What might cause GS not to capture?

[Noel 4,487]

Two of my PMDG 738 for MSFS flights have left me puzzled as to why the glide slope was not captured.  Correct frequencies dialed in evidenced by the markers indicate GS and LOC. LOC captured but the plane just continues is slow descent right thru the glide slope and I can't tell why.  Speed seems good, it just for some reason doesn't capture.  It could be I've forgotten to enable something other than simply hitting the Approach button after the loc/gs indicators appear and I'm always careful to hit Approach when the GS indicator is well above the plane so that it is intercepted after the localizer is.  What should I look at?  But...most of my flights have captured the GS just fine.

Thanks

Edited September 9, 2022 by Noel

[Bob Scott 7,716]

What's the pitch mode annunciator at the top of the PFD showing when it happens?  Are you getting the white "G/S" armed indication prior to intercept?  Does it ever go into G/S mode (green "G/S")?

[Noel 4,487]

6 minutes ago, Bob Scott said:

What's the pitch mode annunciator at the top of the PFD showing when it happens?  Are you getting the white "G/S" armed indication prior to intercept?  Does it ever go into G/S mode (green "G/S")?

Yep--on these two failed captures G/S is white and stays that way as it doesn't capture.

[Bob Scott 7,716]

If you've got a green LOC and a white G/S and it flies through the glideslope intercept without capture, that sounds like a bug rather than a feature.

[Dave_YVR 968]

Sounds like you are trying to intercept from above, not a good idea unless you are manually flying the approach. I don't think the AP logic will even do that. Always intercept the GP from below.

[Noel 4,487]

12 minutes ago, Dave_YVR said:

Sounds like you are trying to intercept from above, not a good idea unless you are manually flying the approach. I don't think the AP logic will even do that. Always intercept the GP from below.

No Dave--I stated I'm always careful to hit Approach when the GS indicator is well above the plane and have already captured the localizer this I'm aware of.

 

 

[LRBS 944]

At which airport, rwy please? Or this is at any airport.

Please post 2 pictures that include also the MCP, one prior G/S intercept and the second one after you passed through it.

 

 

[Noel 4,487]

This was at KBUF ILS23.  As mentioned this was one of two flights (diff airports) that resulted in no GS capture.  I've done maybe 8 or so flights now in the 738 and the others were fine.

[LRBS 944]

It will help a lot with pictures when this is happening.

[LRBS 944]

Something is not right.

I just did a test flight LNAV/VNAV transition to ILS23 in BUF and this is what I get.

As I mentioned some pics will help somehow.

[Noel 4,487]

Yes, something is not right.  I'm duplicating the flight so in a little while will see how it goes and if it fails again will take a screenshot.  As said, 8 of 10 have been fine in this plane.

[Noel 4,487]

LRBS, thanks.  While here next time up can you see if AI Radio Comm (ATC) is working in the PMDG 738?  Not working here, is in the AS CRJ and the B78X HD.

[LRBS 944]

26 minutes ago, Noel said:

Yes, something is not right.  I'm duplicating the flight so in a little while will see how it goes and if it fails again will take a screenshot.  As said, 8 of 10 have been fine in this plane.

I think that it might be an intermittent issue with PMDG?

Some people are complaining about the same related issues and each time I tried to duplicate I wasn't able to see those issues, some did something wrong, and some didn't. 

Very strange, it could be related to their code. At the same time, apparently, Asobo is optimizing their code, I don't remember exactly which it is, but sometimes I can notice weird things on their planes compared to other developers.

I hope you sort it out.

Oh, I don't use the ATC.

[Noel 4,487]

Just landed at KBUF 23 and this time no problem capturing the GS, so for me, it's intermittent.  I've done 100's of landings in this plane in P3D so I'm pretty familiar with it.  I always aim for +5 degrees of pitch during final approach and adjust flaps to keep it there while keep.  Since I use ATC I often use FLC during descent rather than VNAV, but that shouldn't have any impact on this near as I can guess.

If it happens again I will get a screenshot and forward it on to PMDG.

Cheers

Edited September 9, 2022 by Noel

[somiller 694]

In hundreds of ILS approaches I've done in the 737-600 or 737-800 I've never had it NOT capture, UNLESS I was above the glideslope before arming approach.

Is it possible you had the ILS tuned for the reciprocal ILS 05?

I'm curious at what altitude, distance, and speed you were when the GS centered but didn't capture.