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Airbus (Fenix) Unexpected Descent Behavior

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22 hours ago, zachlog said:

As the A/C was approaching the TOD, I changed the ALT from 34000' to 10000' and as always pushed the ALT button to initiate the descent.  The A/C started descending with mode OP DES (per the FMA) but the flight's phase did not change from CRZ to DES (PERF page), the VS window displayed a descent rate of -1700, Thrust Mode was THR/IDLE and in the scratchpad message "NEW CRZ ALT 10000" displayed in white font. 

I don't believe the distance to the TOD is the issue, but the distance to the airport:

I've seen this happen with the Fenix on two STARs where the TOD is quite early (LENDY8 into KJFK and ROBUC3 into KBOS).

Edited by voske

Marc

  • Author
1 hour ago, voske said:

I don't believe the distance to the TOD is the issue, but the distance to the airport:

I've seen this happen with the Fenix on two STARs where the TOD is quite early (LENDY8 into KJFK and ROBUC3 into KBOS).

I read the post you are referencing.  It's a touchy feely post that doesn't really provide a firm answer. 

  • Amir says that "The aircraft will, at a specified distance from the destination runway threshold - no longer engage managed descent, but instead assume a step descent to a lower level - hence the new CRZ FL message."  OK, what's this distance ?  I don't see it.
  • Secondly, when flight management determines the TOD, shouldn't this distance from the runway threshold be taken into account so that TOD does not occur too early ?  I would think "yes" but.....
  • There are also a couple of other comments about initiating managed descent too early, e.g., 50 NM or more.  This was not the case in my case.  The magnification levels on both the NDs were set appropriately so that I could cross-check the start of the managed descent.  There was no guess work.

I think somewhere there is a better answer to this issue, we'll probably come across it one day.  Today I did a flight using the identical managed descent initiation approach as the problem flight in this post.  It worked just fine. 

 

zachlog

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