A320 High & Low Altitude Idle - IAE Engines

[Bubi352]

I have observed now with certainty that on an idle descent, the FF, N1 and N2 parameters are modulated significantly passing through 17,500 feet. It seems there is a high altitude and low altitude idle.

Above approximately 17,500 feet, those are the parameters I had on a constant basis:

FF: 1000 pph
N1: 44%
N2: 74%

When the changeover happens, the parameters change to the following:

FF: 720 pph
N1: 36%
N2: 68%

Now, what I am trying to figure out is why? Why this changeover? Why does it need a higher idle at higher altitude and a lower idle at lower altitude?

[tubby linton]

Air bleed demand? Are the figures from a CFM?

[Bubi352]

No, IAE engines. Those are the numbers I extrapolated on several idle descent at 300 knots. Those numbers have been very consistent on several A320s we fly all with IAE engines. The changeover occurs in a few seconds so it's definitely modulated.

[Turbine D]

My guess would be at higher altitudes, the engine requires both more fuel flow and engine speeds at idle due to the thinner air. Then at the crossover point observed, the idle speed and fuel flow is reduced as the air becomes denser, Could be wrong…

[Bubi352]

That's what I am thinking too. To prevent an engine flameout but wanted to see if someone knew exactly.

[pattern_is_full]

Getting back to tubby's point - not just engine flameout at higher altitudes, but passenger flameout.

The engines provide pressurization bleed air as well as thrust. Without enough N, they can't keep the pressure up. Until below 17,500 feet or whatever, where a lower bleed pressure is adequate.

Concorde had a minimum N(forget if it was 1 or 2) of 94% in descent, because a certain amount of power was required to keep the accessories running adequately, regardless of whether the engines would flame out.