Apart from the cost angle, which seems to be the biggest factor in airlines, is it technically possible to bring cabin atmosphere (includes pressure, temperature, humidity) etc. at par with ground level, at the cruising altitude of most airliners (say 35000 ft).

I just want to understand what stops airliners from providing this kind of comfort to the passengers. I am assuming there might be issues of hull integrity in order to maintain higher cabin pressure etc.

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    Just for clarification, all of this is outside of the control of the airline. It's all a matter of aircraft design decisions made by the manufacturer, not operational decisions on the part of the airline. The airlines just follow the operational guidelines set by the manufacturers. – reirab Jul 27 '15 at 14:17
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    "...is it technically possible to bring cabin atmosphere...at par with ground level" Yes. "what stops airliners from providing this kind of comfort to the passengers" Cost. – Adam Davis Jul 27 '15 at 16:36
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    As the customer of the airframe manufacturers, it's not really outside of airliner's control - if airliner passengers demanded sea level pressurization from the airlines, the airlines in turn would demand it from manufacturers (which would, of course, drive up costs). And once one manufacturer offered it, they all would have to in order to remain competitive. It's just that the status quo is good enough for most passengers so no one is demanding greater pressurization (though Boeing may have started a trend toward greater pressurization with the Dreamliner's 6000ft level). – Johnny Jul 27 '15 at 18:39
  • The Airbus A380 has a cabin altitude of 4,990 feet (1,520 m), lesser than the Dreamliner , 6,000 feet (1,800 m) – Firee Jul 28 '15 at 14:01
  • The SyberJet SJ30 is one plane which offers 'sea level cabin' (zero cabin altitude) up to 41,000 ft (due to its 12 psi differential pressure) – Firee Jul 28 '15 at 14:04

I'll touch on the humidity a bit.

This is actually a big issue for planes; they tried to make it a bit better on the Dreamliner and were able to bump it up a bit

Humidity is improved, yet still dry as the desert. Humidity levels on the Dreamliner are 10%-15% — better than 7% on other planes on long trips

source

but it's still a big issue. There are two main reasons they can't really increase it that much.

First off, water is very heavy. Since planes pump in dry exterior air they would have to humidify it which means they need to carry enough water to do so. This weight would greatly affect the useful load of the plane. Now you could try to recycle some of the ambient moisture, but on a large scale that takes power, which takes fuel, which comes back to the weight issue.

The other issue (although not as often thought about) is the longevity of the airframe. The extremely dry conditions that airliners see for most of their time actually helps to prevent corrosion on the airframe as well as rust on any rustable parts. By pumping moisture into the airframe you will run the risk of it causing real damage to the metal components.

The temperature is a simple energy problem. To keep a plane hot you need to heat it faster than the exterior temp is cooling it (or evenly to keep it a constant temp). Insulation can help with this but you are still going to see some cooling. On a rudimentary level you can apply Newton's law of cooling here and keep in mind the exterior temperature is in the -40 (and below) area (according to today's charts).

On to the pressure,

This, of course, is a big one, but, in reality, it's not a big deal. While it may seem a bit irritating, 8000ft atmosphere equivalent is still more than breathable. The FAA does not require continuous O2 until 14,000 ft for unpressurized planes, so an 8000 ft equivalent cabin is more than fine. Now from a comfort standpoint, your ears may pop and you may feel some discomfort, but it's more than safe.

From a purely technological standpoint we can do this, but from an engineering use case standpoint it's better to sacrifice some of the pressure to build a lighter, thinner plane. This saves on fuel and material costs down the line. There is great hope that carbon fiber may bring a change to all this and 8000 ft equivalency will be a thing of the past. Boeing has been pushing this with the 787 having only a 6000ft pressure level which they claim alleviates problems. I have not yet flown one so I don't know first hand how much better it really is. Then again I fly unpressurized stuff often and have become somewhat acclimated to it.

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    Cabin "heaters" in jets don't heat, they cool. The cabin air is taken from the engine intakes, where compression heating raises the temperature to 100C or above; it is then cooled to a reasonable level by running it through a heat exchanger. – Mark Jul 27 '15 at 22:44
  • @Mark: The air needs to be cooled, but for the cabin itself it is still heating. It does indicate that it is a non-issue though since just compressing the outside air makes it hot enough for the purpose. – Jan Hudec Jul 28 '15 at 7:10
  • Temperature is not so much an energy problem (compression for pressurization heats the air more than enough) as it is a problem of defining what the temperature should be. Usually 21-22°C is considered normal temperature and that is what the air conditioning is set to throughout the year. If there is 30°C outside before you board, it feels cold inside, but then you may land somewhere else where there is 10°C and then at least the shock is not as big. And on another day there can be -30°C and the 21°C will feel pleasantly warm. – Jan Hudec Jul 28 '15 at 7:21
  • I have flown the Dreamliner several times, and I can tell you it feels much more comfortable in terms of cabin pressure, even at altitudes in excess of 40,000 ft – Firee Jul 28 '15 at 13:53
  • The air in the cabin is usually taken from the engine, where it is highly compressed. So, doesn't compressed air usually contain higher amounts of moisture? – Firee Jul 28 '15 at 13:58

Weight and strength.

  • At 10700 metres (35000 feet), the ambient pressure is about 24.8 KPA (3.6 PSI).
  • The pressure inside the aircraft is about 75.8 KPA (11 PSI), assuming 8000 feet cabin altitude.
  • Imagine a cabin door approximately 2 metres by 1 metre (18 square feet).
  • The force on the door is about 3970 KGs (8750 pounds or 4.3 tons).
  • Imagine now that the cabin is pressurised to 100 KPA (14.5 PSI).
  • The force on the door is now 5830 KGs (12850 pounds or 6.4 tons)!

And that's just one door. Now imagine the extra force for all of the other doors, the windows and the fuselage itself. The additional force would be huge. To build a fuselage that strong would be very heavy.

The temperature can be controlled no matter what the cabin altitude and most people can cruise at 8,000 feet with no significant health effects. Therefore, it is just not necessary, or cost effective, to build an aircraft strong enough to fly with a cabin altitude of zero.

  • I agree with almost all of this answer, though I wouldn't go so far as to say that there are no ill effects. Maybe no significant health effects would be a better way to word that. Anyone who has spent 14-20 hours at cruise altitude flying halfway around the world can attest to the nasty feeling resulting from spending that long at such low humidity and pressure. This is discussed further in this question about the 787. – reirab Jul 27 '15 at 14:31
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    Where did those force numbers come from? I'm trying to replicate the calculation (as $F=A_{\text{door}}(P_{\text{in}}-P_{\text{out}})$), but I'm getting around 150 kN if the cabin is at ground level and 100 kN if the cabin is at 8000 feet (in lbs-force, that's 23,000 lbs-force at 8000 feet and 34,000 at ground level). – cpast Jul 27 '15 at 14:34
  • @cpast Good point. Just as a quick sanity check, (14.5 - 3.6) / (11 - 3.6) = 1.473, so the force should 'only' be 47% higher using the pressures listed in the answer. Also, unlike lbs, kg is not a unit of force. – reirab Jul 27 '15 at 14:42
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    @Simon But isn't that assuming that the strength determining failure mode in an aircraft is the internal pressure? Is it? i.e. When you do the thickness calculations for various failure modes, e.g. ground handling, buckling etc. is the skin thickness really set by the "pressure vessel" mode of failure? – curious_cat Jul 27 '15 at 15:52
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    @curious_cat It is the determining factor, or more precisely, the pressurisation/de pressurisation cycles which apply repetitive stress to the fuselage. If the pressure were increased, the stress would be increased. – Simon Jul 27 '15 at 20:02

In addition to needing structural strength to hold in a greater amount of pressure, the airframe will undergo repeated stress cycles due to alternating conditions of equal pressure (on the ground) and low outside pressure with high inside pressure (at cruise altitude).

Making the stress cycles deeper (by increasing the pressure difference without changing airframe structure) can cause more rapid failure of the fuselage. Reducing the stress means increasing the material you put into the fuselage, but almost surely not by just adding a few reinforcements and (I think) not generally even by just making everything stronger. To safely increase the pressure difference, I think you'd need to substantially redesign all the parts of the airframe that hold in the pressure (potentially down to individual panels and rivets), and then sacrifice an airframe to run a lengthy test of pressurization cycles to verify that it doesn't fail.

It seems reasonable to do such a thing for a completely new model (such as the Dreamliner), where you already have to do that amount of design and testing for some level of pressurization, but I think you'd need a stronger incentive to redesign an older model in that way, and retrofitting existing aircraft would (I suspect) be prohibitively expensive.

TL;DR: with a completely new aircraft design, maybe (if you consider it worth the cost); with existing airliners, no.

  • Some small aircraft manufacturers, like Gulfstream, claim to provide cabin pressurization equivalent to 4,000 ft at 40,000 ft – Firee Jul 28 '15 at 13:56
  • @Firee Sure, that would be another example like the Dreamliner (but more so), in which you can design a higher cabin pressurization into a new aircraft type. And I suppose there are quite a number of airports you can fly between where 4000 feet would be "at par with ground level". – David K Jul 28 '15 at 14:21

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