VATSIM / IVAO / PILOTEDGE Users - Be aware of an important bug!

If I let my team roll out code of this caliber, I would have been out of a job a long time ago.

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It seemed to go way off on altitude once I switched to standard pressure at FL180. Live weather temps still all over the place.

It’s very unfortunate the hotfix didn’t resolve the problem. Hoping we can hear some more from the team about what’s up.

Hey @Bishop398 - can you please comment on this as I know you mentioned a fix was coming… guess it didn’t happen in this patch?

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@moxiejeff The fix was part of this patch, yes. The temperature data seems to be affecting the fix and I’m unsure as to why the altitude temperatures are not resolved. A good number of hours of flight time was spent testing the temp fix and this behavior was not observed.

For now for online flights I would still recommend Clear Skies. That’s all the information I have at the moment.

-Matt | Working Title

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GPS altitude (in a real aircraft) is true altitude above sea level which is almost never the same as indicated altitude. They would only match if conditions (both pressure and temperature) were exactly ISA.

I will verify this tonight using the sim var watcher, but what I think is happening is that the simconnect variable INDICATED ALTITUDE (which should always match whatever is shown on the altimeter) is now receiving the value of either pressure altitude or true altitude. AFAIK, INDICATED ALTITUDE is the simconnect variable that external client programs for VATSIM have always used.

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Interesting, I had thought that the altitude calibration given by the airport compensated for both pressure and temperature at ground level, in which case I’d expect differences only if there were pockets of different temperature and pressure between the airplane and the ground. Guess I was wrong!

I’m a bit surprised though, since the difference from GPS altitude is showing up in my tests so far at about 50 feet per 1000 feet (500 feet off at 10,000 feet). That’s a much greater difference than I expected.

Do you know if real-life ADS-B reports pressure altitude instead of GPS altitude as well?

This presumably means that you can’t accurately tune the barometer by dialing in the known elevation of the airport, either, without doing manual calculations for the temperature. Is this correct?

This is actually exactly the case. Right now, you should definitely observe that field elevations once baro is set should be correct, which was not the case prior to the patch (they could be off by potentially 1000+ ft). I believe the wrong temperature data is causing a number of issues above that, though.

-Matt | Working Title

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Ok, did some checking on the source for fs2ff and the “GPS altitude” I receive in ForeFlight is pulled from the SimConnect variable “PLANE ALTITUDE”. This is higher than the altitude reported on the altimeter (in my testing in daytime in Southern California summer, by 500 feet at an indicated altitude of 10,000).

At the airfield they both match.

I think it’s PRESSURE ALTITUDE which Vatsim uses since INDICATED ALTITUDE is always what’s in PFD. So even if you fly at FL300 with baro 993 hPa INDICATED ALTITUDE will show 30000ft, but PRESSURE ALTITUDE shows 32000ft or so.

When ISA is OK, INDICATED ALTITUDE and PRESSURE ALTITUDE is close each other. But when ISA jumps to +65 INDICATED ALTITUDE will still show “correct” altitude, but PRESSURE ALTITUDE is way wrong.

Volanta for example reports PLANE ALTITUDE

Here’s couple screenshots from FL300:

+6 ISA

+57 ISA

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I departed Colorado Springs and yes the displayed altitude matched field elevation and everything was reporting correctly in VatSim, STKP & Volanta (~6,100ft), where before it would be off.

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The thing that strikes me here is that ambient pressure has somehow not really changed even though the temperature is way higher. That’s going to really throw the whole thing off, that data just doesn’t make sense. In the two shots, the difference between real altitude and pressure altitude only changed by like 12 feet (which I would expect given the small ambient pressure change). So some data is totally wonky.

-Matt | Working Title

When I stopped climbing to FL300 ISA was +57c and when it changed to +6c, Ambient pressure spiked to 312 along with the altitudes, then slowly started going lower. I think I took the +6c screenshot bit too early, Indicated altitude and Ambient pressure was same after a while than in +57c screenshot, Pressure altitude and plane altitude was different and temperatures of course between the two screenshots.

Thanks Matt. Jayne confirmed this as well. Hopefully it’ll get fixed soon. Appreciate it

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I work professionally as an avionics tech, specializing in air data systems, so if I may interject…

Ambient pressure should not change significantly with temperature. Ambient pressure is the weight of the air column above the measurement location. Temperature does have some effect, but it is relatively minor. (It becomes more significant at very cold temperatures) A simple barometric altimeter should always read very close to field elevation when the baro set is adjusted to current pressure (corrected to sea level) whether the OAT is 5C or 30C

Density altitude on the other hand is very strongly affected by temperature, but that is not displayed by any instrument, but must be calculated using pressure altitude and temperature.

Density altitude is critical to calculating aircraft takeoff and landing performance. As I write this, I am sitting in the cockpit of a CRJ-200 parked in a hangar at KELM. The current corrected pressure is 29.96 (0.04 in/hg above standard), which gives the correct field elevation of 955 feet MSL on the altitude display. SAT is 23C and the calculated density altitude right now is 2,115 feet MSL.

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Yes, pressure altitude is what should be reported by the transponder to VATSIM. Not sure if that is true or not, but that’s what it should be. Indicated altitude is what is displayed on your altimeter. When the two differ, that’s where problems can occur.

They will differ below the transition altitude/level when the pressure is different than standard, but ATC (at least in the real world) accommodates that. Temperature should not play into this as neither the altimeter, nor pressure altitude have any temperature compensation. They depend solely on pressure. (Now, your actual height above the ground will change with temperature due to the altimeter error for not accounting for the temperature effect - which is really a density effect - but that’s not what’s being discussed here.)

When the ambient pressure fits the ISA standard pressure relationship with altitude, then the indicated altitude and the pressure altitude should be the same. When you set 29.92/1013.25 or STD on the altimeter’s baro setting, like you would do above the transition altitude, you are telling the airplane to fly at the altitude associated with the ISA standard pressure relationship with altitude. That’s what a flight level is. By definition, the indicated altitude should then equal the pressure altitude, regardless of temperature or the sea level pressure. This is where MSFS is still incorrect. And why you will continue to get questioned by ATC, whether it’s VATSIM/IVAO/etc or the built-in MSFS ATC.

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Aah, so “pressure altitude” is always calculated calibrated to std pressure, whereas “indicated altitude” is the same thing but calibrated to whatever pressure the pilot entered (either given by airport, or std above transition alt) → and is what’s displayed on the altimeter?

Wikipedia says mode C transponders send pressure altitude (though says nothing about ADS-B); this would seem useful above transition altitude but does it mean there is no way for ATC to get true altitude of a plane below transition altitude except for radar?

Thanks for any clarification! I’m trying to fill in gaps in my knowledge. :slight_smile:

Yes, exactly. When you change the baro setting on the altimeter, what you are doing is moving that standard pressure vs altitude curve up or down. It keeps the same shape and change in altitude for a given change in pressure.

ATC does the same correction to pressure altitude below the transition altitude that the pilot does. They essentially have the same way as the pilot to dial that curve up or down based on the QNH. That way, if the pilot dials in the wrong QNH, either purposefully or inadvertently, they can catch it!

Ah, that makes perfect sense, now that I see all the pieces together. ATC only needs the uncorrected pressure altitude, since they know the local baro pressure (of course!). Thanks for the data. :slight_smile:

Now to wait for the fix. :wink:

Yes, exactly like I said in my post. Indicated altitude simvar is what is shown in the altimeter, doesn’t matter which baro setting you have.

And I’m 99% sure that vPilot reads PRESSURE ALTITUDE and reports that altitude to Euroscope.

Unless Euroscope calculate altitude from indicated altitude, that’s possible too.