Another VNAV question

[sidfadc]

Hi guys

I posted in another thread about VNAV in the climb.  This time it’s about VNAV capture at top of descent, no matter what I seem to do the aircraft stays in VPTH on the PFD and won’t go green into VPATH.  These are the checks I have done to troubleshoot

1.  Ensured the descending altitude at each way point has been confirmed in the FPL.

2.  Armed the VNAV button on the AP console way in advance, I think there is a rule where you have to arm with 2 mins of TOD but I always do it way before that anyway.

3.  Entered the bottom of descent altitude on the PFD.

Every looks good coming up to TOD, I get an aural warning as I approach.  I see the arrow on the PFD come down but it doesn’t automatically capture.  I always have to chase it with VS and then it captures.

Any ideas?  I must be doing something wrong can’t figure it out.

Cheers

 

Edited February 23, 2020 by sidfadc

[RobW05]

11 minutes ago, sidfadc said:

Armed the VNAV button on the AP console way in advance, I think there is a rule where you have to arm with 2 mins of TOD but I always do it way before that anyway.

That might be the issue. You have to arm it within 5 minutes prior to TOD. It will be ignored in case you do it earlier. 

Other than than it's pretty much straight forward:

- set your VNAV profile

- set an altitude preselection lower than your current altitude (typically the altitude you have been cleared to descend to by ATC) 

- arm VNAV within 5 minutes prior to TOD (simply do it when you hear the "vertical track" callout) 

[bunchy]

Yes that's right.

VNAV will reset after 5 mins if it hasn't reached TOD within that time.

Stu

[sidfadc]

Ah I think that’s been my issue, I have been arming it way too early!  Good tip to do it when you hear the vertical track call out!  Cheers guys will give it a go