Problem with FMS descent

[ezfox]

I have issues with the T/D and descent profile of the FMS.

If the FP doesnt contain a FIX between the T/D one normally would expect (about 60miles before the destination),

the FMS places the T/D shortly before the last FIX (even if its still 200miles to destination) and starts the descent.

It descents then to 10.000ft, holds this altitude to the destination and continues the descent when apropriate.

 

Last example i have was the flight from MTPP to TXKF over  ETBOD UB891 PTA M596 NUBUS L459 TALSU L458 BDA.

Last FIX before BDA (target altitude set by FMS 8453ft) is GECAL. Cruise altitude was FL350.

The FMS places the T/D short before GECAL, the FIX gets an altitude set by FMS of FL284.

The airplane descents to 10.000ft, the vertical deviation value on the FMS then starts to rise even higher then current altitude (so if we would follow its path we would dive into ocean).

Later the vertical deviation value resets to 25.000ft (CRZ alt minus current alt) and decreases again while the plane continues to fly at 10.000ft.

Short before BDA it resets again to 0 and then the plane follows the descent path one normaly would expect.

Please let me know if you need further informations.

 

greetz, Enrico

 

Edited January 18, 2017 by ezfox

[Litjan]

Thank you for the detailed report. I am unhappy to say that our VNAV calculation is only able to handle very basic scenarios for now. If you have restrictions on your descent path, you are well advised to use FL CHG or V/S modes to meet them (like most real pilots do, too).

We are planning to update VNAV to provide a more robust functionality (as we learned that this is of utmost importance to simulator pilots ).

Unfortunately VNAV is a very complex structure, and getting it right is not trivial at all - so it will be a while.

Cheers, Jan