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Has "effective Cabin Pressure" Changed Recently?  
User currently offlineChase From United States of America, joined Nov 2005, 1054 posts, RR: 0
Posted (4 years 7 months 6 days 21 hours ago) and read 4362 times:

I'm sure I have my numbers off a bit, but as I recall, most A/B pax aircraft are pressurized to the same air pressure that you'd experience if you were standing on a mountain about 8000' high. And I seem to recall that one of the 787's selling points was that it would allow a higher interior pressurization, equivalent to an altitude of about 7,000'.
Someone recently told me that these number had changed recently and that all pax aircraft are now pressurized to a greater pressure than they were a few years ago. Frankly I'm very skeptical of this but figured I'd ask here since you all are sure to know.
Thanks.

11 replies: All unread, jump to last
 
User currently onlineHAL From United States of America, joined Jan 2002, 2560 posts, RR: 53
Reply 1, posted (4 years 7 months 6 days 21 hours ago) and read 4332 times:

An 8000 foot cabin altitude would only be at the very highest cruising altitudes. On a more typical flight in the 767 that I fly, I'll see cabin altitudes more in the 4000 to 6000 foot range, depending on what flight level we're at.

The only changes might be on the 787 as I do think that aircraft will be using a higher differential pressure (lower cabin altitude). But any legacy aircraft from Boeing or Airbus currently operating will have about the same differentials as in the past.

HAL

[Edited 2010-01-25 11:08:31]


One smooth landing is skill. Two in a row is luck. Three in a row and someone is lying.
User currently offlineAircatalonia From Spain, joined Nov 2007, 554 posts, RR: 0
Reply 2, posted (4 years 7 months 6 days 21 hours ago) and read 4237 times:



Quoting Chase (Thread starter):
about 8000' high

I think it is less than that. I would say 8000ft is too high for people with breathing conditions.


User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 3, posted (4 years 7 months 6 days 21 hours ago) and read 4218 times:

Quoting Aircatalonia (Reply 2):
I think it is less than that. I would say 8000ft is too high for people with breathing conditions.

People living in MEX and LPB would beg to differ   

But yes that is a typical cabin altitude when cruising above FL400 for most commercial planes.

[Edited 2010-01-25 11:45:06]

User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 4, posted (4 years 7 months 6 days 15 hours ago) and read 4080 times:



Quoting Chase (Thread starter):
I'm sure I have my numbers off a bit, but as I recall, most A/B pax aircraft are pressurized to the same air pressure that you'd experience if you were standing on a mountain about 8000' high.

It ramps in as a function of cruise altitude. 8000' is usually the max, but you'll only get to that at the upper end of the plane's altitude capability.

Quoting Chase (Thread starter):
nd I seem to recall that one of the 787's selling points was that it would allow a higher interior pressurization, equivalent to an altitude of about 7,000'.

6500' max. The ramp is similar, so at equal altitudes a 787 should have lower cabin altitude than an aluminum equivalent.

Quoting Chase (Thread starter):
Someone recently told me that these number had changed recently and that all pax aircraft are now pressurized to a greater pressure than they were a few years ago.

Untrue. Altering the pressurization schedule will drastically affect the fatigue life of the fuselage.

Tom.


User currently offlineLimaNiner From United States of America, joined Oct 2006, 400 posts, RR: 0
Reply 5, posted (4 years 7 months 6 days 13 hours ago) and read 4035 times:



Quoting Tdscanuck (Reply 4):
Altering the pressurization schedule will drastically affect the fatigue life of the fuselage.

Suppose you didn't care too much about the fatigue life, e.g., you got a hold of a second-hand low-cycle, high-hours frame that you wanted to turn into the most comfortable private jet you could, and you realistically were only looking at another thousand cycles or so, max... say the Google boys' 767.

Could you safely crank up the pressure differential, fatigue life be damned?


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 6, posted (4 years 7 months 6 days 13 hours ago) and read 4033 times:



Quoting LimaNiner (Reply 5):
Could you safely crank up the pressure differential, fatigue life be damned?

In theory, yes. Although fatigue life is a really (really really) non-linear function of stress, so small changes in cabin pressure could do horrible things to your durability.

I'm not sure how you'd certify it though...you'd need some mechanism to alter the certification basis to encompass the new pressure schedule (which impacts a lot of stuff, like smoke penetration, beyond just fatigue life). In theory, you could pay the OEM to do this but I don't think it would be cheap.

Tom.


User currently offlineLaddie From United States of America, joined Jan 2010, 582 posts, RR: 8
Reply 7, posted (4 years 7 months 6 days 12 hours ago) and read 4017 times:



Quoting LimaNiner (Reply 5):
Could you safely crank up the pressure differential, fatigue life be damned?

It would be simpler to fly at a lower altitude to get a lower cabin pressure. You would pay for it in higher fuel burn, however.

Remember that the 8,000' cabin altitude only comes into play at the end of a very long flight when most of the fuel is burned off and the airplane is above 40,000'. In other words, it just doesn't come into play often enough to start tweaking cabin pressurization schedules or flying at non-optimum altitudes.


User currently offlineDw747400 From United States of America, joined Aug 2001, 1259 posts, RR: 1
Reply 8, posted (4 years 7 months 6 days 9 hours ago) and read 3956 times:



Quoting Laddie (Reply 7):
It would be simpler to fly at a lower altitude to get a lower cabin pressure. You would pay for it in higher fuel burn, however.

Exactly. We once ran a G550 coast to coast at FL220 because the boss had a head cold.



CFI--Certfied Freakin Idiot
User currently offlineJetlagged From United Kingdom, joined Jan 2005, 2546 posts, RR: 24
Reply 9, posted (4 years 7 months 5 days 23 hours ago) and read 3870 times:



Quoting Tdscanuck (Reply 4):
Untrue. Altering the pressurization schedule will drastically affect the fatigue life of the fuselage.

Not just reduced fatigue life. Increasing the maximum differential pressure would bring the bring it closer to the proof limit, reducing the safety margin - possibly below the legal limit.



The glass isn't half empty, or half full, it's twice as big as it needs to be.
User currently offlineComorin From United States of America, joined May 2005, 4896 posts, RR: 16
Reply 10, posted (4 years 7 months 5 days 16 hours ago) and read 3767 times:



Quoting Dw747400 (Reply 8):
Exactly. We once ran a G550 coast to coast at FL220 because the boss had a head cold.

Would you have had to fly slower too because of air density at FL220?


User currently offlineRwessel From United States of America, joined Jan 2007, 2346 posts, RR: 2
Reply 11, posted (4 years 7 months 5 days 15 hours ago) and read 3742 times:
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Quoting Comorin (Reply 10):
Would you have had to fly slower too because of air density at FL220?

Very likely. Take a look at page 1-10 of:

http://www.smartcockpit.com/data/pdf...eam/G550/misc/G550-Limitations.pdf

You see that at FL220 the G550 hits its Vmo limit of 340kts, which at 22,000ft is 463kts true. At 32,100ft, the MMO of .885 works out to 517kts true.

Given that it's a relatively short flight for a G550, the "normal" high altitude flight would likely have been flown near max cruise, rather than the slower maximum range speed, which would require the slower speed at FL220. If they *were* planning to flight the high altitude route at low speed (presumably to save fuel in this case), it's plausible that they could have run the low altitude route at the same speed. In either case they burned a bunch of extra fuel at 22,000ft.


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