Direct To Causes VNAV Issues

[xterminator24]

I was doing a flight today with the following routing. It was the second time flying this arrival and both times I ran into the exact same problem:

KCVG  BNGLE4 BNGLE RIKLE WWSHR DORET J584 SLT FQM3 KEWR  (ILS04L)

I was flying without ATC so I entered a couple of altitude restrictions on the arrival while in flight. The altitudes I used were based on the "Expect" notations on the chart at RACKI (13000) and SWEET (7000). Both altitudes seemed to work fine after pressing EXEC.

This arrival has a vector section after SWEET which seemed to work correctly. LNAV and VNAV was still engaged.

The problems arose when I tried to go DIRECT to the IAF of KILMA. When I pressed EXEC after moving KILMA to the top of the LEGS page, I received a message "UNABLE CRZ ALTITUDE" and the plane immediately tried to speed up to 290 kts. I had to disable VNAV, adjust the speed manually, and start using V/S to adjust the rest of the descent. 

X-Plane log and FMS Debug files are attached.

Arrival Chart:

http://imgur.com/LGMEJFq

 

 

IXEG_FMS_debug.txt

Log.txt

[jamarns]

I am having the same problem, I'm not sure if I'm just misunderstanding how the FMC/VNAV is supposed to work or if it's a bug, when I try to go from VECTORS to the next point in my STAR the aircraft tries to accelerate to 280Kts even though the FMC has 171Kts in as the speed restriction. I have made a video to illustrate. 

https://www.youtube.com/watch?v=xGNvJsSPS8Y&feature=youtu.be

 

[Litjan]

3 hours ago, jamarns said:

I am having the same problem, I'm not sure if I'm just misunderstanding how the FMC/VNAV is supposed to work or if it's a bug, when I try to go from VECTORS to the next point in my STAR the aircraft tries to accelerate to 280Kts even though the FMC has 171Kts in as the speed restriction. I have made a video to illustrate. 

https://www.youtube.com/watch?v=xGNvJsSPS8Y&feature=youtu.be

 

Unfortunately the video is "private", so I can´t really see what is going on...

[jamarns]

Ok try now. 

 

Geez if I fly a plane as well as I operate Youtube it's no wonder I'm having problems!!

[xterminator24]

Good Video

10 hours ago, jamarns said:

I am having the same problem, I'm not sure if I'm just misunderstanding how the FMC/VNAV is supposed to work or if it's a bug, when I try to go from VECTORS to the next point in my STAR the aircraft tries to accelerate to 280Kts even though the FMC has 171Kts in as the speed restriction. I have made a video to illustrate. 

https://www.youtube.com/watch?v=xGNvJsSPS8Y&feature=youtu.be

 

Good video. That's exactly what I am seeing. You can see the plane speeding up and if you look at the speed bug, it's at 290. 

[Litjan]

Yeah, I watched the video - it is a really complicated situation for the FMC - it still thinks that you are in climb mode and wants to go to CLB speed after recalculating the route after the DIRECT TO.

I don´t really know what went wrong at that point, but in that case I would suggest to simply revert to a more simple autopilot mode like MCP SPD and V/S or FL/CHG.

We will look at these kind of situations (descents involving several speed and altitude restrictions) more closely in the future, there is still a lot of work to be done...

Jan

 

[jamarns]

Hi Jan,

Thanks for the response, my routing for that flight is as follows, I've been able to reproduce this one fairly regularly.

CYVR YVR J514 CILLI Q983 IGVEP CYYC

Departue 08L using RICH5

Arrival ILS17R using IGVEP3

 

If there's any other information I can provide to help with this let me know.

Also thanks for the stellar support, you guys rock =)

Edited September 10, 2016 by jamarns