beep747

max alt with 1 pack failure

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Hi,

Short after take off, I got a PACK L failure message. I followed the non normal checklist but the failure remained. As my cruise alt was fl 380, I searched the forums because I know that this limits the max alt in many airliners.

I read however that one 777 engine can supply both packs and that a reduced cruise alt wasn't necessary. Others stated that their company policy did prescribe a reduced cruise alt. 

I reached 38000 feet with normal cabin pressure and normal cabin temps. So, what is the correct procedure?

Jos Denis

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15 minutes ago, beep747 said:

So, what is the correct procedure?

If it isn't in the QRH, then it's likely something that's part of a company-written and FAA-approved (/overseeing body-approved) procedure. Not everything is left to the FAA (/oversight authority), or the manufacturer. Aviation seems formulaic and rigid - black and white - to many, but in reality, it's actually very grey. Company policies are allowed to be more restrictive (and occasionally, where approved in specific cases, sometimes more permissive) than the generic regulation/AFM-set-limit. As such, hearing that a company says "if you lose a pack, your max alt is X" isn't too surprising, even if the AFM says "doesn't matter."

So...the "correct" answer, if the QRH,/ECL/AFM don't specify is "whatever you decided to do."

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Pack failure is an interesting one in this regard.

The QRH is ultimately the document to follow after dispatch (the exact definition of which varies but broadly -- doors closed/pushing back). Before this point, the MEL applies...

...and you may well find in some instances that the MEL limits cruise altitude, whereas the QRH does not! In other words -- if you get a PACK L failure message before pushback, the MEL applies and your max alt may be limited. Get it a minute later after pushback, the QRH applies and there is no limitation...

Someone with more technical knowledge of Boeing's thinking in writing the manuals than I may be able to offer some greater insight as to why this could be. I might theorise that this could be because the QRH assumption is that for the duration of a single flight there will be no adverse effects, whereas operating for perhaps multiple sectors with a pack MEL'd might place excessive stress on the remaining operating pack and thus lead to a greater chance of failure. Just a guess, however.

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Thanks for the explanation. I followed the steps on the non normal checklist and there was no mention of a restriction. Just set PACK L to off.

Jos Denis

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The MEL is something missing from our sim world, and I'd like to see one for a B77LR/F/W.  I suspect that because the packs are in themselves redundant (dual controllers in each pack) that you're not looking at a single point failure potential even with one pack out of service and this brings me to the original event of a pack failure.  Was that something programmed with service based failures, random failures or did you program the failure in any way to occur?  Did you try the A/C reset?

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It was a random failure. I tried resetting the A/C as the non normal checklist  suggested. 

Jos Denis

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