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Apparently jet engines can take several seconds, up to ten or fifteen seconds, to go from full power to idle.

Why is this? Can't a simple fuel valve shut off fuel flow? Or reduce it to whatever flow rate corresponds to idle? Either of these, if done with a valve, should happen almost instantly as far as I can tell.

This would surely cut thrust, regardless if the engine still spins for a while. Is there something dangerous about this idea? To me it seems like a fuel valve can close and open again as many times as it wants.

Note: I'm interested in turbojets and turbofans. And I care about throttling down only.

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    $\begingroup$ To prevent compressor surge. See (maybe duplicate): Why do turbine engines take so long to spool up?. $\endgroup$
    – mins
    Oct 1, 2015 at 6:57
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    $\begingroup$ Inertia. But thrust will drop immediately if the fuel supply is cut. No heating, no thrust. $\endgroup$
    – Peter Kämpf
    Oct 1, 2015 at 6:57
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    $\begingroup$ @mins I read that but I don't see how a surge could happen when throttling down suddenly. Maybe the opposite could happen, a sudden drop in pressure, but that doesn't sound dangerous like a spike in pressure is. $\endgroup$
    – DrZ214
    Oct 1, 2015 at 7:02
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    $\begingroup$ @DrZ214 of course, all of this is ignoring that in modern high bypass engines, the bulk of thrust (like 80% or more) is generated by the movement of air by the fan at the front - to cut that thrust to zero immediately, you need to stop that fan or gear the fan blades so they can be feathered. Stopping the fan means a hefty brake (those fans weigh tonnes and have a high RPM) and feathering the fans means more weight and complexity. Airlines don't want either of those, so it hasn't ever been a problem that has needed to be solved. $\endgroup$
    – Moo
    Oct 2, 2015 at 8:39
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    $\begingroup$ @PeterKämpf: While inertia will keep the shaft & everything attached spinning for a short while, the air passing through the engine does serve as a massive brake. Obvious, from an energy point of view : the spinning engine contains only a fixed amount of energy, and accelerating air through the engine drains that energy. High RPM means a lot of energy, but also a lot of air being accelerated, trust produced while the engine spools down, and thus energy carried away. $\endgroup$
    – MSalters
    Oct 26, 2015 at 15:31

3 Answers 3

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The simple answer is rotational inertia, once you get a heavy mass spinning fast that heavy mass will have a tendency to continue spinning fast unless something else is acted upon the object. Piston engines have a great deal of friction between the piston walls and pistons (how they hold the explosion in) this helps them slow down faster than a jet in which the turbine is made to easily handle this thus the joints are very well made and very free moving. While cutting fuel will keep the engine from continuing to generate thrust the turbine will continue to spin until a force causes it to stop (drag and friction really).

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Jet engines have little mechanical margin, and operate at high temperatures. That means all components expand by more than the mechanical margins when heated. This is no problem when everything expands and contracts together in a controlled fashion. This may be counterintuitive but is easily demonstrated. Cut a circular hole in a metal plate, and heat up the plate. The hole will grow with the same expansion rate as the surrounding material.

Now what happens if we suddenly stop the fuel flow? The compressive heating of air flowing into the engine doesn't suddenly stop, but all the air flowing through the engine does cool it. This means that the turbine in the back does cool as relatively cool air flows through it. Not only does this cause thermal stresses, but the turbine will shrink.

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  • $\begingroup$ Are you saying that even if the throttle control is pulled to idle, some ECU will continue to feed fuel into the engine to avoid rapid cooling? $\endgroup$
    – feetwet
    Apr 21, 2018 at 16:12
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Basically it is that the more weight the moving object has (the turbine in this case) the more time it takes to slow down because of the energy stored in a moving object. It is the same in all object which have a large mass. In an automobile, there is the flywheel which stores some of the energy the engine produces so that a little hiccup in the pistons would not interrupt the spinning. With the jet engine, the large mass of the turbine also keeps the increase or decrease of speed at a constant rate so that the engine would not produce "jerks" of power when the throttle is changed.

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    $\begingroup$ This is a start of an answer; can you expand on it? $\endgroup$
    – CGCampbell
    Nov 11, 2015 at 3:06

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