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Bubi352
13th Feb 2015, 00:58
I am trying to find more on the engineering side what is exactly happening when approach idle is selected. I can see on the EWD that FF increases which results in a N2 increase and N1 increase. Are there any data or reference to it?

Amadis of Gaul
14th Feb 2015, 01:26
It's all PFM, Pure Bleeping Magic.

Drifter72
14th Feb 2015, 04:46
I guess what happens is the same as you advance Thrust a bit more - only this time it is done automatically so we will not forget to do it.
Why - due to the certification reasons. Engines must be capable to accelerate from 15% to 95% within 5 seconds in case of G/A.
And the airframe must be capable to achieve 3.2% landing climb gradient (all engines) within 8 seconds.
To achieve that we need slightly increased idle to allows engine to accelerate rapidly from idle to G/A thrust - since idle in all other situations is driven by fuel efficiency and is lower and acceleration would take more than 5 sec for engine and more than 8 seconds for airframe.

Although...even with approach idle active, if you are high and fast (read un-stabilised - but within the limits at 500'), idle may be below certification threshold. This is also parameter that shall be taken in to the consideration and that is why it shall be also monitored by FDM.

If the question was purely technical - what actually happens with FADEC or fuel control or thrust control or similar...than disregard what I wrote.

Metro man
14th Feb 2015, 11:03
Flaps 1 (slats only) in the air will give you approach idle which is worth taking into account as you are normally trying to slow down at this stage.

Turning on engine anti ice will give an increase in idle speed as well.

Bubi352
14th Feb 2015, 16:09
I read somewhere if my memory served me right that fuel flow and N2 vanes angle are commanded to increase (essentially similar to a prop aircraft) which results also in an N1 increase thereafter. Can anyone confirm this?

Fursty Ferret
15th Feb 2015, 11:50
Flaps 1 (slats only) in the air will give you approach idle which is worth taking into account as you are normally trying to slow down at this time.

Indeed. My experience is that you can achieve a better rate of descent clean at GD with engines at flight idle, than at flap 1 and S speed.

tdracer
16th Feb 2015, 04:38
There are multiple idle schedules programed into the FADEC, and each idle schedule has different minimum parameters. The FADEC will pick between those minimum parameters (burner pressure, N2, and occasionally N1) and select the one that provides the highest thrust.

Speaking for Boeing, NOT Airbus (although AB is probably similar in concept), approach idle is automatically selected when Engine/Cowl Anti-Ice is ON, or flaps in landing (20 or greater, or 25 or greater, depending on the model). On some installations, selection of Continuous ignition will also command approach idle.