777 Pack Inoperative

[Bamba Zonke]

Can anyone please explain why, with one pack inoperative, the RTOW on the 777 is reduced? With less bleed demand, shouldn't it be higher?!
The MEL even suggests switching the other pack off, in order to provide a higher RTOW. Go figure..

[mutt]

I thought that they changed that....... if you have one pack supplying the complete demand this is the amount of thrust that you have to base the takeoff calculation upon, hence more air, less thrust, less takeoff weight.

But as i said, our initial 777's had that restriction, but i havent seen it in the latest version of the AFM.

[Bamba Zonke]

This limitation is still present in our MEL (Cathay). So, if only one pack is supplying the air conditioning demand, then it will actually require more bleed air than if both packs were running? More bleed equals less performance. Now I get it! Thanks!

[flyhardmo]

In the pack inoperative dispatch, the functioning pack maintains a high flow mode to compensate for the inoperative pack. The Fan Air Modulation Valve will also open fully on the engine supplying bleed, because the single pack operation in high flow mode requires cooler input bleed air. The operation of one pack in high flow mode therefore requires that the EPR on that engine be further reduced (relative to normal pack flow) to maintain EGT margin at the higher fan and core bleed level. Because the thrust management system will not permit any significant thrust asymmetry to be set, both engines will be throttled back to the lower level.
The net effect is that both engines are producing less thrust in the Pack Inoperative dispatch even though only one is supplying bleed air, at a higher rate, to supply the single operating pack. Also, after the assumed engine failure, the operating engine maintains the lower EPR value initially set, so the thrust in the one pack inoperative case is lower throughout the takeoff manoeuvre. :ok:

[Bamba Zonke]

Thanks very much for clarifying this! Excellent. Cheers!