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Right now at my university we are busy with a project for designing a conceptual aircraft. Our aircraft is in the CS-23 range, this airplane is a small cargo aircraft with the option for carrying injured passengers (so like an ambulance aircraft).

Our aircraft will have a cruise height at 10,000 ft. We want to make this aircraft pressurised but our question is therefore, is it possible to pressurise the aircraft from inside below the normal 8,000ft. So for example, we are flying at 10,000 ft but we want to have the inside pressure having the same as on sea level if that is possible, or is it only possible to pressurise the inside at 8000ft.

Thank you in advance.

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    $\begingroup$ @GHB, aircraft cruising at 10,000 does not need either pressurization nor supplemental oxygen. 10,000 ft is considered safe. Rare medical conditions where it might be a problem are more easily handled by portable oxygen; such patients are likely to need a ventilator, at least available, anyway. $\endgroup$
    – Jan Hudec
    Commented May 13, 2016 at 10:06
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    $\begingroup$ @Simon: If the cabin is hermetically sealed to maintain the pressure when the door was closed, two very bad things will happen: You'll run out of oxygen in a matter of hours, and the humidity from exhaled breaths will fog up all the windows and begin to condense on all the instruments. Pressurized airplanes continually bring in fresh air and cycle out the air already in the cabin. The airplane is not literally sealed tight. It just has a compressor (and heater and humidifier) between the outside air and the cabin. $\endgroup$
    – abelenky
    Commented May 13, 2016 at 12:20
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    $\begingroup$ @abelenky I know. I was simply saying that it is entirely down to the designer what happens using all the usual compromises. There is nothing preventing someone from designing an hermetically sealed fuselage pressurised to sea level. $\endgroup$
    – Simon
    Commented May 13, 2016 at 12:41
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    $\begingroup$ 8,000' is not a "Normal" cabin pressurization altitude. It is the MAX allowable cabin altitude for a pressurized aircraft. Take any pressurized aircraft and fly it below it's max cruising altitude and the cabin alt will be far below 8,000'. Flying at 10,000 feet you will be able to maintain the cabin much lower than 8,000' with even a very modest cabin pressure differential. One exception to this is when flying to Bogota at 8,361'. Airline SOP requires you to keep the cabin at 8,361' for the entire flight. I would always keep the cabin at 6,000' and then raise the cabin prior to landing. $\endgroup$
    – Mike Sowsun
    Commented May 13, 2016 at 12:46
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    $\begingroup$ @JanHudec If you are transporting someone who was injured or became ill while scuba diving, and they are not currently suffering decompression sickness, you might be able to transport them safely without using a decompression chamber if the cabin is pressurized to sea level, but you might not want to expose them to any lower pressure. Oxygen is not the problem here, it is purely and simply (as far as I understand it) a matter of the ambient partial pressure of nitrogen. $\endgroup$
    – David K
    Commented May 14, 2016 at 13:33

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If you design the aircraft, shouldn't you be answering that question? If it is part of your aircraft's mission to fly at 10.000ft whilst maintaining sea level cabin pressure, then you should design it that way.

Why would it only be possible to pressurize the inside to 8000ft? Not to 9000ft, 7000 ft or any other arbitrary altitude?

Of course the construction of the aircraft needs to be able to withstand the forces caused by pressure difference between the inside of the cabin and the outside. And the pressurization system needs to be able to maintain the pressure at sea level, so compensating for any air leaks in the pressure vessel.

For an initial feasibility study you could look at the pressure difference on the hull when an aircraft is flying at FL430 whilst maintaining cabin pressure at 8.000ft. Note thate the Boeing 787 maintains the cabin altitude at 6.000ft, while operating at 43.000 ft.

Compare that to the pressure difference between 10.000ft and sea level. Looking at those pressure differences should give you a feeling whether or not it is easy to achieve your objective.

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  • $\begingroup$ Yes, this. Specifically the first paragraph. $\endgroup$
    – Michael Hall
    Commented Feb 22, 2021 at 3:43
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The SyberJet SJ30 business jet flies way higher, around FL410, and it is pressurized at sea-level. Composite materials help in coping with extreme cycles.

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    $\begingroup$ An even more extreme example would be the International Space Station, which is maintained at sea level pressure while in near vacuum (although of course, I assume pressurization cycles are only an issue for the airlocks) $\endgroup$
    – Sanchises
    Commented May 13, 2016 at 11:26
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    $\begingroup$ @sanchises, ISS has advantage of not going through any cycles. $\endgroup$
    – Jan Hudec
    Commented May 13, 2016 at 21:26
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    $\begingroup$ @Jan yes, isn't that what the second part of my comment addresses? $\endgroup$
    – Sanchises
    Commented May 14, 2016 at 12:17
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Pressurized aircraft such as LearJets are designed to maintain sea level pressure up to a certain altitude, say 28,000 feet. Above that altitude, the cabin pressure begins to fall so that at 43,000 feet in this example, the cabin pressure is about 5,000 feet of altitude. However, air ambulances can tell the tower what altitude they will fly so that they can maintain sea level pressure in the cabin.

Airambulance.com

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  • $\begingroup$ Not just medevac, but any aircraft's pilot can tell ATC (not just tower) what altitude they can accept—ATC will provide separation at any altitude. If vertical separation is not available they can vector aircraft around each other. Just because an aircraft has a specific service ceiling does not mean they have to fly that high 100% of the time. $\endgroup$
    – randomhead
    Commented Feb 21, 2021 at 22:00

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