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-   -   747-400: 3 engine taxi? (https://www.pprune.org/questions/598299-747-400-3-engine-taxi.html)

Uplinker 14th Aug 2017 10:50

747-400: 3 engine taxi?
 
I don't fly the type myself, but saw a Virgin 747-400 taxi in with Eng 3 stopped, and all the gear doors open and dropped.

Was this simply fuel saving - in which case why only one engine off, rather than two - or was it a fault?

Dan Winterland 14th Aug 2017 11:57

Very common to shut an engine down on taxi in to save fuel. On the 744, the number 3 is the obvious choice as the number 3 hyd system is the only one not used by any braking system.

Was a VS 744 SOP when I flew them 15 years ago.

eckhard 14th Aug 2017 12:15

When I started in BA, we only shut down No 3 but then after a few years the procedure was to shut down Nos 2 and 3. Only an SOP for taxi-in after landing although at JFK in the snow, we sometimes started all 4 on departure and then shut down No 2 and maybe also No 3 for a while when the delays were excessive.

Uplinker 15th Aug 2017 04:32

I don't know where my copy of "Flying the big jets" is, with its schematic of a 747 classic hydraulics, but I am guessing there are no PTUs or Electric Hyd pumps to maintain systems when engines are shut down?

On the A320/321, we run the elec yellow hyd pump to keep systems alive during single engine taxi.

TowerDog 15th Aug 2017 10:03

The 747 classic sports an electric hydraulic pump for the Nr. 4 system to maintain brake pressure with the motors shut down.

wiggy 15th Aug 2017 10:17

As others have said taxiing in with number 3 shutdown was a common SOP for many years at various airlines, at our outfit shutting down number 2 as well came along afer I left the fleet..


but I am guessing there are no PTUs or Electric Hyd pumps to maintain systems when engines are shut down?
Don't remember losing any hydraulic system with number 3 shutdown - Now I might be wrong 'cos it's "been a while" so this is a strictly as I recall it but I think even with 2 and 3 shut down their respective hydraulic systems will still be powered by the respective demand pumps ( Air Driven Pumps using Bleed air from the main Bleed duct, rather than using Bleed air of it's "own" engine...) .

As Tower dog mentions there was also an electric pump on system 4 for ground use (e.g during push back).

mustafagander 15th Aug 2017 11:51

Wiggy is correct. The B747 series has Engine Driven Pumps and Demand Pumps for each of 4 systems. If any engine is shut down and the DP is armed, it will run and all 4 hyd systems are powered. Beware of monkeying around without DPs - it bites. Taxi on 3 is quite normal and the decision on which inbd engine to shut down is influenced by the direction of turn onto the bay.

Akrapovic 16th Aug 2017 01:07

Almost.
3 is shut down purely because it's the only engine that doesn't supply to the brake system. Nothing to do with turning in. (Also, it's shutdown after a three minute idle, and at LGW there are certain stands that we must have all 4 running to avoid jet blast when taxying on three engines)

esreverlluf 16th Aug 2017 05:35

Wiggy and Mustafa are correct!

Engines 2 & 3 each have an electric demand pump, so hydraulics and braking systems are not affected by having either one shut down. SOP at my airline is exactly as Mustafa says. Engine cooldown time before shutting an engine down is also dependant on engine type (GE or RR) in our operation.

darkbarly 16th Aug 2017 10:26

Doors extended could be a fault. Like other types, after alternate gear extension on the 744, doors do not close. Engine shutdown may be a red herring. Post maintenance taxi maybe?

Uplinker 17th Aug 2017 09:27

It could have been, I really don't know. This was at MAN and we had pushed back from 213 or thereabouts and the 744 was taxiing in from the runway direction with Eng 3 not running, but there are also maintenance hangars over there.

In the BAe146, we used to shut down both outers as we taxied in.

Akrapovic 17th Aug 2017 15:45

Sounds to me like a standard taxi in. Eng 3 shutdown after three-minute cooling period.

Not sure about your reference to gear doors, but on the 747 some remain extended with the gear down.

BBK 19th Aug 2017 13:07

Uplinker

To answer your question why only shutdown one engine the reason, I suspect, is that it might struggle on two engines without exceeding the maximum recommended thrust setting. Anyway the SOP is as Dan W stated, (well remembered Dan!).

Dan Winterland 20th Aug 2017 12:58

Thank you BBK. It's been a long time and three types since!


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