# Can cabin pressure be lower than outside pressure?

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?

• 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 '14 at 19:18
• Here's what happens if it is ... youtu.be/Zz95_VvTxZM or youtu.be/JsoE4F2Pb20 – Brian Drummond Jan 2 '15 at 11:50

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.

• 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 '14 at 11:21
• @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 '14 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 '14 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 '14 at 13:34
• @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 '14 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.

Can an aircraft cabin have a lower pressure than outside

Yes, absolutely is CAN. It's not desirable, but if the seals are all tight enough & you descend quickly enough, it can happen. Because the effects on the structure would be bad, aircraft typically have a negative pressure relief valve to prevent this from happening. By definition, you don't need to prevent something that can't happen, so yes, a negative pressure differential is possible.

What you want to avoid, and with modern pressurization controllers, you probably can't reach, is a situation where you descend quickly enough to "catch the cabin" and the cabin altitude matches the aircraft altitude. (Which is to say, you're depressurized -- with or without the negative pressure relief valve opening -- a few thousand feet above the ground.) At that point (assuming you're still descending quickly), the cabin rate goes from something mild, a couple hundred feet-per-minute, to something much greater, perhaps a couple thousand feet-per-minute. If you aren't used to dealing with that sort of a rate of change, it is disconcerting & uncomfortable, and for somebody who is flying with a minor head cold who could keep up with a gentle rate of change, it could be pretty painful. And since nobody wants to scare Grandma, this is undesirable!

Thankfully, with modern pressurization controllers, you probably won't ever get there -- they'll manage the rate to keep things on schedule regardless. With an older analog controller, it's possible, although probably pretty rare.

As explained in other answers, the modern controllers start to pressurize so that the cabin altitude goes slightly below the airport elevation (i.e. the cabin pressure rises slightly) at some point before liftoff (when throttles are first advanced, typically -- it's all automatic). And on landing the cabin is likewise slightly pressurized, and that pressure bleeds off slowly after you have weight-on-wheels. This is done so that the system transitions into and out of pressurization gradually, instead of immediately upon liftoff or on landing. The outflow valve moving at full speed can give you a pretty good "pop," and that's not comfortable, so gradual transitions are designed in.

It's all about keeping the paying passengers comfortable & happy!

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.

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'.

• Weird, you guys actually need to dump before opening the canopy? The T-38 won't go back to ambient on its own descending through 8k? Pretty cool looking airplane though, and **** you for having AB's on your trainer. The T-45 could hit the number if was in mil and dove from like 40k. Fastest I ever heard a stud get was .96. – Rhino Driver Nov 24 '14 at 18:36
• Yah it will go back to ambient passing 8k, but you could land before it fully reaches ambient. With the non-PMP models you could hit 1 pretty easily but with the PMP mod you lose a little bit at cruise. I hit .98 at altitude in MIL in a dive. – user3309 Nov 25 '14 at 15:19
• Are you sure the Ram Dump isn't there mainly to prevent opening the canopy with positive pressure inside, which would result in at least your ears popping, and perhaps a more violent opening of the canopy that is good for the mechanism that moves it up & down? – Ralph J Mar 11 '15 at 23:09