737 Low & High Idle

[Vandelay]

Hi. I was wondering if anyone can clarify the reason for low and high idle on airliners, particularly in the 737 (CFM). Is it for quicker spool up time or actually to decrease stress on the brakes on the ground?

Thanks a lot.

[BmPilot21]

Both reasons are correct I believe. High idle is required to improve spool up times in flight. However, on landing, as you say, low idle reduces braking activity.
Two points:
1. There is a 4 second delay after toucdown before low idle to improve reverse spool up time.
2. High idle is a pain in the descent as it makes the 737 very difficult to 'go down and slow down.' High idle is 32% N1, and for comparisson, you only need 55% on final approach with flap 30, and 50% with flap 15!

Low idle is 22% N1.

[QAVION]

"Is it for quicker spool up time or actually to decrease stress on the brakes on the ground?"

It's not so much a case of "high and low", but "high and normal". High is for approach so that spool up time is less for go-around... and, for many aircraft, when bleed air demand is greater than normal (such as when anti-icing is required).
Low/minimum/normal (or whatever you like to call it) is for economical flight/taxy. I'd be more inclined to think that better braking is just a non-intentional side effect.

The time delay from touchdown to normal idle varies depending on the aircraft type.

Rgds.
Q.

[Capt PPRuNe]

Closing this thread until Vandelay fixes email as all notification of replies are bouncing back to me.
:mad: