VATSim xPilot Client Altimetry Discussion

[Ross Carlson]

On 1/10/2022 at 3:59 PM, Graeme_77 said:

To make this work, rather than sending the altitude relative to 29.92, the online clients send the actual altitude displayed on your altimeter. If it's right for the pilot, it's right for the network.

A slight correction on this ... online clients don't send indicated altitude. They send both true altitude and pressure altitude. Other pilots' sims will render your aircraft at the true altitude. ATC radar clients will display your true altitude in your data block if you are below the transition layer. If you are above the transition layer, the ATC client will display your pressure altitude in the data block.

If I'm understanding the the 650's more realistic altimetry simulation correctly, then it should only affect what a controller sees if the aircraft is below the transition layer. Above TL, the controller will see your pressure altitude, which should match your indicated altitude, assuming you have set 29.92/1013 in your altimeter, as you should when flying in the flight levels. (Once the bug discussed in this thread is fixed, of course.)

Edited January 15, 2022 by Ross Carlson

[Graeme_77]

That’s not the behaviour we’ve been seeing. VATSim sends whatever altitude is showing on the XP Altimeter, and it’s not related to the custom altimetry. The issue was the Challenger altimeter not feeding the subscale back into X-Plane, (so the Challenger may be flying 29.92 with XP telling VATSim 30.12, for example.) This has been resolved for the next update.

Edited January 15, 2022 by Graeme_77

[Ross Carlson]

I'm the developer of several of the VATSIM clients (vPilot, VRC, vSTARS, and vERAM) and it works as I described. Note that vPilot is only for P3D/FSX/MSFS, so I can't speak for the developers of the various clients that work with X-Plane, but if they are sending indicated altitude instead of pressure & true altitude, they are going against the VATSIM protocol specification. And it seems like we would have noticed issues way before now. I know for sure that the xPilot client is sending two different altitudes because I can see in the data stream that the two fields in the data packet (true alt and pressure alt) have different values. (I can't say for sure that one of them isn't indicated alt, but as I mentioned, that would be a violation of the protocol spec.) I was watching a couple pilots today when I was trying to track down reports of a similar issue with MSFS. I noticed a couple X-Plane users who were several hundred feet off of their cruising altitude on my radar scope. They were both flying the Challenger 650. I watched their data packets in the stream of data from the server. (One of them pointed me to this thread.)

Edited January 15, 2022 by Ross Carlson

[Graeme_77]

I understand Ross. As I explained, X-Plane and the Challenger Altimeter are not sync’d in the release version, so even if the pilot was seeing 29.92, the VATSim client would see something else.

In that respect, the VATSim client is not sending pressure altitude, as it seems to send values other than 29.92. Perhaps this is something to investigate with the XPlane client developer?

The next release of the 650 will correct the synchronisation with the X-Plane default parameters.

Edited January 15, 2022 by Graeme_77

[davidbarrocasr]

Release date of this new version? @Graeme_77

Edited January 15, 2022 by davidbarrocasr

[Ross Carlson]

4 minutes ago, Graeme_77 said:

In that respect, the VATSim client is not sending pressure altitude, as it seems to send values other than 29.92.

Which field in the data packet are you referring to here?

[Graeme_77]

Here’s the xPilot source where it reads the datarefs, you can see it’s the altitude shown on the altimeter, not normalised to standard pressure altitude. The dataref in question changes based on baro subscale.

Edited January 15, 2022 by Graeme_77

[Ross Carlson]

Yeah, so that's a bug in xPilot. You can even see that he's storing it in a field called AltitudePressure.

[Graeme_77]

Yes, it seemed like a silly way for the client to do it, but the objective for the test team was to determine why the 650 was behaving differently from other X-Plane aircraft, and we have done so and the developers corrected it.

We had assumed a VATSim client would have been tested to behave as the network wanted, given that altimetry and ATC go hand in hand. If you let me know when VATSim has fixed their client I’ll update the explanation.

Edited January 15, 2022 by Graeme_77

[Ross Carlson]

A little googling suggests that there is no dataref for pressure altitude. If that's the case, I guess indicated alt is the next best thing. It's just not realistic, because if the pilot doesn't dial the right altimeter setting, the controller won't see them at the wrong altitude, and it'll affect where other pilots see them, too.

[Ross Carlson]

I suppose xPilot could derive pressure altitude by looking at indicated altitude and the kollsman window setting?

[Graeme_77]

Something for VATSim network to discuss with the xPilot developers perhaps?

But, to reiterate, from the next release the Challenger 650 will behave like other X-Plane aircraft with regard to the altimeter subscale issue. 

[Ross Carlson]

18 minutes ago, Graeme_77 said:

We had assumed a VATSim client would have been tested to behave as the network wanted, given that altimetry and ATC go hand in hand.

I suppose this kind of thing could easily escape detection during testing because as long as the pilot is setting their altimeter correctly, indicated altitude is the same as pressure altitude above transition layer. And above transition layer is the only time the pressure altitude value is displayed, in ATC clients. Pilot clients only use the true altitude value, for rendering aircraft models in the sim.

 

Edited January 15, 2022 by Ross Carlson

[Ross Carlson]

I checked with one of the other pilot client devs (swift) and they derive pressure alt from true alt using local sea level pressure. That's probably what xPilot should do too, to be compliant with the spec and provide more realistic behavior. I've asked Justin about it.

[Ross Carlson]

34 minutes ago, Graeme_77 said:

If you let me know when VATSim has fixed their client I’ll update the explanation.

I'll try to remember to do that, but I may not if enough time passes between now and then. You could update it now anyway to reflect how it works in all other VATSIM clients, and in other online networks. (I also develop clients for a couple other networks, and none of them use indicated alt on radar scopes, for the obvious reason that it's not realistic.)

[Graeme_77]

I’ve marked the comment as specific to xPilot.

[Pils]

1 hour ago, davidbarrocasr said:

Release date of this new version? @Graeme_77

Unknown at this time.