Originally Posted by
8che
1. Because the hydraulic C1 and C2 air driven demand pumps need air to work. The APU cannot supply 2 packs and hydraulics for take off. If 2 packs are on with APU that would conflict with the right
bleed air. The left and right air systems have to be isolated for the APU to give the bleed air relief and hence the take off performance improvement.
2. the DSP plays no part in any non normal procedures for B777 and is stated as such in the FCTM. Only an EICAS message indicates a problem. For the flight control check you are looking for the EICAS FLIGHT CONTROL message to generate with the full and free movement. Missing indications on the DSP are no indication of a failure and as such should not be used for diagnosis. Again it’s all in the FCTM.
Many thanks. That makes sense now. My systems knowledge isn’t quite there yet. Appreciate you sharing yours.