Expert: Marc Ebelini - 3/25/2007

Question
Hallo!

Many jet engine shutdown procedures require you to run the engines at a certain RPM above idle (usually 60 to 70%) for a certain time prior to shutdown.

I am curious as to the reason of this procedure.

Thank you in advance.

Answer
All engines including piston type have unique metallurgical problems that must addressed. In all cases it is the problem of uniform cool down. Temperatures within certain stages of engines must be balanced for a uniform cool down.
With piston engines it's mainly the cylinders. However in jets we have three main stages. The compressor, burners and rear turbine. At lower percentages the heat is retain in the rear turbine and burner. Shutting down at lower percentages will retain a higher heat. Since I am not familiar with all the engines of today. Shutdown procedures will vary from engine to engine determined by test and field experiences. Shutting down at idle speeds the turbine will slow down quickly without allowing some of the heat to be exhausted. If I recall the length of run time at any of the recommend shutdown percentages is also important. As an example. The clearance between the turbine blades and the outside case are very close, so should the case cool down to fast before the blades do, the clearance gap could shrink to a dangerously close clearance causing damages to the engine. You must understand that all metals expand and contract at different rates. So this becomes important when dealing with close tolerances. Has this answer your question satisfactorily