A320 - Speed reduction in THR IDLE

[FullyFullyReady]

Hi All

Have had a look through FCOM to no avail to try and find the answer - if in descent in THR IDLE is there any logic to determine the vertical speed applied in the event of an airspeed target reduction until the IAS is attained?

Thanks
FFR

[BEL1000]

I am not very sure if I understand your question. But if it says THR IDLE when AP is on then the way to reduce speed is to reduce rate of descent until reaching target speed then descending at planned rate again. If your question is how to slow down if it is throttle at idle power.

Depending on rate of descent the throttle may increase power again if the rate of descent is not high enough. The AP will automatically adjust rate of descent or climb to reach the target speed then climb or descend at the planned rate.

AP means Autopilot if you do not know.

[FlightDetent]

I think there's no such thing target vertical speed for decceleration in THR IDLE / OP DES.

I recall seeing something about target decceleration rate, similar to 10 kt per 10 s, but have no time to look it up now. The pitch command would be continiously adjusted to achieve and maintain that particular rate.

[BEL1000]

No there is not a target vertical speed for deceleration in THR IDLE with an optimised descent. What I meant by "planned rate of descend/climb" is that I mean that MCDU keeps "telling" what rate of climb/descent AP should adjust to.

[vilas]

FullyFullyReady
I can't remember where I read it but it is energy exchange something like 70% for speed and 30% descent.

[sonicbum]

Hi vilas,

thanks for the info, if you manage to get hold of the documents that would be very useful

[TSIO540]

Originally Posted by sonicbum

Hi vilas,

thanks for the info, if you manage to get hold of the documents that would be very useful

FCTM T/O or climb section explains the 70/30 acceleration logic. I can't recall reading it for descent but from operational experience you will tend to see RoD around 700fpm. In THR IDLE you would also be in DES or OPN DES where pitch controls speed. You can read about that as it relates to speed control by searching "open descent" in your manuals.

[*Lancer*]

Must be something more to it: If you pull speedbrake while decelerating the ROD won't change.