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Understandably, airliner pilots - particularly those operating in extremely cold climates and airports - should preheat the engine at idle for 3 minutes or more so they do not apply full thrust and subsequently take off and fly dangerously on cold or frozen oil.

one answer to a question that refferenced to a day that electric towing becomes feasible and as fast and as good as engine powered taxiing stated that electric taxiing would not work because an engine should not be immediately shut down despite supplying outside solar electric power to the aircraft for airconditioning and hydraulic functions because engines should not be shut down immediately

But why should pilots keep the aircraft's engines running and not shut down immediately post landing? How do ground support work on the aircraft when its engines are running and how do passengers disembark in places that aren't equipped with jetties? Someone made the argument for the need to keep the engines running after landing when I suggested an electric towing system that could be installed along the runway for taxiing pre and post flight to save tonnes of fuel.

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  • $\begingroup$ "How do ground support work on the aircraft when its engines are running and how do passengers disembark in places that aren't equipped with jetties" that looks like a random question in the middle of the sentence. Could you please clarify? $\endgroup$ – Federico Apr 25 '18 at 13:23
  • $\begingroup$ I assume for safety, engines are shut down completely when passengers and ground support are on the ground $\endgroup$ – securitydude5 Apr 25 '18 at 13:28
  • $\begingroup$ I don't really understand your question. Are you saying that pilots should or should not keep the engines running on the ground? Engines are running after landing because the aircraft has to taxi to its stand, but after that I have no idea why you think they're still running. But I may have misunderstood you, so please consider asking your question in a shorter, simpler way. $\endgroup$ – Pondlife Apr 25 '18 at 13:47
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    $\begingroup$ I am saying one answer to a question that refferenced to a day that electric towing becomes feasible and as fast and as good as engine powered taxiing stated that electric taxiing would not work because an engine should not be immediately shut down despite supplying outside solar electric power to the aircraft for airconditioning and hydraulic functions because engines should not be shut down immediately $\endgroup$ – securitydude5 Apr 25 '18 at 14:02
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    $\begingroup$ @securitydude5 I actually think you have a valid question, though to help others understand, it would be useful to link to the question/answer you mentioned and make it clear your scenario is at an airport where aircraft are towed once clear of the runway. $\endgroup$ – Notts90 Apr 25 '18 at 14:35
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Especially once you get into the bigger jets/bigger engines where such fuel saving would make sense, you need to consider reverser usage and engine cooldown time: the larger the airplane, the more likely it will need to use full-thrust when deploying reversers to slow down after landing.

Shutting down an engine imediatelly after running at full thrust is generally a bad ideea, and most engines have a specified number of minutes to be run at idle after using full-thrust reverse, before being allowed to be shut down. By that time the plane is mostly at the gate, so any saving from the electrical towing is greatly reduced.

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  • $\begingroup$ I believe reversers are not used very often $\endgroup$ – ElmoVanKielmo Apr 25 '18 at 22:27
  • $\begingroup$ Normal SOP at my company is full reverse every landing, as it is a safety-related item. In practice, the manual allows you to only deploy the reversers (always!) but stay at idle thrust if full reverse is not needed. Mind you, with auto-brakes the reversers will not reduce landing distance much, it just means the brakes have to work harder. If you have a contaminated runway or a brake fault, then reversers might save your life $\endgroup$ – Radu094 Apr 26 '18 at 8:20
  • $\begingroup$ My belief was false then $\endgroup$ – ElmoVanKielmo Apr 26 '18 at 13:35
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The engines supply power for the aircraft's systems: hydraulics for the steering and brakes, electrical power for the control system, electrical power and air conditioning for the interior. If you shut off the engines, you have to start an APU first (if one is available on the plane, not a given).
You can switch off most engines, but generally one has to be running.

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In addition to Hobbes' answer you have to consider that "immediately after landing" the aircraft is on the runway. Without engine power it is unable to vacate it, nor to taxi to the gate. This would have severe repercussions on the airport schedules (waiting for a tow-truck to show up, link, and pull the aircraft away is a massive waste of time).

There are ideas around that aim to change this, but until those solutions are certified and adopted, the current status is that engine power is needed for taxiing.

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