If the PMDG 737-600 is on a locked VNAV Path, is this as good as a GS or GP.
I came in about 700 feet high over Rifle County Airport in Colorado (KRIL), even though it showed a locked VNAV Path.
I'm not sure if this is my fault, this airport's GS setup, the Sim's fault, or PMDG's fault.

All I know is, I've really seen a lot more ILS issues since about SU9 than prior versions.
In SU 8 it seemed like almost every ILS was coming in about right. Not sure though, could be luck of the draw.

 

Edited October 25, 20223 yr by Alpine Scenery

11 minutes ago, Alpine Scenery said:

If the PMDG 737-600 is on a locked VNAV Path, is this as good as a GS or GP.
I came in about 700 feet high over Rifle County Airport in Colorado (KRIL), even though it showed a locked VNAV Path.
I'm not sure if this is my fault, this airport's GS setup, the Sim's fault, or PMDG's fault.

All I know is, I've really seen a lot more ILS issues since about SU9 than prior versions.
In SU 8 it seemed like almost every ILS was coming in about right. Not sure though, could be luck of the draw.

 

It's not as precise as a GS, but it should not leave you 700 feet high. Altitude dial was set to 0? Baro was set correctly? Airport elevation on legs page was correct?

Altitude was set to the airport elevation (actually 100 feet below), yes the legs data was correct including descent altitudes, and I manually hit the DESC button in the FMS on the way down at the top of descent point. It seems like the issue is you need to hit the IAF right at the exact elevation or it comes in high (but I cannot imagine that would be true in real life, since the point of VNAV Path is to have some auto-correction on the vertical guidance). I was probably only 250 feet high at the IAF and still 15+ miles out, so that is a bit silly.

Ahh well, next time I'll do a regular ILS landing, VNAV isn't worth it.

 

Edited October 25, 20223 yr by Alpine Scenery

VNAV works well in the PMDG.  I probably do more LNAV-VNAV approaches in it than ILSs.

There's always the possibility that this particular approach has a coding issue, but more likely, you reverted from VNAV Path to VNAV Speed without realizing it.  Vnav Speed works more like level change.  If you get high enough on the path, the mode will switch to Vnav Speed, and you'll need more drag at that point to get back on Path.  

The real airplane does do this; gotta watch the FMA. Our procedure is to not set field elevation in the MCP until in Path, and if you revert to Speed after that, to re-set the MCP to the next stepdown altitude and manage drag to get back on Path.  Once back in Path, reset MCP to field elevation. 

This reversion to Speed will only happen further out on the arrival but if it happens and you don't realize it, you'll be stuck high, like you were.  

2 hours ago, Alpine Scenery said:

If the PMDG 737-600 is on a locked VNAV Path, is this as good as a GS or GP.
I came in about 700 feet high over Rifle County Airport in Colorado (KRIL), even though it showed a locked VNAV Path.
I'm not sure if this is my fault, this airport's GS setup, the Sim's fault, or PMDG's fault.

All I know is, I've really seen a lot more ILS issues since about SU9 than prior versions.
In SU 8 it seemed like almost every ILS was coming in about right. Not sure though, could be luck of the draw.

 

Did you at any point press the "App" button on the MCDU?

 

6 hours ago, MDFlier said:

Did you at any point press the "App" button on the MCDU?

 

I am a little rusty here but would that action make it and IAN approach instead of lnav/vnav?(FAC/G/P)

Is i recall it both IAN,RNP, LNAV/VNAV is considered none precision approaches hence the higher MDA.

at which point is missed approach altitude set in LNAV/VNAV approaches? I normally set MDA and being below it change to Missed approach altitude.

Michael Moe 

Edited October 25, 20223 yr by Michael Moe

Any of the above is possible, as I may have hit approach just to see what happened, but I don't recall (it was late). That said, I believe the issue is the descent path over the mountains changes from a slow descent to a steep descent and I was going too fast, therefore it just couldn't descend fast enough in VNAV-path mode. It seems like they have this mode "capped" and it isn't able to make large adjustments like a regular ILS using a glideslope auto-adjusts, I guess it expects a near reference approach speed to maintain the descent, but no it never told me I was in VNAV-speed mode, it always said path and I was watching pretty closely.

I will try again somewhere else, Colorado is renowned for having airports with too steep approaches. The worst one is the VOR-DME-C approach at Aspen with a 9.6 degree slope, but I'm not sure how that differs from a GS approach or if it differs at all.
 

 

Edited October 25, 20223 yr by Alpine Scenery

1 hour ago, Michael Moe said:

I am a little rusty here but would that action make it and IAN approach instead of lnav/vnav?(FAC/G/P)

Is i recall it both IAN,RNP, LNAV/VNAV is considered none precision approaches hence the higher MDA.

at which point is missed approach altitude set in LNAV/VNAV approaches? I normally set MDA and being below it change to Missed approach altitude.

Michael Moe 

It would make it an IAN approach.

Out of habit, I recently hit 'App' while doing an LNAV/RNAV approach. I did not intend to, but was a muscle memory thing as most of my approaches are ILS approaches. When I did, it deactivated VNAV and I went high on the approach just as you did. Made me wonder if that's what Alpine did without realizing it.

Edited October 25, 20223 yr by MDFlier

4 hours ago, Alpine Scenery said:

but no it never told me I was in VNAV-speed mode, it always said path and I was watching pretty closely.

If it remained in Path the whole time then there's a coding error on this approach. There's no way to be that high off the path and still be in Path mode. 

As far as speed management, the FMC tells you what speed it would like to be at when crossing each fix.  Its glidepath is drawn with those expectations, so you'll make your life easier if you follow the box's speed schedule.  But still - there's no way you could wind up 700 ft high without reverting to Vnav Speed.

You also couldn't have pressed approach and accidentally been in IAN mode if you're sure the FMA was annunciating Vnav Path the entire time. 

9 hours ago, Michael Moe said:

Is i recall it both IAN,RNP, LNAV/VNAV is considered none precision approaches

I thought RNP approaches were precision approaches. I could be wrong, but RNP AR approaches will have MDA's in the 200's and while having really intricate approach paths. 

36 minutes ago, Bdub22 said:

I thought RNP approaches were precision approaches. I could be wrong, but RNP AR approaches will have MDA's in the 200's and while having really intricate approach paths. 

RNP approaches are not considered precision approaches, nor are LPV, though in many cases, both approach types can have minimums as low as a corresponding ILS to the same runway