VNAV Window not showing the next waypoint with an altitude contraint

[Ed Moore]

I'm sure this is because I don't understand something!

Was flying EGBB > EGPB, FL270. Selected ILS27 as approach whilst on ground at Birmingham. During the cruise, looking at the EGPB approach charts, I decided to add a hold at SUM at the end of the route before the CI27 from the start of the approach sequence. I wanted the aircraft to calculate a TOD to get me to SUM at 5000' so I put 5000A in the scratchpad and added it to both SUM and the hold at SUM.

I expected that the MFD VNAV window would therefore show me SUM as it was the next waypoint in the flight plan that contained a altitude restriction, but it seemed to only want to show me what it had shown originally which was the FF27 altitude of 2100'.

As I passed KOKAL I did get a magenta 5000 in the PFD so I set 5000 in the MCP and made sure VNAV was armed. Got a TOD chime and a magenta diamond on the glidepath indicator showing that the desired path was above me - however the diamond never descended and then I noticed the green TOD marker was behind me on the map so I went manual at that point and VS'd myself down.

All a bit of a mess - where did I go wrong?

 

[Ed Moore]

Here's another screen showing a bit later - this is after I put myself in a 2500fpm VS descent. As you can see it's got the 5000' restriction on the PFD but no sign of it in the VNAV window. I'm also going way too fast I realise!

[Graeme_77]

Thanks Ed, will check it out shortly and see what's happening there.

[dirmer3]

I am also having trouble getting the vnav to work correctly to descend to altitudes restrictions on the STAR.

[amyinorbit]

Hi Ed! Thanks for the report.

I believe your issue was that you entered "5000A" -- "at or above 5000ft". Therefore, the VNAV solver starts at your most restrictive constraint (the @2100ft one for CF27), then projects the 3° profile back. 5000A doesn't require the solver to put you at 5000ft, but anywhere above that, so it won't show in the VNAV window, because VNAV is not aiming for 5000ft, just checking that whatever profile it comes up with passes SUM above 5000ft.

If you want to make sure that VNAV gets to SUM at 5000ft, you can just enter "5000" as the constraint, and once you do so it will show up in the VNAV window. I hope that makes sense! 

[amyinorbit]

9 hours ago, dirmer3 said:

I am also having trouble getting the vnav to work correctly to descend to altitudes restrictions on the STAR.

Could you please create a separate post, and add more data? We can't do much of anything without a log and screenshots of the issue  Thanks!

[Ed Moore]

2 hours ago, amyinorbit said:

Hi Ed! Thanks for the report.

I believe your issue was that you entered "5000A" -- "at or above 5000ft". Therefore, the VNAV solver starts at your most restrictive constraint (the @2100ft one for CF27), then projects the 3° profile back. 5000A doesn't require the solver to put you at 5000ft, but anywhere above that, so it won't show in the VNAV window, because VNAV is not aiming for 5000ft, just checking that whatever profile it comes up with passes SUM above 5000ft.

If you want to make sure that VNAV gets to SUM at 5000ft, you can just enter "5000" as the constraint, and once you do so it will show up in the VNAV window. I hope that makes sense! 

Like all the best answers, that is SO OBVIOUS in retrospect. Thanks so much Amy.

[amyinorbit]

More than happy to help!