Aircraft does not follow speed and altitude restrictions properly during approach - Serious VNAV problem

[ekosif]

Hello, 

I will try to explain with help of a few screenshots:

As can be seen from above, there is a altitude restriction at SISPI, which is at or below FL280. But aircraft descend profile is not following it and it starts descend just before the waypoint. In this case, it is not possible to be at/below FL280.

 

Second case during the same approach, again as can be seen, even though I have already passed 11000 feet restriction and also another 11000 feet restriction in front of me, VNAV shows FL142 is on track. Also, during this time, aircraft is not descending and stayed leveled, so it is not overshoot issue.

 

 

Also again during same approach, even though there is a restriction on the next waypoint which is between 5000 and 6000 feet, VNAV is not following it. I switched to OP DES to catch it but it shows I am 4700 feet below at this point according to VNAV. 

 

 

And also here, there is a speed restriction there (not below 210 know, it is at 210 knot) aircraft activates the approach mode and disregard the speed targets.

If you also look at the VSD, it shows I will climb first and then I will descend again.

Edited August 7 by ekosif

[stagre]

Hi Ekosif,

I've been working on tracking the VNAV system and documenting its behavior for the dev team. I see you're flying an arrival here, but you could let me know the specifics of which arrival? I will try and fly it myself to see if I can recreate the issues.

Best,

Steven

[ekosif]

2 minutes ago, stagre said:

Hi Ekosif,

I've been working on tracking the VNAV system and documenting its behavior for the dev team. I see you're flying an arrival here, but you could let me know the specifics of which arrival? I will try and fly it myself to see if I can recreate the issues.

Best,

Steven

Hello!

Thanks for your interest.

It was the flight from Antalya(LTAI) to Istanbul (LTFM), SISPI1A arrival, ILS 35R. 

I am also attaching OFP here in case it helps.

Thanks!

LTAILTFM_PDF_1754520514.pdf

[stagre]

1 minute ago, ekosif said:

Hello!

Thanks for your interest.

It was the flight from Antalya(LTAI) to Istanbul (LTFM), SISPI1A arrival, ILS 35R. 

I am also attaching OFP here in case it helps.

Thanks!

LTAILTFM_PDF_1754520514.pdf 833.57 kB · 0 downloads

Thanks! I will see if I can recreate.

[dectenor1]

On 8/8/2025 at 12:29 AM, stagre said:

I've been working on tracking the VNAV system and documenting its behavior for the dev team.

I have made various reports on issues relating to VNAV stuff, F-PLN page predictions and how these are displayed, managed speeds, and again how these are displayed, constraint behaviour etc., etc. Please feel free to reach out for any info that may help in this regard. VNAV related things need a LOT of work currently!

Best

[stagre]

8 minutes ago, dectenor1 said:

I have made various reports on issues relating to VNAV stuff, F-PLN page predictions and how these are displayed, managed speeds, and again how these are displayed, constraint behaviour etc., etc. Please feel free to reach out for any info that may help in this regard. VNAV related things need a LOT of work currently!

Best

Hi Dectenor, 

Yes, I've seen your thread as well and been going through it. Definitely agree the VNAV needs work and that is something I am working with our RW A350 pilots on helping to improve.

Best,

Steven