5
$\begingroup$

From an article on WHO's website:

Although aircraft cabins are pressurized, cabin air pressure at cruising altitude is lower than air pressure at sea level. At typical cruising altitudes in the range 11 000–12 200 m (36 000–40 000 feet), air pressure in the cabin is equivalent to the outside air pressure at 1800–2400 m (6000–8000 feet) above sea level.

Why aren't cabins completely pressurized, but instead to 6000-8000', seeing that many passengers wouldn't have to endure sometimes painful popping in the ears?

$\endgroup$
  • 1
    $\begingroup$ If cabins were pressurized to sea level, what would happen when they land at the many airports that are well above sea level? $\endgroup$ – jamesqf Jul 14 at 18:04
  • 1
    $\begingroup$ @jamesqf - I had a friend suffering from severe altitude sickness in the La Paz airport - her condition improved after takeoff. $\endgroup$ – quiet flyer Jul 14 at 18:29
  • 1
    $\begingroup$ @quiet flyer: Sure, that could well happen at high-altitude airports. But my point was mainly about the OP's ear popping. Instead of encountering it on takeoff, the passengers would just experience it when they land and the plane is depressurized from sea level to field elevation. Though one would think it would be easy enough to gradually change pressure during the flight... $\endgroup$ – jamesqf Jul 15 at 20:18
7
$\begingroup$

Two reasons: Longevity and weight. Which really come down to just weight.

Airframes have a limited fatigue life, measured in flight cycles. The main driving factor for airliner airframe wear is pressurizing and depressurizing them. Each millibar of difference between cabin pressure and outside pressure effectively consumes some percentage of the airframe's fatigue life.

Reducing the cabin altitude means increasing this pressure difference, and thus consuming more of the airframe's life. This could be compensated for with sturdier construction, which adds weight. It would also consume a little more bleed air, requiring slightly heavier packs, which, as well as weight, means a loss of efficiency.

Luxury business jets often maintain a lower cabin altitude, such as 4,000 ft. This eats into their flight cycles, so they can still be switched to the usual 8,000 ft for flights without the owner/VIP inside.
Carbon fiber has a much longer fatigue life, so CFRP fuselages can afford to lower the cabin altitude to 6,000 ft. This pressure altitude can also be maintained in other airliners at flight levels well below their ceiling.

The optimum compromise point is subject to a lot of debate. The highest cabin altitude that could be permitted is 15,000 ft, above which hypoxia-induced loss of consciousness can occur. The regulatory bodies have settled at 8,000 ft, so that's what the manufacturers targeted with most of their aluminum airliners, and that's where airlines prefer to run them even if they have a choice, to get more life out of their planes.

$\endgroup$
  • 1
    $\begingroup$ '+1' for the last paragraph. $\endgroup$ – William R. Ebenezer Jul 15 at 14:06
  • 1
    $\begingroup$ Replace weight by money and you have it even further... $\endgroup$ – tsg Jul 15 at 15:02
5
$\begingroup$

The higher the pressure differential between inside and outside, the more stress is put on the plane, which reduces lifespan, and the stronger it needs to be, which increases weight. More weight means more fuel burn and shorter range. All of these factors would combine to increase the overall cost of operation--and eventually fares.

Many passengers don't even notice the higher cabin altitude, especially the frequent fliers who account for the vast majority of airline revenue, so there is little financial incentive to change.

$\endgroup$
  • 1
    $\begingroup$ Is there any reason for choosing 6000 to 8000 feet? If it is some kind of optimization between "not too much structural stress" and "not too much passenger discomfort", how did we get to this figure? $\endgroup$ – William R. Ebenezer Jul 15 at 3:06
  • 2
    $\begingroup$ @WilliamR.Ebenezer Above 10k you get noticeable cognitive impairment. So that's a hard upper limit. $\endgroup$ – ratchet freak Jul 15 at 12:11
  • 1
    $\begingroup$ @ratchet freak: Maybe folks who live at sea level might experience problems at 10K ft, but plenty of us have adapted to higher elevations and consider them normal. $\endgroup$ – jamesqf Jul 15 at 20:22
0
$\begingroup$

Because the extra air would add extra weight!

In all seriousness the weight of the extra air might well be on par with a layer of paint, or heavier, and we are often told that airliners are sometimes left partially unpainted (e.g. on undersides) to save weight.

Of course the extra structure required by the extra pressurization would also add weight.

$\endgroup$
  • 1
    $\begingroup$ On the other hand, if we could pressurize with a lighter-than-air gas like hydrogen or helium, then the more the better. (Kidding) $\endgroup$ – quiet flyer Jul 14 at 21:32
  • $\begingroup$ Yeah, that cabin full of suffocated passengers would be a problem. $\endgroup$ – CrossRoads Jul 14 at 22:18
  • $\begingroup$ +1, the statement is correct. Air weighs a surprising amount over large volumes, analogous to the discussion here. $\endgroup$ – Koyovis Jul 15 at 7:37
  • 1
    $\begingroup$ @Koyovis it is a true statement, but it is also a poor answer that buries the lede and devotes much more space to comical trivia than to the actual reason. $\endgroup$ – AEhere supports Monica Jul 15 at 8:45
  • 2
    $\begingroup$ On an A320 the pressurised volume is 330m³. That's about 80kg per 0.2atm. $\endgroup$ – Sanchises Jul 15 at 22:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.