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VNAV Descent setup.

Posted: Sat Jan 18, 2020 9:24 pm
by rsyferd
Hello,

I'm having trouble understanding how to set up the AP to follow the VNAV descent path. I've had the AP start and follow the descent path upon reaching TOD without any issues in the past, today it flew right past TOD without starting the descent. I had set the MCP ALT to a lower altitude as before and had and had a VALT indication above the PDF as before. What did I miss?

Re: VNAV Descent setup.

Posted: Mon Jan 20, 2020 10:34 pm
by rsyferd
Had the same issue today. The aircraft over flew the TOD without initiating a descent.
First pic TOD is about 5 miles out. At 5 miles before TOD, VNAV has calculated a 1500 FPM descent and provided magenta bug on the VS display. Descent profile diamond has shown up to the right of the ADI. A TOD message has shown up just below and to the left of the ADI. I've set my ALT to 2,000 ft. The AP is in VALT mode.

Second PIC, the aircraft is about to pass the TOD. Just after that, the TOD and all the other indications I've mentioned above disappear, and the aircraft remains at FL280. Is there a missing step? The approach and its ALT restrains are entered and confirmed in the legs page as before which is required for the VNAV to calculate the TOD is the first place I believe.

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2020-1-19_12-48-42-39.jpg (234.93 KiB) Viewed 1226 times
2020-1-19_12-49-27-102.jpg
2020-1-19_12-49-27-102.jpg (235 KiB) Viewed 1226 times

Re: VNAV Descent setup.

Posted: Fri Jan 24, 2020 6:36 am
by rsyferd
Any help on this?

Re: VNAV Descent setup.

Posted: Fri Jan 24, 2020 12:48 pm
by KenG
Did you press the VNAV button, did PATH arm in white?

Re: VNAV Descent setup.

Posted: Sat Jan 25, 2020 12:31 am
by rsyferd
KenG wrote:
Fri Jan 24, 2020 12:48 pm
Did you press the VNAV button, did PATH arm in white?
When do I press the VNAV button. Just before I cross the TOD?

Re: VNAV Descent setup.

Posted: Sat Jan 25, 2020 11:27 am
by KenG
Anytime prior to TOD would be fine.