lehpron
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Pressurization, Factor Of Safety, Altitudes, Etc

Wed Mar 08, 2006 11:23 am

I'm running off of memory, so please correct me, I may be wrong about stuff.  

Sea level air pressure is about 101 kPa or 14.6 psi, outer space is presumed zero pressure, so techincally, if a volume was design to withstand 14.6 psi, it could go into space...

So why are aircraft like 777 have their fuselage pressurized to 12 psi**? If there was a 50% factor of safety, then 8psi would be an operational max. If they wanted to maintain sea level pressure, they could fly at an altitude that was (14.6 - 8 =) 6.6 psi, FL215. If there was no factor of safety, then FL395 feet.

Do I have that correct?

For example: If I had a plane where the max operating pressure in the cabin (not ultimate) is 10 psi, and the plane were pressurized to 5000 feet, what altitude could it go to regardless of what the wings could do?


** my source was the old "21st Century Jet" special on PBS from 1994. During the pressurization test they went to 12 psi and a leak occured in the rear left exit door, they fixed it.

[Edited 2006-03-08 03:26:35]
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Jetlagged
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RE: Pressurization, Factor Of Safety, Altitudes, Etc

Wed Mar 08, 2006 11:09 pm

I may have missed the point, but it should be obvious that the 12 psi test would include the required safety factor. 12 psi may not be the ultimate pressure limit of the design, either. A typical airliner might have a operating differential pressure limit of between 8 and 9 psi. I don't know the precise figure for a 777.

You appear to be assuming that airliners are pressurised to maintain sea level pressure as high as possible, but this is never the case. Even if the aircraft is only cruising at 20,000 feet, where a sea level cabin is theoretically possible, cabin pressure would still be controlled to be less than that, on a schedule based on cruise altitude and field altitude. If you maintain sea level cabin pressure to the differential limit as the aircraft climbs, once that limit is reached, the cabin will then have to climb at an uncomfortably high rate. The idea is to have the cabin climb continuously at a lower rate, so it reaches it's target level at the same time the aircraft reaches cruise altitude.

To maintain a sea level cabin with an 8 psi differential, the highest the aircraft could fly would be 20,200 feet, so you weren't far out. With a 12 psi differential, 40,150 feet cruise altitude would be possible.

Your theoretical aircraft could cruise at 44,150 feet with a cabin diff of 10 psi and a cabin altitude of 5,000 feet.

Hope that helps.
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Bellerophon
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RE: Pressurization, Factor Of Safety, Altitudes, Etc

Thu Mar 09, 2006 12:36 am

Jetlagged has given the correct answers in a good post.

He also, correctly, was careful to refer to differential pressure, not cabin pressure, in his reply, as it appears (to both of us) that it was differential pressure that Lehpron was actually asking about.

We need to be very clear in our own minds about the difference between these two terms, and to be sure to use the correct one, as otherwise discussions become very confused!

Flight crew almost always talk about differential pressure, not cabin pressure. If they do refer to cabin pressure, they usually refer to it as an equivalent altitude. So, whilst you might hear “Our cabin altitude is 6,100 ft”, you will rarely hear “Our cabin pressure is 11.74 psi”.

Differential pressure is the important limit on the airframe, we all know what happens to an over-inflated balloon. Cabin pressure is (almost) irrelevant, unless it drops below 10.00 psi, when it becomes very important!  Wink

Knowing the Cabin Altitude was 6,100 ft, at a Diff Press of 10.7 psi, you could easily calculate that the cabin pressure must be 11.74 psi, the outside air pressure was 1.04 psi and the aircraft flying at 60,000 ft.

Figures that occurred regularly not so long ago!


Regards

Bellerophon
 
joness0154
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RE: Pressurization, Factor Of Safety, Altitudes, Etc

Thu Mar 09, 2006 12:42 am

The problem with high pressures is that it puts a strain on the airframe, which lessens its uselful lifespan. Also, the packs would be working their ass off to keep pressures that high.
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Grbld
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RE: Pressurization, Factor Of Safety, Altitudes, Etc

Thu Mar 09, 2006 2:18 am

Quoting Joness0154 (Reply 3):
Also, the packs would be working their ass off to keep pressures that high.

Right about the first point, wrong about this one: One pack can keep sufficient cabin pressure at any altitude, for two (or more) it's no biggie at all. Just keep the outflow valve(s) in a more closed position in order to keep the pressure at a higher level.

The Gulfstream G550 has a max pressure differential of a whopping 10.15 psid. The 737NG and 757 have a limit of 9.1 psid but normally don't exceed 8.3 psid (737) or 8.6 psid.

You'll see that at FL410, the cabin altitude in a 737NG will be around 8,500 ft, while at FL510 in he G550, cabin altitude is no more than 6,000 ft (amazing!).

The G550 has a much stronger structure and better seals (also easier since it's simply a lot smaller). Even though the limits seem mighty close, you'll have to realize that at those altitudes, outside air pressure doesn't change much, so a bit of extra strength can give you a much lower cabin altitude at the same level.

Air pressure at just 18,000 ft is about half of that at sea level.


Grbld
 
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Starlionblue
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RE: Pressurization, Factor Of Safety, Altitudes, Etc

Thu Mar 09, 2006 3:06 am

Quoting Grbld (Reply 4):
. Even though the limits seem mighty close, you'll have to realize that at those altitudes, outside air pressure doesn't change much, so a bit of extra strength can give you a much lower cabin altitude at the same level.

Air pressure at just 18,000 ft is about half of that at sea level.

This is an important point. Unlike underwater pressure differentials, atmospheric pressure differentials are very gentle.
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zeke
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RE: Pressurization, Factor Of Safety, Altitudes, Etc

Thu Mar 09, 2006 5:41 am

Quoting Lehpron (Thread starter):
For example: If I had a plane where the max operating pressure in the cabin (not ultimate) is 10 psi, and the plane were pressurized to 5000 feet, what altitude could it go to regardless of what the wings could do?

SL = 14.6959 psi
5000 ft = 12.2277 psi

The differance between the cabin and outside pressure is 10 psi

12.2277-10=2.2277 psi

As Jetlagged said, 2.2277 psi is equal to an altitude of 44154 ft in the 1976 Standard Atmosphere.
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AirWillie6475
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RE: Pressurization, Factor Of Safety, Altitudes, Etc

Thu Mar 09, 2006 8:54 am

Quoting Grbld (Reply 4):

You'll see that at FL410, the cabin altitude in a 737NG will be around 8,500 ft

I don't think any plane has a cabin altitude higher than 8,000ft.
 
Grbld
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RE: Pressurization, Factor Of Safety, Altitudes, Etc

Thu Mar 09, 2006 3:40 pm

Quoting AirWillie6475 (Reply 7):
I don't think any plane has a cabin altitude higher than 8,000ft.

My bad, it is indeed 8,000 ft @ FL410 for the 737NG.

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