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August 2, 2005 12:11 PM   Subscribe

I have read that airplane cabin pressures are regulated to match the pressure at about 8000 ft. Why is the pressure not regulated to approximate ground level conditions?

I'm having difficulty finding a definitive answer, but I would guess it's one of the following:
  • too difficult/expensive to regulate at that pressure
  • decreased oxygen keeps passengers calm
  • reduces metal fatigue on the fuselage, thus cutting maintenance costs
  • the fuselage is simply not designed to withstand higher differential pressures
  • the airlines hate us
posted by Galvatron to Travel & Transportation (11 answers total) 1 user marked this as a favorite
 
Reasons 2 and 5 are facetious. 4 is flat out wrong, and 3 is a minor issue. 1 sort of has it -- it's a fuel-saving measure. Air compressors and air conditioners need energy to operate, which is drawn from the engines.
posted by randomstriker at 12:27 PM on August 2, 2005


I found this which shows the effect on 3: (BBJ is Boeing Business Jet)
In all 737's the pressurisation system ensures that the cabin altitude does not climb above approx 8,000ft in normal operation. However in 2005 the BBJ will be certified to a reduced cabin altitude of 6,500ft at 41,000ft thereby increasing passenger comfort. he payback for this is a 20% reduction in airframe life cycles, ie from the standard 75,000 down to 60,000 cycles. This is not a problem for a low utilisation business jet but would be unacceptable in airline operation where some aircraft are operating 10 sectors a day.
posted by smackfu at 12:28 PM on August 2, 2005 [1 favorite]


Well...OK...4 is not completely wrong, in that an airframe built to withstand higher internal pressures would end up being heavier, and thus less efficient. Again, it is a way to cut down on operating costs.
posted by randomstriker at 12:29 PM on August 2, 2005


randomstriker, is the "fuel-saving measure" answer just speculation, or do you have insider information? I would think that the fuel budget for powering air pumps is negligible relative to the fuel budget for moving the plane from point A to point B.
posted by Galvatron at 12:48 PM on August 2, 2005


I would think that the fuel budget for powering air pumps is negligible relative to the fuel budget for moving the plane from point A to point B.

Which would in turn be negligible to the fuel budget for moving the plane from the ground to cruising altitude and back to the ground!
posted by Pollomacho at 12:54 PM on August 2, 2005


#4 is correct.

Airliners are built to withstand pressure differentials of 8 to 9 psi. Maintaining sea level pressure (14.7psi) inside the cabin at 30,000 feet (4.3psi) creates a pressure differential of 10.4psi, which exceeds the 8 or 9 psi differential limit of the plane.

The 9psi limit is a design compromise -- the plane could be made stronger by adding more reinforcement to the fuselage, but this adds weight. Adding weight has lots of negative consequences for an airplane -- one of which is that it would require more fuel to fly. So yes, in a way the lower cabin pressure is a money saving measure, but it's not because of the fuel required to operate the compressors. It's because a heavier airplane costs more to operate. It's due to decisions made by engineers at Boeing/Airbus/etc and not by penny pinching airlines.

Other than adding weight, there is another way to make an aircraft stronger: manufacture it using stronger materials. This is Boeing's strategy with the 787 in development. It will be built with more composite materials that are lighter and stronger than the metals they replace. Boeing claims that this increased strength will allow higher cabin pressure.
posted by blue mustard at 1:52 PM on August 2, 2005


It's due to decisions made by engineers at Boeing/Airbus/etc and not by penny pinching airlines.

The engineers may have made the decisions, but they were made to make the planes cheaper up front and cheaper to operate, and thus make them more attractive to the airlines.

This is Boeing's strategy with the 787 in development. It will be built with more composite materials that are lighter and stronger than the metals they replace. Boeing claims that this increased strength will allow higher cabin pressure.

Or, presumably, it would allow the same cabin pressure to be maintained using lighter, cheaper planes.
posted by blm at 3:13 PM on August 2, 2005


Salon's Patrick Smith, of Ask the Pilot fame says, "Maintaining a cabin at sea level itself is unnecessary and would put undue stress on the airframe, and so interior pressure is kept at a certain height above sea level" in his 3 June, 2005 column. His 18 July, 2002 column deals with the same topic, and debunks number 2 and fuel savings.
posted by QIbHom at 3:56 PM on August 2, 2005


Air has a density at sea level of about 1.2 mg/cm^3, 1.2 kg/m^3. If a jet was approximated as a 70m long, 11 m wide and 10 m high (about the size of box that would hold a 747-400 fusealage) that would be 7700 cubic meters so at sea level that would be 9240 kg. At 8000 metres altitude the density of air decreases about 45%, or to 4160kg. The difference would be 5080kg. That's only about 0.3% the weight of a 747-400 so the savings due to decreased internal atmospheric pressure would be pretty minimal.

I just donated blood and drank a 6 pack of guiness so my math may be out to lunch.
posted by substrate at 4:47 PM on August 2, 2005


I think Ask the Pilot guy is just being an apologist. Who appointed who to decide if it is "necessary"...

The argument that blue mustard makes is completely convincing. Take a look at a pressure vs. altitude chart (6.9 kPa/psi) to see that his numbers are about right:

Sea level - 14.7 psi
8000ft - 10.9 psi
30,000ft - 4.3 psi

So the differential is 60% higher if you require sea level cabin pressure. Obviously a 60% increase in pressure is structurally significant!
posted by Chuckles at 6:27 PM on August 2, 2005


The new Boeing passenger 787 is going to be pressurized to a higher level for passenger comfort, according to Boeing's press releases. What makes this possible is the composite airframe, which can be stronger and lighter than metal airframes.
posted by ikkyu2 at 7:16 PM on August 2, 2005


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