Old 744 Forum

Archived posts

Small VNAV issue

Page: 1

Author Post
Member
Registered: Sep 2014
Posts: 165
Location: Near Paris
Hello again Hardy,

As I was playing with VNAV to try and understand it a bit better, I encountered a strange issue where there was a fast oscillation between VNAV SPD and VNAV PTH, as if the system hesitated and did not know which one to choose.
I don't know exactly what causes this, but I got you a situation which should allow you to reproduce this issue very quickly.
First, this is how I created this situation (to give you the context):
I launched your situ Basic 017 - 10 nm to TOD as a starting point. Immediately after starting it, I go to VNAV DES page, and put 320 in LSK2L as SEL SPD (I know it is more than what the aircraft can do at that level), and execute. Then I dial down altitude on the MCP to 20000 (although I think you also get the issue without it) and then you wait ! Around FL300 in descent, I saved a situ, which is the one I put a link to at the end of this post.
Okay, now that you know how I got here, here is what happens:
The aircraft is at 320kt descending and the next contraint is at DENUT, 280kt and FL240B. At some point, the aircraft will start reducing to 280kt. Because of this it will find itself too high, switch to VNAV SPD, then realize it can't do the constraint and ask for DRAG REQUIRED. At this point, I immediately add full spoilers. Then the aircraft is gradually going back closer to the path. And at some point between FL250 and 240, when I expect it to revert to VNAV PTH, is when you see this very fast oscillation between VNAV SPD and VNAV PTH for a few seconds.
I remember you worked a lot on VNAV logic in the last round of updates, but I don't know if this is something you mentioned.

As usual, I'm at your disposal if you want some precisions.

I hope I won't make you loose time on this one. Normally, with the situ I created, it should just take a couple of minutes to get the issue. All you have to do is launch it and add full spoilers when required and wait, you should see it.

All the best.

Charles





https://www.dropbox.com/s/bq14cpsuxmexgst/Basic%20017%20-%2010%20nm%20to%20TOD%20bis.situ?dl=0
Moderator
Registered: May 2009
Posts: 5140
Hi Charles,

I see no oscillation on my system. As soon as the idle path can no longer be followed, VNAV PTH changes to VNAV SPD and that remains engaged until crossing DENUT which is overflown at FL246.

Thereafter it returns to VNAV PTH because there are 100 nm to go at a shallower path angle, and the aircraft is just 600 feet above the FL240B constraint when that remaining route segment starts, hence easy to follow.

Edit: The above test was without spoilers. Just did it again with spoilers fully extended and now I see what you mean: yes, this is a known issue; this mode oscillation may last for a second or two shortly before waypoint sequencing if the conditions are -- coincidentally -- exactly like in this situation (first idle path waypoint, high descent rate, less than 400 ft deviation etc. pp.).


Cheers,

|-|ardy
« Last edit by Hardy Heinlin on Thu, 28 May 2015 12:28:36 +0000. »
Member
Registered: Sep 2014
Posts: 165
Location: Near Paris
That is really strange. I ran the situ 3 times on my laptop (on windows 8.1) before posting and had exactly the same issue every time.
I just ran it on my main sim rig (windows 7) and same thing.

I just made a video of the issue so that you can see it. I captured the whole thing so that you can see if there is something wrong. Just go to 2:15 and watch the FMA if you want to see it happen.

https://www.dropbox.com/s/uo9gvtt8wparwt1/Desktop%2005.28.2015%20-%2014.47.30.02.mp4?dl=0
Moderator
Registered: May 2009
Posts: 5140
Did you see my edit above? :-)
Member
Registered: Sep 2014
Posts: 165
Location: Near Paris
No I did not.
Now I do :mrgreen: !

Thanks !
Moderator
Registered: May 2009
Posts: 5140
That mode flip-flop is now removed in 10.0.5-beta9 (item 0.5.0042):

http://aerowinx.com/forum/topic.php?id=2939


|-|ardy
Member
Registered: Sep 2014
Posts: 165
Location: Near Paris
Fantastic !
Will give it a try ASAP.

Charles

Page: 1

Old 744 Forum is powered by UseBB 1 Forum Software