Question in regard to the 747 two or three engine taxi operations

[Arzenim]

Hi there,

I am interested to know whether the APU has to be running and does it have to be on bus when two or three engine taxi operations are commenced.

Also are there any other procedures in that regard other than the fact that you have to acknowledge thermal warm up and cooldown times (5 minutes if I am not mistaken) and monitor the engines a little closely ?

Thank you very much :)

[Intruder]

Short answer is "No."

As far as practical considerations go, there is little savings in a 3-engine taxi with the APU running. Also, you have to consider whether you will have a clear path behind you for a crossbleed start (likely prohibited while taxiing), or will be taxiing long enough to make a difference before restart, before you shut down the APU.

FOD is always a consideration if you will have to use more than idle thrust, not to mention negating much of the fuel savings.

[negativeclimb]

Usually for taxi in we shut down two engines to save some fuel, engine and to decrease the constant thrust that affects brakes wearing. APU on only once approaching the gate.

for a taxi out no, all engines running.

[BalusKaptan]

APU not required for 2 or 3 engine taxi though many prefer to have it running when on 2. Crossbleed starts not really a problem anymore with respect to jet blast. PW4060s and Genx have sufficient air output at idle that you don't need to increase the thrust for a crossbleed start.

[Arzenim]

Thanks guys :)