A word of acknowledgement to Murmur for his (usual) contribution to better understanding how a simulator models some physics concept / effect /...

One of the announced features of XP12 was geopotential height variation due to Temperature deviations from the standard atmosphere T=15ºC at mean sea level.

The well known "from High to Low, watch out bellow..." addagio applies to both pressure and T variations, although most flight simulators, with a few exceptions going to Aerowinx and somehow even DCS and IL2 Great Battles ( these two combat sims in a rather scriped way, but they did model it ), never modeled it other than for the pressure side of the variation.

Well, XP12 after some (me ncluded) suggested LR to model such an important feature, came with it's own model of this varation, and apparently, at least as mentioned in the documents, MFS does it too, although I haven't been able to properly test it in that sim yet.

This:

"Geopotential Height Variation"

with T is rather important in aviation when operating in cold weather conditions, requiring special procedures mentioned in some approach crharts and even automatic compensation done by some FMS systems:

FMS Cold Temperature Altitude Correction (uasc.com)

In XP12 you can test the effect with a situation in which you vary the surface temperature at a given aerodrome, while keeping pressure constant. This corresponds to a situation in which IRL at a given airfield with a significative annual temperature amplitude, you can find two day, one in Summer he other in "deep" Winter where the reported QNH is exactly the same BUT! the difference in T is high enough to make the effect come to play.

In XP12 you will probably be able to test the effect using Real World Weather and making and registering tests along one year at such an airfield. But it's a lot easier and possible to observe the effect comming into play by simply using the weather menu and manually setting the values of QNH ( constant ) and T.

At the Airport Reference Point ( ARP ) to which the QNH value refers, if you look at the altimeter there should be no variation in the indicated altitude since you are not changing QNH vaules. The differences will be reflected for instance at reference points along a glidepath, where in cold Winter days and while flying for iinstance an ILS approach you will be able to see that your altimeter reads higher than expected at a given point in the glidepath, while in hot Summer days it'll read lower than true height.

Murmur, after I told him yesterday that XP12 was probably not modelling this correctly, made some tests to show me I was wrong, and indeed ALL IS GOOD 🙂

The problem with my initial tests, was that the airfiled point I was making my tests at was probably at a different height from the ARP used for the QNH callibration at that aerodrome.

Out of curiosity, for you XP12-only user (like me.... 🙄 - well... there's the gliders comming you see...and I was missing the 737 and the Fenix...), when MFS was released it did include such effects too, although users started complaining about problems when flying online in VATSIM - a problem I believe should also be experienced by XP Vatsim users unless their "transponders" include a small trick..  ASOBO said they did work on it, but I am unable to properly test the end result because as they stated, the effect is present only when fling with Real World Weather, so, we have to find the right spots, and with Winter arriving there will be some available, so I hope, for testing...

Edited October 30, 20223 yr by jcomm

It's not necessary to wait for winter to test it. One method would be:

.) find an airport where the temperature is significantly different from the vertical ISA profile (+15C @ sea level, +8.5C @ 1000m, +2C @ 2000m, etc.);

.) fly over it at some altitude (say 5000ft or 10000ft) using real weather and the altimeter set at QNH; or, if the airport is at a significant elevation, just stay on the ground and note field elevation.

.) disable real weather; set manual weather at ISA temperature, and set sea level pressure at the same QNH of real weather.

If the indicated altitude changes, then it means pressure deviations due to temperature deviations are modeled, otherwise not.

 

Yeah Murmur I know😁and that was the method I used before together with the display of the environment variables in Dev mode, but I had some difficulties checking the results against rw correction tables. 

Also see this older post by @JRBarrettwhen I questioned the same about MFS:

 

Edited October 30, 20223 yr by jcomm

1 hour ago, jcomm said:

Yeah Murmur I know😁and that was the method I used before together with the display of the environment variables in Dev mode, but I had some difficulties checking the results against rw correction tables. 

Also see this older post by @JRBarrettwhen I questioned the same about MFS:

 

Based on another post by Murmur, it may not work correctly when using a weather preset and entering only ground temperature, but it definitely works when using LiveWeather with all atmospheric pressure and temperature levels (surface and aloft) supplied by the MeteoBlue model. I have tested it extensively.

In summer, when temperatures at the flight levels are often ISA +10 (or more) it is not uncommon to see aircraft flying at a true geopotential height as much as 2000 feet above indicated altitude. This can be easily seen on the tracking site https://globe.adsbexchange.com which shows all transmitted ADSB parameters for aircraft in flight, including barometric altitude (from the aircraft’s air data computer), and true altitude (from the aircraft’s GPS).

Plugging the real time atmospheric environment variables from MSFS into an altimetry calculator shows their calculations are spot on. Good to know that XP12 is now doing the same thing.

 

Edited October 30, 20223 yr by JRBarrett

I really don't know how it get's implemented in XP12, or MFS for that matter...

Apparently, as you say, it should well be the case that MFS is calculating "true altitude" based on their GRIB feeds by Meteoblue, but then they will always have to interpolate for the lower and ground levels, specially when havng to blend that forecast data with observations.

In the absence of GRIB data MFS must revert to an ISA pressure and temperature lapse rate, so, it's not possibe to test under those circumstances.

OTOH X-Plane 12 models it even in Manual Weather mode, but I've seen the altimeter showing slight variations even when I think the spot my aircaft is at differs less tha 1 m from the ARP. On some airfields though, as Murmur was able to clearly show in a short video recorded at Portland, there is practically no altimeter variation between ISA-30 to ISA+30.

Edited October 30, 20223 yr by jcomm

3 minutes ago, jcomm said:

Apparently, as you say, it should well be the case that MFS is calculating "true altitude" based on their GRIB feeds by Meteoblue, but then they will always have to interpolate for the lower and ground levels, specially when havng to blend that forecast data with observations.

In the absence of GRIB data MFS must revert to an ISA pressue and temperature lapse rate, so, it's not possibe to test under those circumstances.

No interpolation is necessary. At the flight levels, with the altimeter set to STD pressure, the aircraft’s indicated vs. true altitude depends only on the ambient atmoshereic conditions that exist at the aircraft’s current altitude. The aircraft does not “know” (or care) what the ambient conditions are at any point above or below its current altitude.

Now, indicated altitude (when flying below the transition level), depends on the current altimeter setting, and that value, (and the MSL elevation of the station from which the setting comes), has to be factored into the true altitude calculation. 

The only time MSFS uses the standard ISA values for temperature and pressure is when using the “clear sky” preset. In that state, it will always set the sea level temperature to 15C and the sea level pressure to 29.92 in/hg. Temperature and pressure at any higher altitude will match ISA for that altitude exactly, so indicated and true altitude should always be the same when using that preset.

When Live Weather is active, the temperature and pressure for any given altitude will come from the MeteoBlue model prediction. I don’t have direct access to the MeteoBlue predicted values (from their NEMS30 model) other than by looking at the sim weather variables being injected at any given moment, but when entering current ambient pressure, current air temperature and current indicated altitude from the sim variables, the geopotential altitude always precisely matches the external altimetry calculator I use.

http://www.luizmonteiro.com/altimetry.aspx

When flying, I use Foreflight, loaded with my current flight plan, and with MSFS current aircraft position injected by the XMapsy app. Foreflight can give the predicted winds and temperatures aloft based on the NOAA GFS model. I have found that those values normally match the MeteoBlue NEMS model quite closely.

I have not tested this in XP 12 yet, but will do so. XP will also output current aircraft position to Foreflight natively. One advantage of XP is that the various datarefs for atmospheric temperature and pressure can be easily displayed on the screen, rather than having to use an external program like the MSFS simvarwatch utility. I believe XP 12 is using the GFS model GRIB predictions to set conditions aloft.

One complicating factor when using MSFS Live Weather is the fact that airport temperature and pressure will come from METAR (when available for a given airport). The transition between METAR and MeteoBlue values for those parameters appears to happen at approximately 1500 feet AGL. AFAIK, XP12 also uses METAR for airports. I’m not sure how (or when) they transition between model GRIB and METAR as an aircraft climbs or descends.

51 minutes ago, JRBarrett said:

No interpolation is necessary. At the flight levels, with the altimeter set to STD pressure, the aircraft’s indicated vs. true altitude depends only on the ambient atmoshereic conditions that exist at the aircraft’s current altitude. The aircraft does not “know” (or care) what the ambient conditions are at any point above or below its current altitude.

[...]

One complicating factor when using MSFS Live Weather is the fact that airport temperature and pressure will come from METAR (when available for a given airport). The transition between METAR and MeteoBlue values for those parameters appears to happen at approximately 1500 feet AGL. AFAIK, XP12 also uses METAR for airports. I’m not sure how (or when) they transition between model GRIB and METAR as an aircraft climbs or descends.

That's what I was referring to when I mentioned the "blend".

I'm not sure if one is used, nor do I know exactly what levels they get from the Meteoblue GRIB data. If it was ECMWF I would know - that's what I do at work 🙂