VNAV descent

[frankbyte]

Hi

I am not sure if this already has been reported (i didn't see any report about that so far):

I'm having difficulties during a VNAV descent. When leaving the cruising altitude (T/D) the plane first follows the VNAV path and descent. But at some waypoints it has difficulties to reach/maintain the altitude. Sometimes it even climbs (!)., So for example when a waypoint has the restriction 210/13000 and the plane is at FL160 5-6 miles away from that waypoint, instead of trying to hurry up descending, it stops and even climbs sometimes. Thats why i always have to disconnect the VNAV mode and descend with FLCH. Or i can achieve it, but the plane has suddenly to descent with 3'000 fpm to reach the altitude of the next waypoint, so that it accelerates too over 250kts on final.

Some other times the plane follows nicely the VNAV path but when passing a waypoint the bar that indicates the "VNAV deviation" suddenly jumps to 2'000-3'000ft down (so that i am suddenly a lot too high).

And a third strange situation is that sometimes when i am already a litlle bit too high over the VNAV path and descend on VNAV mode with the speedbrake sout, the deviation jumps to a negative deviation of 13'000-14'000ft (!!!), which means i would have been a lot too low (!) even this is absurd since i was already a little bit too high a second before.

So i'm really having difficulties maintaining a clean VNAV descent to the landing or the final approach point.

I attached a Log of my last flight but can give you more informations of course if needed.

Thanks and cheers
Franz

 

GizmoLog.txt

Edited May 8, 2016 by frankbyte

[Litjan]

Hi Franz,

I see the same thing on some STARs and transitions... we will need to start "tuning" the VNAV calculation process over the next weeks - for now we concentrate on clearing the crashing...

Jan

 

[jailer]

13 minutes ago, Litjan said:

Hi Franz,

I see the same thing on some STARs and transitions... we will need to start "tuning" the VNAV calculation process over the next weeks - for now we concentrate on clearing the crashing...

Jan

 

Jan, I had reported an issue when changing any constraints before and you had asked me if I could reproduce it for you, but now I see that the thread has been moved to "solved" and is locked. 

I made a video that illustrates the issue.  If you skip to the 2:30 mark, you'll see that there is an altitude constraint of 13000A at EASEL; however, there is a 16000 constraint at JOCKE a few waypoints later.  If the EASEL waypoint constraint is deleted, the descent path immediate jumps until EASEL is crossed, at which point the descent path goes back to normal.  This is not correct because the 13000A should basically be ignored as it is redundant with a higher constraint further down the legs.

(I understand this is low priority, real men don't use VNAV, etc. etc., but I did promise to reproduce so here you go.)

 

[Litjan]

Thanks for the feedback, Jailer! And no, its not solved, we just "mass locked" those threads, because people kept posting their stuff there and we were worried that we might not see them...

Jan

 

[SkyCoyote]

Great aircraft I'm enjoying takeoff, climb, cruise and descent, but approach not so much.

My last flight EDDM 26R using INPU1N SID to EGLL ILS27R with LAM transition.

The route: EDDM INPU1N Y102 UPALA UL610 BATTY UL608 DENUT UL610 GILDA  LAM C127R ILS27R EGLL.

My Nav data comes from Navigraph.

 

The trip was inconsequential until I reached the EGLL ILS approach where the VNAV and LNAV left me poorly aligned with the runway, High and right. The altitude set on the MCP was well below ILS capture altitude.

 

 

I have yet to make a successful Instrument landing with the 733.

GizmoLog.txt

[poodster]

1 hour ago, SkyCoyote said:

Great aircraft I'm enjoying takeoff, climb, cruise and descent, but approach not so much.

My last flight EDDM 26R using INPU1N SID to EGLL ILS27R with LAM transition.

The route: EDDM INPU1N Y102 UPALA UL610 BATTY UL608 DENUT UL610 GILDA  LAM C127R ILS27R EGLL.

My Nav data comes from Navigraph.

 

The trip was inconsequential until I reached the EGLL ILS approach where the VNAV and LNAV left me poorly aligned with the runway, High and right. The altitude set on the MCP was well below ILS capture altitude.

 

 

I have yet to make a successful Instrument landing with the 733.

GizmoLog.txt

If you think lnav and vnav are going to leave you off the ils course, switch to loc and then once you intercept the glide slope, switch to app. I've made many ils landings. I feel like you are doing something wrong. 

[kneighbour]

1 hour ago, SkyCoyote said:

The trip was inconsequential until I reached the EGLL ILS approach where the VNAV and LNAV left me poorly aligned with the runway, High and right. The altitude set on the MCP was well below ILS capture altitude.

 

 

I have yet to make a successful Instrument landing with the 733.

This seems to be my experience as well. 

[SkyCoyote]

1 hour ago, poodster said:

If you think lnav and vnav are going to leave you off the ils course, switch to loc and then once you intercept the glide slope, switch to app. I've made many ils landings. I feel like you are doing something wrong. 

That doesn't sound like anything out of the Ops Manual to me.

[poodster]

1 hour ago, SkyCoyote said:

That doesn't sound like anything out of the Ops Manual to me.

Are you trying to do a RNAV app or ILS. Because if you are donig ils, then you have to switch to the loc.