RNAV approaches in the ATR (and I would venture to guess in all stock planes/systems) do not give you vertical guidance to the touchdown zone of the runway, but rather to the Airport Reference Point (ARP), which is usually halfway down the runway. So if you are on an RNAV approach with vertical guidance, on glidepath, on speed, rather than landing at the 1,000-foot markers, you land halfway down the runway. It's probably this way for all stock airplanes.

For example, if you fly the RNAV 10 approach to TTPP, you are getting vertical guidance to here:

What this also means is that all of the distances you get get are going to be wrong as well. If you see below, TALUS is the final waypoint before the RW10 waypoint, which is the runway threshold. Per the chart, it should be 5.1 NM from TALUS to RW10. However, because you are not getting vertical guidance to RW10, but rather the Airport Reference Point (which is approximately halfway down the roughly 2-mile runway), your distance indicated inside TALUS will be off by (you guessed it) about a mile. This will also cause your vertical path to be flat, in other words 2.-something degrees rather than the standard 3 degrees.

Here is a video showing the situation in detail:

I sent a Zendesk report, which they acknowledged. I have zero hope of this ever being fixed. I don't know of anyone else who has mentioned it. People are probably just confused if anything and don't quite know what's happening. It took quite a bit of digging to figure it out. I would share it on the MSFS official forums but I am not a member.

Edited August 14, 20232 yr by mryan75

Whenever I fly an Rnav approach, I hand fly it from about1,000 feet agl. 

1000 AGL autopilot off, hand fly approach - solves all the problems for me

Guys, that's not the problem.

If the OP is correct then this is a big issue. I watched the video but I'd need to see more examples. 

I've been struggling with SU13 perf issues and a complete re install yesterday so i have not had a chance to fly properly but i have not observed these issues and RNAV approaches is about all i ever do fly. Mind you im flying PMDG/FENIX products with the occasional Kuro 787 and for the 787 yeah that may occasionally miss vertical guidance but more often than not it hits the marks. I disconnect around MDA which is usually less than 1000 ft AGL So i think it would be easier to notice if i am high on the approach. So unless this is new bug or i didn't notice the glidepath on my RNAV approaches are typically spot on.

 

Will test when i get home and finish my reinstallation and report using the three Aircraft afore mentioned.

Edited August 14, 20232 yr by Maxis

12 minutes ago, sd_flyer said:

1000 AGL autopilot off, hand fly approach - solves all the problems for me

How would this be a solution?  LPV approaches have near ILS minimums; whether you're coupled or handflying at 1,000ft, if you're following incorrect guidance, the guidance is still incorrect, right?

That said, I'm not sure I'm convinced about this bug.  I use the PMS50 750 to fly a lot of low RNAVs, in the 310 and now the Comanche, and I do not end up high. There are times in fact that I end up feeling low on the breakout because of MSFS's unrealistic placement of trees off the end of the runway.  If this were true, you'd be awful high on an RNAV approach to a large airport ..

10 minutes ago, mspencer said:

Guys, that's not the problem.

If the OP is correct then this is a big issue. I watched the video but I'd need to see more examples. 

Yeah, the one thing that I've been meaning to do is to fly the G1000 172 into Miami on an RNAV approach, and either taxi the plan upon landing to the ARP, or if that won't work (because the G1000 will sequence to the missed approach or suspend), I will try and fly over the ARP to see if the distance on the PFD goes to 0. That will be confirmation. But vertical guidance on RNAV approaches in the ATR is a mess. And I'm sure this is why. I just haven't tried it in any other stock plane.

Interesting with regard to the ATR, but does the OP thinking that this is generally true for "all stock airplanes"? The ATR isn't an ASOBO product, but rather an outfit called S&H Software and it may well make use of a custom FMS implementation. In that case, that problem is specific to the ATR.

BTW, what happens if you try to fly an ATR RNAV approach to a runway that is not coincident with the ARP? If as seems to be the case that the FMS does not correctly identify the location of the touchdown zone, you would find that the LNAV isn't working correctly as well.

Good luck.

2 minutes ago, Stearmandriver said:

That said, I'm not sure I'm convinced about this bug.  I use the PMS50 750 to fly a lot of low RNAVs, in the 310 and now the Comanche, and I do not end up high.

I'm assuming using Navigraph data, which would be correct. I think this issue is limited to stock aircraft, or possibly all aircraft if using stock nav data. I know you know what you're talking about, and what I think is happening is that on approaches with a runway waypoint, that waypoint for whatever reason is being ignored by the MSFS systems. In the example above, it's indicating a distance to RW10 waypoint, but the distance is actually to the ARP. Not sure how they managed that one.

3 minutes ago, Maxis said:

Mind you im flying PMDG/FENIX products with the occasional Kuro 787 and for the 787 yeah that may occasionally miss vertical guidance but more often than not it hits the marks.

If this bug is real it won't be a factor for the Fenix or PMDG as they use their own navdata that does indeed draw a GP from the threshold.  The question would be about the MSFS navdata.

2 minutes ago, jrw4 said:

Interesting with regard to the ATR, but does the OP thinking that this is generally true for "all stock airplanes"? The ATR isn't an ASOBO product, but rather an outfit called S&H Software and it may well make use of a custom FMS implementation. In that case, that problem is specific to the ATR.

You may very well be correct.

2 minutes ago, Stearmandriver said:

If this bug is real it won't be a factor for the Fenix or PMDG as they use their own navdata that does indeed draw a GP from the threshold.  The question would be about the MSFS navdata.

Fair.. Longitude test then it is.. Oh wait .. the 787-10 is stock AC as well .. 

Edited August 14, 20232 yr by Maxis

2 minutes ago, mryan75 said:

I'm assuming using Navigraph data, which would be correct. I think this issue is limited to stock aircraft, or possibly all aircraft if using stock nav data.

Interesting point; yes I'm using Navigraph data but would that make a difference?  The 750 isn't using custom navdata the way the PMDG or Fenix does, it's using the base sim navdata.  Navigraph does update that more often than Microsoft does, but it's the same format data, right?

2 minutes ago, Maxis said:

Fair.. Longitude test then it is

That would be awesome.

4 minutes ago, Stearmandriver said:

Interesting point; yes I'm using Navigraph data but would that make a difference?  The 750 isn't using custom navdata the way the PMDG or Fenix does, it's using the base sim navdata.  Navigraph does update that more often than Microsoft does, but it's the same format data, right?

I'm pretty sure Navigraph updates the 750's data. I think if you're using Navigraph, all the navdata in the sim is Navigraph. I just can't figure out how or why this issue would be happening. It's odd. It's as though the system thinks "Waypoint->waypoint->waypoint->airport", with "airport" being the ARP, when it should be waypoint->waypoint->waypoint, etc.

It may turn out to be an error specific to the ATR as @jrw4 mentioned above. I hadn't considered that. I'm going to fly the TTPP RNAV10 in the 172 G1000 and see what happens.

Edited August 14, 20232 yr by mryan75