Hi,

For the 747-8, which number of engines should be turned off during taxiing to save fuel (inner or outer ones?) ? 

I saw one clip when the Capt, just switch on #2 and #4 during pushback and taxiing.

And if with only two engines running, should the APU on too ? 

Thanks

Edited December 19, 20187 yr by hkhoanguyen

i think its different depending on airlines SOP for example the  airline i fly for virtually says only 2+3 to be turned off during taxi

On the -400 it is always the inboards (3 and potentially also 2) which would be shut down as the 1 & 4 hydraulic systems run normal brakes and steering. Unlike many twin engined aircraft, there is no requirement to have the APU running before shutting down the inboard(s) on taxi in. If you are doing it on taxi out (not really worthwhile unless your taxi out time is expected to be greater than about 30 minutes) then you would need the APU bleed air at some point to start the other engines before takeoff so little point in shutting it down just to start it up again a couple of minutes later!

The system architecture may well be different on the -8 and therefore different engines may be used.

Worth considering aircraft weight and (not applicable in FS but in real life very much so) things like taxiway slopes, system defects and apron layout (considering where your jet blast is going to go and the associated risk to ground equipment etc) before deciding to do a reduced engine taxi as at higher weights and/or with upsloping taxiways/aprons the amount of thrust required to manoeuvre on 2 or 3 engines may be excessive.

Assuming flat ground etc, the -400 can be taxied comfortably with 2 and 3 shut down up to about 300 tonnes, and with No. 3 shut down up to about 350 tonnes. Someone with -8 knowledge would have to chip in with equivalent figures!

Beyond simply the systems which the outer engines can power, as others have said, on four-engined aircraft, it's almost always the outboard engines which would be running if taxying with some engines shut down, since being further out on the wing, they can create more leverage for asymmetric thrust to assist a turn should that prove necessary. It's also worth noting that because the outboard engines have slightly greater ground clearance, owing to the wing's dihedral angle, they are slightly less prone to picking up FOD, which is another advantage to having those rather than the inboard ones running.

6 hours ago, Chock said:

 they can create more leverage for asymmetric thrust to assist a turn should that prove necessary. I

A neat summary and intuitive for those of an engineering background. The greater clearance for FOD damage is less obvious but equally important.

11 hours ago, skelsey said:

On the -400 it is always the inboards (3 and potentially also 2) which would be shut down as the 1 & 4 hydraulic systems run normal brakes and steering.

On the B744 the No3 engine is normally the only engine which will be shut down after landing, leaving the other three running until the aircraft arrives on the Gate. This is mainly due to the fact that the No2 engine's hydraulics power the alternate brakes and the stabiliser trim (which is normally set to 6.0 prior to shutdown to ensure proper refuelling of those fitted with a Stabiliser Fuel tank).

 

11 hours ago, Chock said:

It's also worth noting that because the outboard engines have slightly greater ground clearance, owing to the wing's dihedral angle, they are slightly less prone to picking up FOD, which is another advantage to having those rather than the inboard ones running.

Although this is generally true, the outboard engines usually hang over a non-paved surface when manoeuvering on narrow taxyways.  Therefore, great care is always necessary when taxying the B744, because too much thrust on any one engine can easily result in a FOD incident.