Taxying

[CaptainCriticalAngle]

Apologies in advance if this is a silly question ...

But why do some aircraft taxi with one engine off until the last minute, then switch it on just before getting on to the runway?

I've seen this on a few turboprops on YouTube while they're taxing.

PS I'm obviously still a student of aviation.

[LadyL2013]

Fuel saving. Why do a taxi on 2 engines when you can do it on one?

[NudgingSteel]

Jets too...you just don't notice unless there's lots of water on the taxiway!

The fuel saving per flight might only be a few kgs...but add that up over an entire fleet over a year and it adds up to quite a saving for the airline. And (for what it's worth) less CO2 emissions too.

[DaveReidUK]

No, taxying or taxiing. :O

[AeroSpark]

Is that a good idea for the engine though? How long does a modern engine take to get to running temperature. I wouldn't have thought going from cold to full takeoff power in such a short space of time would do much for the engine.

[Talkdownman]

Ah, but what about gas turbine...

[RHS]

Guys will often start both engines early on the first flight of the day to get the oil temperatures up and the engine warm. As you say, and in my opinion taking a cold engine and applying take off power is not particularly smart.

For the rest of the day though particularly with short turnarounds the engines never really totally cool down which eradicated the cold engine problem.

On the aircraft I fly if departing into icing conditions the oil temperature must be above a certain temperature which can take 15 minutes to be reached. So come winter you're going to see a lot of two engines running. Also on icy taxy ways etc it's a good plan to have both engines running.

[chevvron]

Easyjet often use this procedure, I noticed it when I flew to Barcelona a couple of years ago (Barcelona 5 Getafe 0. Yes we managed to get tickets).