UA839 SYD-MEL (6/5/12)

[Jayden Laing]

Question for the engineers/pilots on the B744's:

Today I noticed UA839 (SYD-MEL) taxying to 16L via Golf & Charlie. When I saw it crossing 34L on taxiway Golf I noticed that the number 3 engine had not been started and engines 1, 2 & 4 were all running. NOW my question is, when UA839 departed, IS/WAS it possible that it only took off with 3 engines operating at the time of departure or did the crew start the number 3 engine whilst holding short of 16L? If it did depart with only 3 engines operating, would there be signifcantly more stress on the airframe/wings rather than if all 4 engines were operating?

This just caught my attention as it was the first time I had seen this & I wanted to pose the question out there to see what anyone has to say.

[Nigel C]

I'm neither an engineer or pilot on the B744, but it used to happen a bit at Sydney when they were taxiing for 34L departure. I presume it was for fuel savings, and they'd always have all the engines going for the departure.

When we started doing the Twy shoulder widening works along Alfa, United were requested to cease 2 or 3 engine taxiing because of the jetblast damage they were causing to the new work areas and the resultant FOD they were spraying all over the taxiway behind them. The FOD obviously caused the closure of the taxiway until it could be cleaned.

It hasn't been an issue along Alfa since the work was completed, but I could imagine it might cause a grass FOD issue on the eastern taxiways if they're going for 16L and cross-bleed start the last engine. If only some of the big jet flight crews were a little more thoughtful as to what's going on behind them....

[Matt_L]

J,

No 744 engineer/pilot but can add a bit to your question.

Firstly, it definitely took off with all 4 engines. Flights with 3 engines for example are termed ferry flights and operate with just operating crew under strict conditions including performance (a ua 744 which had a birdstrike a few years back flew Syd SFO on 3 with just crew as an example!)

Secondly, it's common for a wide range of airlines, but from what I've seen very often it happens in the US , taxxying on 1 engine for a twin or 2-3 for 744 for example. This is done to save fuel on long taxis or long queues (such as to 16L Esp as it wouldve waited at 16R for a while to cross!)

Furthermore, engine 1 and 4 on the 744 deliver the main source of hydraulic pressure for nosewheel steering, brakes and this is likely why eng 3 inboard is the one not started till close to takeoff (which is about 5 mins to lineup from what I understand).