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Seems like it could save money perhaps. Has this been considered in the past?

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    $\begingroup$ How would they load & unload freight & luggage? $\endgroup$
    – Ralph J ♦
    Jan 29, 2020 at 17:18
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    $\begingroup$ Probably the fastest 4 answers I have seen... This one was low hanging fruit, thanks for the easy pitch! $\endgroup$
    – Michael Hall
    Jan 29, 2020 at 17:25
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    $\begingroup$ Can you imagine all the wailing babies in the vicinity of the airlock? $\endgroup$
    – rclocher3
    Jan 29, 2020 at 22:31
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    $\begingroup$ @MichaelHall Gotta let people get their rep up (: $\endgroup$
    – Conor Henry
    Jan 30, 2020 at 8:49
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    $\begingroup$ Could you edit this question to explain how you think that might save money? $\endgroup$
    – Tanner Swett
    Jan 31, 2020 at 13:22

6 Answers 6

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I believe there are quite a few misconceptions here:

  • When an aircraft is "pressurised", it means that at higher altitudes, the pressure inside the aircraft is higher than the pressure outside. At lower altitudes, the pressure is exactly the same inside and out.

    Originally, aircraft weren't pressurised, and at higher altitudes the low pressure is a problem for the people on board (lack of oxygen, mostly), and for some hardware (due to the pressure differential).

    So the pressure is artificially increased inside the aircraft beyond a given altitude (or more precisely, beyond a given outside pressure): when the aircraft takes off, the pressure goes down as the plane reaches higher altitude, until that pressure is reached, and from then on, the pressure outside continues to decrease while the pressure inside is maintained at that safe level.

    So there's no reason to pressurise an aircraft on the ground. It has the same pressure inside as outside.

  • Even if it were useful, when on the ground, there are plenty of openings other than just the passenger door(s). Other doors are opened for catering and technical staff, cargo doors are open, and so on. So it would be impossible to maintain a different pressure in the aircraft and outside without completely breaking down all operations.

    Also, having an "air lock" would require doors on each side. One door would be opened, people would enter the air lock, the door would be closed, the pressure would be changed, the other door would be opened, people would exit the airlock. Then the door would be closed again, pressure brought back to the other level, and the first door would be opened again before more people would be able to enter the lock. This would significantly hamper the flow of people embarking or disembarking.

    Additionally, people are already annoyed enough by the changes in pressure on board planes which take quite a few minutes. Doing that in a few seconds to ensure relatively quick movement would just be horrible.

    Finally, consider that many aircraft do not use jetways but stairs on the tarmac.

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    $\begingroup$ Great comment. Especially the point near the end. You could only change the pressure altitude in the airlock about 500 feet per minute (1000 ft/min at best). Even if you had an airlock big enough to accommodate the entire plane load of passengers, that would add about 15 minutes per plane load in DFW. Or, you would have to pressurize (depressurize in the case of DFW) the entire building. $\endgroup$
    – Dean F.
    Jan 29, 2020 at 17:37
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    $\begingroup$ Another point is that pressurization in flight is a continuous process, exchanging fresh outside air for cabin air that's full of exhaled CO2. So you'd have to run the plane's pressurization system (or the airport's) on the ground as well, costing even more money. $\endgroup$
    – jamesqf
    Jan 29, 2020 at 18:01
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    $\begingroup$ Pressurizing an entire building isn't impossible, @DeanF. (I know, you didn't say that.) Look at all the stadiums that have air supported roofs*. There are loads of revolving doors to keep the pressure inside, and the ear pop while entering is very annoying, but they're used on a regular basis. *The "Hoosier Dome" (later renamed "RCA Dome") in Indianapolis, and the "Humphrey Dome" in Minneapolis are two that come immediately to mind. $\endgroup$
    – FreeMan
    Jun 22, 2021 at 15:55
  • $\begingroup$ @FreeMan - I never said pressurizing the entire building was impossible. I essentially said it was impractical. Remember, regardless of whether you pressurize the air around the passengers at the jetway or at the front door of the building, it would be a drastic pressure change depending on where the building was. DFW is roughly 500 feet Pressure Altitude compared to 8000 feet cabin pressure altitude. The pressure difference between the inside and outside of the domes you mentioned was minimal. $\endgroup$
    – Dean F.
    Sep 7, 2021 at 22:04
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Apart from the practical implications, aircraft pressurise their cabins with the bleed air coming from the jet engines. This air is compressed by the engines and used for the combustions process. So there are no seperate air compressors. If you want to have the aircraft compressed at the ground, you will need to either keep the engines running or add equipment to the aircraft. The first thing is not beneficial and the latter will increase the fuel consumption of the aircraft.

Have a look at the following picture: enter image description here

From the picture it can be seen that the pressurization level of the cabin equals the pressure level at 7000ft. The means that if you keep this pressure then on the ground there is an underpressure in the aircraft, aircraft doors and seals are not designed with this in mind. The aircraft door for example is designed such that it can't be opened mid flight since the pressurisation of the cabin pushes the door outwards in the seals. This is the reason why aircraft doors and emergency hatches first move inwards before opening to swining outside of the aircraft.

To summarize, aircraft pressurization is there for the comfort of the passengers and is therefore kept at a comfortable level 7000ft. If the aircraft flies lower then, the inside pressure equals the outside pressure. And seconly, aircraft are not designed for the cabin to be of a lower pressure than the outside of the aircraft.

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    $\begingroup$ Comfort and brain function/health. $\endgroup$
    – Dean F.
    Jan 31, 2020 at 17:07
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The only reason planes are pressurized is to allow the passengers the ability to breathe at flight altitude as if they were on the ground. If the plane is still or already on the ground, there would be no purpose in pressurizing it.

Most metal bodied aircraft pressurize their cabins to about 8000 feet cabin pressure altitude. If the outside air is at a pressure altitude below 8000 feet, the engines are just supplying air conditioning and circulation to the cabin, not necessarily added pressure.

The average human can breathe perfectly well below 15000 feet. Some may feel mild discomfort above 10000 feet. The aircraft engines breathe air and eat fuel best at altitudes of two to three times that. The airlines save money by flying at altitudes where they get the best fuel efficiency. Unfortunately, our bodies are not efficient like engines at that altitude. We would pass out or pass away. The average human has between 6 and 60 seconds of useful consciousness at flight altitudes.

The way to save money is to reduce the self-loading-cargo’s reliance on oxygen. Putting all the passengers asleep while wearing positive pressure oxygen masks would eliminate the need to pressurize the entire cabin 😜.

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    $\begingroup$ But that many oxygen masks, which would need to be inspected & maintained for the life of the aircraft, would probably be more expensive than building a pressurized hull. $\endgroup$
    – jamesqf
    Jan 29, 2020 at 17:59
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    $\begingroup$ Sooooooooooo, you’re saying just put them to sleep with no mask. OK. $\endgroup$
    – Dean F.
    Jan 29, 2020 at 18:03
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    $\begingroup$ By the way, jk. $\endgroup$
    – Dean F.
    Jan 30, 2020 at 5:45
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Below around 8000' MSL aircraft pressurization is roughly equal to the outside air pressure. Since most airports are below this elevation, there is no pressure differential once the airplane is on the ground, therefore no need or even possibility for an airlock.

It takes very little bleed air to pressurize when airborne so there isn't a significant cost in doing so. Furthermore, jetways aren't sealed so the additional cost in equipment and procedures to implement this plan even if it were possible or desirable would far outweigh any savings.

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    $\begingroup$ Your first comment about the differential pressure being roughly equal below 8000 is inaccurate. I think you may have some misconceptions about how these systems work. These systems typically start pressurizing before or just after takeoff. The pressure at, say 5000 MSL after taking off from sea level, might be around 500 ft cabin altitude. $\endgroup$
    – J W
    Feb 1, 2020 at 2:20
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    $\begingroup$ You are correct, I don't fly pressurized passenger planes. I'm ex Navy. The EA6B was ambient to 8k. And the Caravan is of course unpressurized. $\endgroup$
    – Michael Hall
    Feb 3, 2020 at 1:25
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    $\begingroup$ Ok, yeah I can imagine that military pressurization systems might be substantially different from pressurized passenger aircraft. $\endgroup$
    – J W
    Feb 3, 2020 at 12:20
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    $\begingroup$ Thanks for pointing out that my answer may not be as universally correct as I hoped it would be. I may edit later to incorporate your comments, but I think the gist of it is sufficient to discredit the idea of a need for airlocks. Although maybe I wont bother now, because there are better answers posted. $\endgroup$
    – Michael Hall
    Feb 3, 2020 at 15:06
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    $\begingroup$ No problem. Your overall point is definitely valid. $\endgroup$
    – J W
    Feb 3, 2020 at 17:20
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Besides the fact that this is unlikely to be kind on passengers to overpressurize, the passenger doors and the jetways aren't the only portal into the pressurized space. The luggage going in the belly and the servicing carts coming in the rear would also have to be accommodated. Trying to load and unload cargo through an airlock would slow things down a lot.

It's possible that such an effort would extend the service life of airframes, but I don't see how it would save any money. New pressurization equipment is needed while on the ground, portals would need extra material to interface with airlocks (increasing weight), and you'd need more time on the ground to move passengers and cargo. Together, those seem to all be extra costs, not money savers.

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Pressurising the cabin of even the largest airliner really doesn't take much energy at all, not least because it is achieved via the ventilation system, so even if all the other answers were irrelevant, it still wouldn't be cost-effective.

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