A320 idle descent

[IFLY_INDIGO]

When we use managed DES at higher altitude, the FMA shows THR IDLE but EWD does not show IDLE in green. When I select OPEN DES, EWD shows IDLE in green the N1 decreases.

It means in managed DES, though the FMA says THR IDLE, it is actually above the achievable IDLE thrust.

Why so? Any light on that?

[Edit: THis doesn't happen all the time ]

[vilas]

The descent profile has several segments one of them is repressurisation segment. When necessary this produces a repressurisation rate during descent. it is a function of the destination altitude and the selected cabin rate. During this the ATHR actually is behaving like in speed mode to maintain speed to a reduced rate of descent to adjust the cabin rate to a/c descent rate. So it is not at idle. After this segment starts the idle path segment when the ATHR stays at idle and AP manages speed.

[safelife]

E/WD stating IDLE is a true statement, while the FMA saying so is more like we-would-use-idle-thrust-here-but-maybe-we-don't.
For various reasons, as explained above.

[sierra_mike]

though it's nowhere written i understood it as sort of an additional buffer, which the AFS can make use of if it gets to high above profile where a speed increase to the upper end of econ speed range won't help any longer.

[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.

-----------------

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.

[AA757]

During a descent in managed mode, the thrust is set at Idle + delta on all Airbus A/C, this is to give some margin to the pilots in case the tailwind is stronger than estimated by the FMS, the theoretical descent path is calculated assuming a descent at Idle + delta. As soon as the pilot flies in selected mode, the thrust reduces to idle.

[IFLY_INDIGO]

During a descent in managed mode, the thrust is set at Idle + delta on all Airbus A/C, this is to give some margin to the pilots in case the tailwind is stronger than estimated by the FMS, the theoretical descent path is calculated assuming a descent at Idle + delta. As soon as the pilot flies in selected mode, the thrust reduces to idle.
Any FCOM reference?

Not sure if this is the case since I have seen many times thrust really set to IDLE in MANAGED MODE also.

[IFLY_INDIGO]

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.

-----------------

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?

[CaptainMongo]

Lurkio - good points all.

When the engines are commanded to operate in a modulated idle condition, is "IDLE" displayed above the EPR gauges?

From FM:

"IDLE Indication Displays green when engines are at idle thrust."


AA757 - I believe you are thinking about the idle input side of the IDLE/PERF inputs entered in the FMGC on the A/C STATUS page. An IDLE input only effects the performance data base in the FMGC - it doesn't have any effect on engine control.

[vilas]

IFLY INDIGO
This has been discussed before. FCTM says some thrust margin is added to cater for anti ice use but FCOM says aircraft will overshoot with anti ice. Take your pick.