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In order to allow adaption to changed pressure, the pressure change in an aircraft cabin is stretched out over time. My observation is that the cabin pressure drops even before the plane takes off, and has not yet reached ground level at the time the aircraft lands. This would make sense, allowing for longer adaption times, but needs better sealing which is able to work both ways.

My question to the systems specialists: Can an aircraft cabin have a lower pressure than outside, and is the technique of stretching out the pressure change as explained above actually used?

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I wonder if modern day hulls can even support a negative pressure diff. Most of them are built on the assumption that outside press is equal or several psi below internal –  Radu094 Aug 24 at 19:18

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Besides the sealing the aircraft would also need a vacuum pump that is able to pull air out of the cabin which means extra weight.

The way aircraft maintain cabin pressure is through a air cycle machine that pushes air into the aircraft and outflow valves. There is no facility to pull air out of the craft.

Any pressure drop you may experience on the ground is probably because the pilot increases air pressure on the ground slightly to test the seals and then releases it before takeoff.

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To create a lower pressure is easy with an eductor pump. Bleed air will provide the energy, and the performance is easily sufficient to suck a little air out of the cabin. The mass of eductor pumps is really low. After all, the pressure drop should only be slight - I do not want to create a vacuum. There is really no such means for regulating pressure? –  Peter Kämpf Aug 24 at 11:21
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@PeterKämpf Why would there be such a mechanism? The only reason that planes need to be pressurized is that humans are feeble and can't cope with low pressure. All you need to solve that problem is a mechanism to increase the pressure if it gets too low. The outside air pressure is never too high for passenger comfort or safety, so why would you need a mechanism to lower cabin pressure below external atmospheric? –  David Richerby Aug 24 at 13:10
    
@DavidRicherby: In order to stretch out the time for the pressure change. Rapid pressure change is uncomfortable; humans need to adapt even if the change is only 0.3 bar. –  Peter Kämpf Aug 24 at 13:31
    
@PeterKämpf and the time is already stretched, planes don't go to 8k ft and then keep cabin pressure stable, –  ratchet freak Aug 24 at 13:34
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@PeterKämpf as mentioned, the time of pressure change is already stretched. In the EMB145 the cabin is maintained at departure elevation for the first 10 minutes or so, then climbed at ~500 fpm or less until reaching 8k cabin altitude at cruise and ~7.8 dPSI. On descent the cabin will descent at ~400 fpm over the course of the descent. The cabin pressure change schedule is independent of the actual aircraft climb and descent rates (except where max delta PSI is limiting, but the scheduling is smart enough that this isnt really an issue). –  casey Aug 25 at 4:19

The A320 family as well as the B737NG series of aircraft start to pressurize during the takeoff roll. The cabin pressure is increased by about 0.1 Psi as this makes pressure changes on the outflow valve during the roll and the rotation less noticeable.

Only during a descent with the outflow valve closed, the cabin pressure could become lower than the ambient pressure. To prevent damage due excessive under pressure aircraft are fitted with inflow valves that open automatically if the pressure differential exceeds a threshold.

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No aircraft is capable of reducing pressure below ambient - that would require pumps going the other way, and there's zero reasons to do it. Your observation of the cabin pressure dropping is an illusion, probably the pressure increasing a small amount as the pressurization system comes online.

It is conceivable that an aircraft could have a negative pressure differential on descent. If it descends very quickly it could have in internal pressure of 8,000 feet (normal cruise pressure) but be below that altitude. The difference would not last long, but it does take time to pump up a widebody - a fully pressurized 747 holds an extra ton of air, engines are at idle and the outflow valve (inflow in this case) isn't all that big.

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To my knowledge there is no aircraft that has the equipment to purposely create a lower pressure inside the cabin. However, as previously mentioned, it is possible to descend at a rate that results in a lower pressure inside the cabin.

In the T-38 we will actually RAM DUMP before opening the canopies for this exact reason. The pressurization schedule is such we are at ambient pressure until reaching 8000' before it begins to pressurize to stay at a cabin altitude of 8000' until 23000'.

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