I'm flying the airplane in the subject because interested in the VNAV development, but I have problems to use it because often the distances between the TOD and the approach are too short, no time and no space to descend to appropriate altitudes. I'm using this Simbrief profile: https://docs.flybywiresim.com/fbw-a32nx/feature-guides/loading-fuel-weight/#development-version

 

Edited April 6, 20224 yr by Claudius_

And the question is...?

BTW You might try a more descriptive topic title like 'FBW A320: problems with VNAV'.

Edited April 6, 20224 yr by tup61

36 minutes ago, Claudius_ said:

I'm flying the airplane in the subject because interested in the VNAV development, but I have problems to use it because often the distances between the TOD and the approach are too short, no time and no space to descend to appropriate altitudes. I'm using this Simbrief profile: https://docs.flybywiresim.com/fbw-a32nx/feature-guides/loading-fuel-weight/#development-version

 

Depends on the winds. You need to enter the wind data from Simbrief into the FMC in order to get a more accurate TOD.

Edited April 6, 20224 yr by jarmstro

2 minutes ago, jarmstro said:

Depends on the winds. You need to enter the wind data from Simbrief into the FMC in order to get a more accurate TOD.

Yesterday I tried a flight from Rome to Munich, LIRF/EDDM, and I had to push the altitude button 50 ml in advance and setting the speed manually. it seems to much to me.

5 minutes ago, Claudius_ said:

Yesterday I tried a flight from Rome to Munich, LIRF/EDDM, and I had to push the altitude button 50 ml in advance and setting the speed manually. it seems to much to me.

Well it's still experimental. Did you have a strong tail wind? It can't properly calculate TOD without the correct wind data.

Edited April 6, 20224 yr by jarmstro

If I remember correctly I had wind from the left, not from the tail, I think that the FBW VNAV is still experimental, but I had doubts about the Simbrief profile, because it was Simbrief to propose that inusual short TOD.

1 hour ago, jarmstro said:

You need to enter the wind data from Simbrief into the FMC in order to get a more accurate TOD.

Afaik that doesn't have any effect at this moment. It's still experimental indeed. 😉

@Claudius_As a real Airbus pilot suggested: you always need to calculate the TOD yourself too in order to verify the shown TOD. With this experimental version that advice is almost mandatory. 😉

There is a simple formula for this: 3 x (altitude difference in thousands) + 10 = (nm to desired alt). I often add a fix with a radius to make the TOD visual on the ND.

Example:
- you are at FL340 and need to be at 5.000 ft at waypoint ABC
- the difference in alt is 34.000 ft - 5.000 ft = 29.000 ft
- 3 x 29 = 87
- 87 + 10 = 97
- so you need to start the descent at 97 nm from waypoint ABC

Depending on the route of your plan you can either add a fix for waypoint ABC and set a radius of 97 nm OR you can calculate how much nm ABC is from your destination (simply add up all leg lengths from ABC up to destination) and add 97 to it to know at what distance you need to start the descent (the MCDU FP page shows the total remaing distance at the bottom).

If you have a tailwind you might want to add 10 extra nm or so.

This won't always result in the perfect TOD and approach but for mit works 9 out of 10 times.

Good topic title BTW! 😉 And I also use that profile: once you add it to Simbrief I think it will be kept up to date with any changes (afaik). Make sure you actually select the correct profile though: I myself kept on selecting the default profile from the list at first and didn't notice the FBW profile was at the top! 😉

Edited April 6, 20224 yr by tup61

At the moment FBW VNAV calculates a closer TOD than the above calculations (which I use). 

Example: En route to ENGM from EGLL at FL390. According to the STAR there is an altitude restriction of below FL120 at the waypoint GM451. Using the TOD calculator in the flypad it suggests a TOD 85nm before the waypoint. I add 10nm to this as I have to slow to 250kts quite early (STAR restriction). 

The VNAV however calculates a TOD 79nm before. Following this I there's no way I could meet the altitude AND speed restrictions of the STAR. 

I usually add a fix in the PROG page of the MCDU. It's worth noting that this is a direct (air distance). Actual ground distance can vary according to the flight plan. Sometimes the only option is to calculate adding up all the legs as mentioned above. 

Check out this guy's channel. 

https://youtube.com/channel/UCCnqAG3Adhufu_RofifxCxg

He's helped me loads. 

Just finished the approach (above), and it's very clear that the FBW VNAV is a work in progress 😄 I decided to follow the VNAV profile. Man... it was all over the place. Looking at the green dot in the PFD (the "yoyo"?) it went up and down constantly - just like a yoyo. One moment it shows I'm 9000ft below profile then the next I'm 2000ft above... Also it didn't follow the speed restrictions as per the loaded STAR. Ended up some 6000ft above profile and had to program a hold to meet the STAR restriction. 

Nevertheless, it'll be very interesting to follow the progress of this. I'm sure the excellent FBW team will nail it eventually!

Edited April 6, 20224 yr by Cpt_Piett

@Cpt_Piett that's my experience too, but not every time the VNAV is wrong, it depends by the STAR and the Airport.

 

It seems that FBW team have pushed a fix to the experimental that somewhat fixes this, perhaps you can try latest experimental?

5 minutes ago, omarsmak30 said:

It seems that FBW team have pushed a fix to the experimental that somewhat fixes this, perhaps you can try latest experimental?

yes I do, I'm very interested on the development of the FBW A320.

I think it also needs to be pointed out is that it is a WIP, as announced by the FBW team. It's being devloped like Lnav was, and will take time. Best thing anyone can do is if you find an issue report it via their discord.

Edited April 7, 20224 yr by Car147

Finally the VNAV seems to be working good, I did a LIRF/LEBL very good flight last night. It's Important to shut off the EFB while flying because it gives some stutters. Anyway, very nice piece of software, very interesting to follow the development and the updates, thank you very much FBW team.