pressure as the plane climbs up to 8000 feet. Theoretically, passed this altitude, he should feel no more changes because the cabin is then held at this pressure for the rest of the climb. However, I noticed that as the plane started its descent from 37000 feet, I immediately felt my ears popping, even though the pressure shouldn't have changed until we passed back through 8000 feet. Can somebody explain this to me?
Another point about this - I took eight flights in the last week and noticed that it didn't happen every time - some times my ears barely popped at all, other times it felt quite sore. Has this anything to do with the regional QNH?
If it's any help, this effect was greatest in the DC-9s. I also flew A-321, B777, and B717.
I know that the cabin pressure during flight is reduced to the equivalent of about 8000 feet. Therefore a passenger should feel a change in cabin pressure as the plane climbs up to 8000 feet. Theoretically, passed this altitude, he should feel no more changes because the cabin is then held at this pressure for the rest of the climb. However, I noticed that as the plane started its descent from 37000 feet, I immediately felt my ears popping, even though the pressure shouldn't have changed until we passed back through 8000 feet. Can somebody explain this to me?
Another point about this - I took eight flights in the last week and noticed that it didn't happen every time - some times my ears barely popped at all, other times it felt quite sore. Has this anything to do with the regional QNH?
If it's any help, this effect was greatest in the DC-9s. I also flew A-321, B777, and B717.
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Your pretty much correct in that the cabin pressure is around 8000ft. However that is not until the aircraft is cruising at around FL390. At Fl 350 is it around 6000 ft. The pressure is reduced gradually from take off elevation to cruising altitude, it doesnt just maintain ambient external pressure to 8000 ft and then hold that. If that was the case your ears would really "pop" especially if the reciprocal were true on the descent.
Modern jets operate on a pressurisation schedule which carefully regulates the pressure during all phases of the climb and descent to provide smooth pressure change gradients. The whole subject does get fairly technical and involves the time an aircraft maintains a given altitude, the landing field elevation, the final cruise altitude, descent and climb modes etc. However this is a fairly simple explanation which should answer your question.
Modern jets operate on a pressurisation schedule which carefully regulates the pressure during all phases of the climb and descent to provide smooth pressure change gradients. The whole subject does get fairly technical and involves the time an aircraft maintains a given altitude, the landing field elevation, the final cruise altitude, descent and climb modes etc. However this is a fairly simple explanation which should answer your question.
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The air for pressurisation is normally supplied from the low pressure compressor side of the engines. At the start of descent the throttles are retarded so the engines are at idle. The pressure and indeed the volume of air supplied from the LP compressor would not be enough to maintain pressurisation with the engines at idle so there is an automatic valve changeover within the engine bleed system to supply the air from the high pressure compressor. It is not uncommon for a pressure "bump" to be felt during this change in bleed source.
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As Jetdriver mentioned, pressurization schedules (differential pressure, climb/descent rates)are programmed into auto controllers with manual backups. As the aircraft descends, the cabin will depressurize as a function of the amount of altitude (aircraft and cabin) to be lost prior to landing. Most jet aircraft land either depressurized or extremely low differential pressures (which is relieved on touchdown). Couldnt open the plug doors otherwise.
One little bit of pressurization trivia. Some modern (corporate) jets are operating at differential pressures over 10.0 psid. Airliners run around 8-9 psid. The folks who design nuclear weapons consider the "ring of destruction" around a blast the .5 psid overpressure ring. The forces involved in a pressurized cabin are enormous - that is why very large things can be forced through very small holes when there is a failure.
Shore Guy
One little bit of pressurization trivia. Some modern (corporate) jets are operating at differential pressures over 10.0 psid. Airliners run around 8-9 psid. The folks who design nuclear weapons consider the "ring of destruction" around a blast the .5 psid overpressure ring. The forces involved in a pressurized cabin are enormous - that is why very large things can be forced through very small holes when there is a failure.
Shore Guy
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Depends on the window. Cockpit windows have to withstand both dynamic and pressurization loads (at some point during the climb/descent in a pressurized aircraft, there is zero differential pressure between the inside and outside - I have no idea how to compute when). Dynamic loads must include protection from bird strikes. They are usually multi-layered laminated plastic/glass with a conductive for (electric) windshield heating. Most are around ¾ to 1 inch thick. By the way, heating a windshield is not just for deicing/ defogging, but to “soften” it (reduce brittleness) for bird strike protection. Many aircraft have, in addition to icing condition restrictions, lower speed restrictions when window heat is inop. For some more info, check out: http://www.ppg.com/gls_ppgglass/aircraft/b757win3.htm
Passenger windows are much thinner (pressurization loads only). Warm air is routed between the inner and outer panes for defogging.
Early Learjets had acrylic windshields with external bleed air heating – and different Vmo’s for different altitudes. Around 300KIAS below 14,000’, 350+KIAS above 14,000 for bird strike protection. I wasn’t very comforted with these numbers when one day at 21,000 ft. a flock of Canada geese went OVER the airplane. Our ground school instructor was fond of saying that the “windshield was stressed for a four pound bird at 306 Knots or a 306 pound bird at 4 Knots”
Hope this helps.
Passenger windows are much thinner (pressurization loads only). Warm air is routed between the inner and outer panes for defogging.
Early Learjets had acrylic windshields with external bleed air heating – and different Vmo’s for different altitudes. Around 300KIAS below 14,000’, 350+KIAS above 14,000 for bird strike protection. I wasn’t very comforted with these numbers when one day at 21,000 ft. a flock of Canada geese went OVER the airplane. Our ground school instructor was fond of saying that the “windshield was stressed for a four pound bird at 306 Knots or a 306 pound bird at 4 Knots”
Hope this helps.
