B787 - CAC inoperative in flight

[Piloteprive]

Hi guys,

Prior dispatch with 1 CAC inop, MEL states that the airplane must remain at or below FL350 if crew rests are occupied.

1 CAC inop after dispatch will not trigger any ECL only a STATUS MSG. Officially, you can disregard the MEL and fly above FL350 with the crew rests occupied.
I wonder why we have this altitude restriction in the MEL? I heard different reasons but i couldn't find any information on paper in order to take proper action if a CAC becomes inop in-flight with crew rests occupied.
Any idea on that ?

Thanks

Max

[STBYRUD]

For the same reason that for example a 737 can dispatch with one pack inop only if the cruise altitude will be limited to FL250 but the QRH does not mention any maximum altitude in case of a failed pack - subsequent failures will potentially result in a loss of pressurisation. In flight a failure of the other CAC on the same side will result in a PACK L / R EICAS, and the checklist will tell you that pressurization may not be maintained 'near maximum operating altitude'.

[BleedingOn]

My thoughts:

Whilst not beholden to the MEL as you mentioned if the failure occurs after dispatch, I would still consider the FL350 restriction valid. I can only imagine it is a ventilation issue.

If on a flight where cabin crew rest was required, I would liaise with the cabin crew to try to get them to start (and therefore finish) their breaks early, maintaining max FL350 whilst that was occurring. Also, although unpopular, minimise their breaks to the legal minimum. CI 0 and other mitigations to ensure the fuel penalty for non-optimal levels was minimised. Once breaks completed, then I would step climb. If 3 pilot ops, I wouldn't have a problem with OFCR as there would only be one pilot in there at a time and it's designed for two so ventilation would be less of an issue (I would probably sleep with my head at the entrance end though!)

[Cough]

BleedingOn - Surely using that logic 3 C/Crew can also rest (half occupancy)??

Not a path I think I'd follow...

[Rads]

The problem is not one of the number of crew in the area but of enough ventilation for smoke removal. Therefore dispatch with CAC u/s and planned requirement for use of OFAR requires fuel planning for Max FL350. I would also suggest the planned use of OFAR and CAC fails in flight sensible to complete required rest as soon as practical descending to or maintaining FL350 or below and then climb to normal cruise. But that's just me, it only says prior to dispatch.

[Cough]

Rads - Best explanation I've heard for this DDG restriction yet. Thanks.

[sleeve of wizard]

From the FCOM, A single CAC provides sufficient air to operate its associated pack in all operating modes.
Therefore there will be sufficient ventilation with only 1 CAC providing air to the pack.
Recirculation fans assist the packs to maintain a constant ventilation rate through the cabin.