Originally Posted by
lurkio
FCOM DSC 70-20 Idle control
The FADEC has the following three idle modes :
Modulated idle
Is regulated according to :
•bleed system demand
•ambient conditions
Is selected :
•In flight, when the flaps are retracted (FLAPS lever at zero position)
•On ground, provided reverse is not selected.
Approach idle
Is regulated according to aircraft altitude, regardless of bleed system demand.
Is selected in flight, when the flaps are extended (FLAPS lever not at zero position)
Allows the engine to accelerate rapidly from idle to go-around thrust.
Reverse idle
Is selected on ground, when the thrust lever is in REV IDLE position.
Is slightly higher than forward idle thrust.
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The repressurisation segment needs some bleed so it will be above actual idle but if you watch it as you descend the modulated idle actually decreases as you get lower. As far as the AFS system is concerned that is idle. However, pull open descent and you get a real idle which will show up on the EWD.
One of the advantages of the integrated systems these days where it works it out for you. Gone are the days of a FE's hand appearing the tweak them thrust levers forward because the cabin isn't descending.
cabin not descending enough, so should airplane not! Therefore, AFS reduces V/S by adding thrust while maintaining the managed speed.
But why Cabin not descending enough?