Aircraft cabin pressure

[Vipers]

A relatively simple question,

On a trans continental flight, lets say from UK to the states, I seem to recall the aircraft flies at about 55,000 feet, but what is the cabin pressure in relation to being up a mountain, ie is it equal to 8000 ft up, or what!

[Jonesy23]

10,000ft usually.

On a 787 it is 8000ft.

[anonymous-user]

http://www.who.int/ith/mode_of_travel/cab/en/index...

Additional question, why don't they just have the pressure set to the equivalent of sea level? Would it not just make the journey more comfortable/safe for all onboard?

[anonymous-user]

Jonesy23 said:

10,000ft usually.

On a 787 it is 8000ft.

That's a bit high

[bertieg]

OneDs said:

http://www.who.int/ith/mode_of_travel/cab/en/index...

Additional question, why don't they just have the pressure set to the equivalent of sea level? Would it not just make the journey more comfortable/safe for all onboard?

it would cause excess pressure on the airframe, meaning the aircraft would have to be built stronger, therefore is heavier and more expensive

[Eric Mc]

No airliner currently flies as high as 55,000 feet. Most cruise between 30,000 and 40,000.

[Slink]

yes they do, when i went to america last year the info channel on where we were and what hight an speed an other stuff said we were going at 560 mph and 56,000 feet

[anonymous-user]

787 cruises at 43000 depending on weight, across the Atlantic it might only be 37000 until later on due to weight or if the winds are favourable lower down, cabin altitude is around 6,000 ft. The composite structure enables this larger differential between cruise altitude and cabin altitude.

747 max cruising altitude is around 43000ft but again wouldn't be doing that across the Atlantic. I think the 747 cabin altitude is around 7000ft like the 767 depending on type?

[anonymous-user]

Slink said:

yes they do, when i went to america last year the info channel on where we were and what hight an speed an other stuff said we were going at 560 mph and 56,000 feet

what airline and aircraft were you on? If you weren't in a military jet it didn't happen.

[anonymous-user]

Jonesy23 said:

10,000ft usually.

On a 787 it is 8000ft.

Feet? Or degrees?

[markmullen]

Slink said:

yes they do, when i went to america last year the info channel on where we were and what hight an speed an other stuff said we were going at 560 mph and 56,000 feet

Concorde used to but conventional airliners don't go that high.

IIRC A 787 is certified to about 45,000 feet.

[Eric Mc]

Slink said:

yes they do, when i went to america last year the info channel on where we were and what hight an speed an other stuff said we were going at 560 mph and 56,000 feet

You either misread, misheard or were given duff information.

Very few civil airliners are capable of going above 45,000, let alone cruise up there with a full load of fuel, passengers and baggage.

The only airliner that was designed to specifically cruise at or above 50,000 was Concorde. Possibly, the Tu 144 could also reach these altitudes as well as it was a beneficial thing to do at Mach 2.

I know that the Vickers VC-10 could exceed 50,000 if lightly loaded - such as on test flights or on position flights with no passengers.

[CharlieCrocodile]

I thought it was 7000'? That's what the PPL books say.

[mattdaniels]

Jonesy23 said:

10,000ft usually.

The cabin is definitely not pressurised to 10,000 feet. 10,000feet is the height at/above which you need auxilary oxygen. The cabin altitude pressure will be more likely around 7000feet

[Vipers]

Many thanks guys, much appreciated.

[mikeveal]

I switched my altimiter on once during a flight in a commercial airliner. The cabin pressure topped out at 6,000ft AMSL.

People generally tend to notice the effects of hypoxia at 10-14,000 AMSL, but it depends on many factors, fitness, aclimitisation to altitude, race, etc. Suffer is a better word than notice, the onset of hypoxia is something you won't notice unless you consciously check for it.

[mattdaniels]

OneDs said:

Additional question, why don't they just have the pressure set to the equivalent of sea level?

The aircraft structure wouldn't withstand keeping the pressure at sea level whilst cruising.

Also for aircraft that land at higher altitude destinations, the cabin pressure is matched to the ambient pressure by the time the aircraft is on the ground. If the cabin pressure was at sea level when the aircraft landed at altitude, people might disembark a bit quicker than expected.

[anonymous-user]

It's also a matter of engine efficiency. To keep the cabin pressurised at sea level would require a much larger volume of bleed air from the engine compressors, and this air is effectively "lost" from the engine, and so results in a reduction in both thrust and fuel economy. Not something commercial airlines like to hear...........

[creampuff]

mattdaniels said:

The aircraft structure wouldn't withstand keeping the pressure at sea level whilst cruising.

It would also mean a larger stress range, which means more fatigue damage and fatigue is a big problem for airframes.

[markmullen]

mattdaniels said:

The aircraft structure wouldn't withstand keeping the pressure at sea level whilst cruising.

Also for aircraft that land at higher altitude destinations, the cabin pressure is matched to the ambient pressure by the time the aircraft is on the ground. If the cabin pressure was at sea level when the aircraft landed at altitude, people might disembark a bit quicker than expected.

Don't tell O'Leary that, he'll use it for even faster turnarounds.