B777-200 one pack inoperative

[Bungfai]

Why does B777-200 with one pack inoperative have to dispatch with FL350 maximum cruise limit? And aircraft with some modification got no limit?

[ghw78]

A number of aircraft have a limit to a certain max alt if you despatch with one pack in op.

The scenario is that on leaving the ground, the remaining pack(s) have to pressurise the aircraft to the normal working diff for the particular model. Where the pack runs out of capability to do this is the the figure published as a max despatch altitude ( most probably with an approrpriate safety factor to allow for degraded performance of an individual air frame/combinations of systems).

If you are at a higher altitude and the pack fails the remaining pack(s) should be able to maintain the required output to keep the cabin where it is by maintaining the required diff pressure.

A narrow body jet from my past had the max despatch altitude of FL250.

[flightleader]

Something to do with smoke removal time limit. Do a mod and the limit can be lifted.

[FE Hoppy]

This is normally to do with the requirement of lb of air per passenger per minute. nothing to do the pressurisation or smoke.

[gas path]

Actually it may be due to leak rate from the apu duct seal where it passes through the rear pressure bulkhead. IIRC there is a check to do if dispatch is required with a pack locked out, not applicable to later a/c or -300.:8

[grounded27]

Those limits also take into account the possibility of remaining pack failure and the time it would take to descend to a breathable altitude. Real world leak rates vary from aircraft to aircraft.

[Bungfai]

Yes! The later aircraft or with modification and -300 are not applicable.
The question is for the older 772 without modification. During cruise above FL350, why the checklist is not telling us to desend to FL350 or lower when one pack fail, or even one pack fail plus smoke warning?

[gas path]

@Bungfai
The a/c is capable of maintaining pressure with one pack so I don't think it's an issue. However 'pumping' it up in the first place is another issue.

[5LY]

I know of a guy who once years ago ignored a max alt. restriction with a pack US on a 737 400. After some time at cruise level the flight attendants started falling over. (fainting). As they were walking about they needed more O2 and because there was no air exchange they became hypoxic. There was enough air to pressuize the a/c, but not enough to provide air exchange, so they were all rebreathing the same air. (170 pax + crew)

So, my guess would be that's the issue, although some of the others mentioned above could also be considerations.