Jet engine throttle-down time

Question

Apparently airline jet engines lose ~80% of their thrust within 1 second after fuel is cut. However this seems to conflict with long throttle-down times noted in another question.

This lead to a couple of questsions:

• How long do jet engines really need to throttle down?
• How do they react to fuel-cut or moving the throttle to idle (see first question)
• Why the difference between the two answers?
• If an engine is throttling down quickly, does the rotation speed of the engine really drop by 80% within 1 second? How does it slow down so quickly, is there so much friction?

I'm mostly interested in commercial passenger transport airline engines, not GA, military, ...

Answer 1

There’s no “throttle down” per se in jet engines. However engine thrust does not vary linearly with spool speed and most engines operate within the 70-100% of maximum speed, with most of the thrust range between 85-100%.

In addition, there is a lag time for the engine to “spool up” from an idle power setting to effective thrust output which has to be taken into account during operation. This is one reason that most jet aircraft have speed brakes. They allow the pilot to maintain a higher thrust setting which allows for quicker response time to thrust lever inputs for a given airspeed at the expense of more parasite drag and fuel consumption. Critical areas of flight such as VFR patterns, instrument approaches, short final, and go arounds, where near immediate thrust response is required makes this a useful flying technique in jets.

Answer 2

For jet engines in general, the thrust produced is not linear to the engine rpm. In fact, the percent of thrust produced decreases by approximately the 5th power for high bypass turbofan engines and 3.5th power for turbojets per N1 rpm. With such considerations, a thrust of 20% of max thrust is achieved at approximately 72% N1 rpm which is still a high rpm. The long throttle down times are because the engines still keep their angular momentum so they will keep on spinning until the friction in the shaft slows them down.