Concept of APPR IDLE - A320

[FlightDirector7]

Hi to all,

I wanted a clarification of the concept of APPR IDLE on the airbus. What does it exactly mean and why is it important ? I recall having "APPR IDLE ONLY" on the status page of a particular failure during a sim session (I'm sorry i cannot recall what the failure was ), but i just wanted to get your thoughts on this particular concept ??

Cheers

[Swedish Steve]

Most jet engines have two idle settings. On old engines it was ground idle, and flight idle. With computor controlled engines, flight idle has changed to approach idle. The reason is that there is a mandated acceleration time from idle to take off of 6 seconds for the go around. So this time is correct from approach idle, and then ground idle can be made much lower to lower idle fuel useage on the ground, and during the descent. Approach idle is typically set when flaps are lowered. After you have started your A320, watch the idle speed increase when you select flaps to take off.

[TyroPicard]

CFM engine - When you select FLAPS after start, the N1 remains the same. Approach Idle is only set in flight, when the FLAPS lever is not in Zero position.
It allows the engine to accelerate rapidly to G/A thrust, and varies with a/c altitude.
The other idle is Modulated Idle, which applies in flight with FLAPS Zero selected, or on the Ground (except in Reverse Thrust). It varies with bleed system demand, and provides less thrust than Approach Idle.

When you have APPR IDLE ONLY as a Status message it means that Modulated Idle is not available, so for descent and deceleration planning the engines produce higher thrust than normal.

[avionneta]

second answer is right

[FLX/MCT]

I recently read the book 'Fly the wing' and captain Webb wrote that the response of the engine also depends on the bleed valve position of the compressor stage. (It should be closed... for a fast acceleration)
In the A320 (CFM) Fcom it is called Variable Bleed Valve (VBV)
What exactly manages this valve?

[TyroPicard]

Luke
It's all done by the FADEC (Full Authority Digital Engine Control), which receives info from sensors, and controls absolutely everything....starting, fuel flow, VBV and variable stator vane schedules, overspeed protection etc.
It has two channels for redundancy, and an internal generator for independent power supply.
The VBV itself is on the engine core between the HP and LP compressors and helps control the airflow through the engine by spilling excess air when necessary. I'm no expert but I think there must be several (lots?) VBV in an annular arrangement around the core.
Try googling CFM variable bleed valve - the first result for me is a link to a very detailed book - wish I'd done it before I wrote this answer!
TP

[Canuckbirdstrike]

Approach Idle is an automated setting from the FADEC that is actuated based on the Flap 1 selection on the A320 to increase the engine idle so that the thrust response time from idle to TOGA meets the certification standard. It is meant to cater to a go around. In days gone by this was done manually by having a minimum thrust setting on approach for non-FADEC aircraft.

[Dream Land]

Steve, Tyro and others, thanks for the great explanations.

[moosty]

The acceleration time for a jet engines is longer in lower RPMs and shorter in higher RPMs. For instance it may take double time from 50 N1 to 60 N1 compared to from 85 N1 to 95 N1 while the thrust difference is very low for low RPMs compared to high RPMs. The difference of thrust between 30 and 40 N1 may be the 10% of difference between 85 and 95 N1. So increasing idle RPM a little bit when you may need high RPMs does not result in high fuel flows (little thrust change) but increase safety margin. In T-37 (tweetbird, military training aircraft, with slow accelerating centrifugal engines) final approach is flown with speedbrake extended, so the RPM is kept high and if a go around is needed retracting the speed brake takes only 2 seconds but saves you more than 10 seconds for reaching full thrust.

[pastyboy]

I came across this post when head scratching recently. The official help is buried in the FCOM. DSC-70-40-40.

[barit1]

FLX/MCT:

I recently read the book 'Fly the wing' and captain Webb wrote that the response of the engine also depends on the bleed valve position of the compressor stage. (It should be closed... for a fast acceleration)
In the A320 (CFM) Fcom it is called Variable Bleed Valve (VBV)

I believe CFMI refers to VBV's as Variable Bypass Valves, or informally as "bleed doors". There is a circumferential array of perhaps a dozen VBV's in the "gooseneck" duct between the LPC and HPC.

What exactly manages this valve?

At high thrust (cruise & above) the VBV's are closed so the total LPC flow enters the HPC. But at lower rpm, the LPC continues to pump - more than the HPC can "swallow" - and there is risk of an LPC stall (surge). So it is reducing N2 that triggers the opening of VBV's, to unload the back pressure on the LPC.