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.