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

Boy, I sure hope ATC software changes IRL don’t ever have this problem! :dizzy_face: :open_mouth: :astonished: :flushed:

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Any ideas from Matt or Asobo on WHEN this will be fixed?

We can’t know exactly, but MSFS will roll out a patch in the coming weeks.

I really hope its sooner than that. Breaking interaction with Vatsim seems pretty bad considering Vatsim are a MSFS partner aren’t they?

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This needs to be released way before several weeks. This is a huge issue that needs to be hot fixed pretty soon.

I’m kinda confused by the known issues statement above. Is this bug simply an error in reporting the altitude to internal & external ATC, or a larger issue with how altitude and temperature interaction is modelled?

I’m particularly confused by point two:

”If temperature is different from ISA the aircraft altimeter does not indicate the correct altitude, most observable when the aircraft is on the ground with the correct QNH set. The altimeter should align with the airports elevation but it won’t with SU5.”

To me this behaviour is correct and not a bug. It’s how altimeters should operate in the real world, unless the variations are beyond normal for a given variation from ISA? (Any deviations from ISA should result in an erroneous reading on the altimeter, equating to approximately 4% height error per 10°C deviation from ISA, I’m not sure of the error MSFS currently produces.)

Real altimeters are not capable of correcting for density changes due to variations in temperature, as they are calibrated for one specific temperature. This is noticeable when flying an ILS as it is a fixed path in space. When cross checking altitudes, temps above or below ISA will produce slights variations indicating high or low. This is why low temperature altimetery procedures exist.

Maybe I’m missing something here, but are Asobo expecting the altimeter to read 100% accurately regardless of the temperature? I hope not.

EDIT: Nevermind.

I’m not a pilot and certainly not an expert. I also don’t think I’m violating any NDA by posting this, but here is a pretty solid explanation of the issue that was discussed. For the record, this isn’t my explanation - it’s from @Nijntje91

What is wrong

I think the main issue is that MSFS is calculating altitudes from Mean Sea Level instead of aerodrome level. The “altimeter setting source” (aerodrome level) should be the starting point from which the altimeter, with aircraft on ground and correct QNH set, reads aerodrome level, all errors (except instrument error) are zero. From the examples I’ve seen, I think the following is going on:

  1. Pressure altitude calculation has an error in it, it should follow the ISA tables.
  2. Temperature error seems to be applied directly to the indicated altitude rather than the temperature affecting the pressure lapse rate. For example when flying with 1013.25 hPa / 29.92 inHg set, the altimeter shall read pressure altitude no matter the temperature. From the examples above you can see that this is only true at ISA, at non-standard temperature the difference between pressure altitude and indicated altitude exactly match 4 ft per 1000 ft per 1C ISA deviation, which is completely wrong!
  3. The temperature error seems to be applied to the aircraft height above sea level rather than the aircraft height above ground level. The altimeter is calibrated according the pressure lapse rate in standard atmosphere, the QNH is calculated using the standard pressure lapse rate in the same way. In other words temperature shall not affect the indicated altitude on ground with the correct QNH set. With QNH set and on the ground the altimeter shall always read aerodrome elevation.
  4. Don’t use the 4 ft per 1000 ft per 1C ISA deviation rule of thumb. Its just that, a rule of thumb, and not even a good one. Its not allowed for use in the real world for temperatures below -15C. Let alone something to base your whole atmospheric model on… Its a little worrying to see Asobo relying on very rough rules of thumb rather than using an exact approach, the same is true with surface wind versus wind above friction layer for example (50%).

This still doesn’t explain the difference between indicated and pressure altitude when 1013.25 hPa / 29.92 inHg is set. With the altimeter set to standard pressure the altimeter should read pressure altitude no matter the conditions. Temperature and pressure need to affect the pressure lapse rate rather than those errors being directly applied to the indicated altitude, causing a disconnect between the indicated altitude and pressure altitude (I assume that is what MSFS currently does).


I think that is a bug, QNH is station pressure reduced to sea level pressure assuming ISA atmosphere temperature, which means the altimeter will read field elevation at non-ISA temperatures since the altimeter is also calibrated for ISA atmosphere temperature. Which is why the cold temperature correction is 0 at field elevation even for a high altitude airport. But that begs the question of what Asobo are even trying to model; if I press ‘B’, am I supposed to get the QNH setting for the nearest local airport or the QNH setting for a fictional weather station at local sea level (in the bottom of a well…?).

Edit: Crossposted with Viralwhite, seems we are on the same track.

I also wonder what the scope is of this “altitude reporting bug”, does the altimeter if set to 29.92 report pressure altitude or something else? With the live weather temperature bug seemingly also still being a thing (are they related?) planning and performance monitoring is quite perplexing.

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Thanks @Viralwhite & @FlyingBear01, your explanation of the bug makes perfect sense. I was miss-interpreting the issue to think Asobo wanted temperature compensating altimeter indications at all times. Thanks for clarifying it for me.

Saying that, the current issue is still extremely poor, hopefully it gets fixed soon.

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Of course not, this only applies ABOVE the altimeter setting source (aerodrome elevation) and not below. On the ground with correct QNH set the altimeter should read aerodrome elevation no matter the temperature.

Edit: sorry, I see this already was addressed.


Using your post to boot hahaha

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That’s ok! Hopefully it gets fixed soon.

So if this issue is related to the live weather is it resolved by using a third party software like Rex Weather Force?

Phew! I hope so!

I try to remember that lots of people play this and many just want to hop in a Cub or Cessna and fly around where they live at maybe no more than 1000ft! But for those of us who love proper airline flying, planning our routes, flying our aircraft “properly” and having the experience and added challenge of being controlled by on-line ATC such as VATSIM, I’m hard pressed to understand how such a massive bug was known about yet they still thought “Ship it!!”. Disappointing, but I was sure they must do an interim fix for this and you’ve confirmed it thank you :slight_smile:


Which Discord/channel is this? Thank you :slight_smile:

I think that would solve the problem. But my Rex weather force is broken after the update. Does it work for you?

Yeah I just saw on their discord that REX isn’t compatible with the update. Guess we’ll have to wait for the hotfix. Hope it’s got some kind of priority to it. I’ve pretty much got to the point where I only fly online.

How? How do they keep releasing “updates” with these known issues? If you know there is an issue, then fix it.

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Imagine release an update with a known bug regarding a partner you proudly announced…