Many questions and observations here have highlighted the idea of using some kind of electric towing system for towing airplane to and from the runway, instead of using fuel to taxi. One of the impediments to this has been the need to preheat the engines during taxi, prior to take-off at full thrust.

Why use the inner engines of a four-engine airliner for taxi instead of the outer engines? Are the outer engines designed to not require preheating prior to full thrust?

If the outer engines do require preheating and are no different from the inner engines, how much fuel do they use to preheat the engines and would it not be cheaper to use electric taxiing then to preheat all engines?

  • $\begingroup$ “The major impediment to this has been the need to preheat engines during taxi prior to take off full thrust.” That statement reflects an incorrect understanding of airline operations. $\endgroup$ – Ralph J Apr 23 '18 at 14:46
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    $\begingroup$ Using outer engines instead of inner engines would create adverse yaw, making it harder to taxi straight and turn smoothly. If it would be cheaper to use electric taxiing, then they would already be doing it. $\endgroup$ – abelenky Apr 23 '18 at 15:01
  • $\begingroup$ Renewable energy is cheaper over time and once its paid for itself, it is free $\endgroup$ – securitydude5 Apr 23 '18 at 15:11
  • $\begingroup$ @abelenky No it doesn't. It's still symmetric. The answer is that they use the inboard engines to reduce FOD ingestion, since they will have to run at higher thrusts and the outboards tend to hang over grass/runway parts that aren't swept as well. (Related to the lack of reversers on the A380's outboards) $\endgroup$ – user71659 Apr 23 '18 at 15:51

How much fuel is used for preheating the outer engines of a four-engine airliner that were not used for taxiing?

It's not a lot. The engines are only run at idle, and only for a few minutes (3-5). Given that fuel burn at idle is only 5-10% of full power operation, and that a 4 engine planes are typically performing long haul missions (8+ hours), the warmup is less than 1/10th of 1% of the total fuel that they will burn during the entire flight.

Are the outer engines designed to not require preheating prior to full thrust?

No. Inboard and outboard engines are typically identical when it comes to the turbomachinery. Both need the same warmup.

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