Altitude discrepancy

[SlabsFly 1]

Hi there

First post here. Not sure if it has been like that for along time/forever, but I just recently noticed a difference in altitude displayed between the display next to the aircraft on the map and on the elevation profile. Discrepancy is marked in red. As far as I can determine, the one in the elevation profile is the INDICATED altitude and the one next to the aircraft is the ACTUAL altitude. One would have hoped that, if you have the QNH set correctly, these two would be pretty much identical. But in the example below and other instances, I have noticed quite substantial differences. I have experimented with adjusting the QNH away from the advised QNH to get the two in synch and the required adjustment is BIG. It is not just a few hPa or like 1/10" HG: it is a lot more.

So I am curious: where does the ACTUAL altitude come from and why is it so very different from the indicated altitude?

Thanks for a great product! I cannot use MSFS without it.

Alwyn

[albar965 1,047]

Hi Alwyn,

I cannot see the image (see https://www.avsim.com/forums/topic/605519-how-to-post-a-screenshot-or-files/ ) but:
Elevation profile uses indicated altitude. Otherwise you might miss altitude constrains depending on transition level and baro settings.

Both indicated, actual and more altitudes come directly from MSFS and are shown by LNM unchanged. Nothing calculated there.
The indicated and actual should match if the baro setting is correct.

Where did you get the baro setting from? LNM progress tab, LNM airport weather report, ATIS or other? Maybe they do not match the sim values.

Will check.

Alex

Edited February 23, 2023 by albar965

[SlabsFly 1]

Thanks for the quick reply Alex. Sorry about the image not showing: will try to fix.

I just quickly tried the same route again from Portland OR (KPDX)  to Seatac (KSEA). ATC gave me a QNH of 29.91, pressed B to set. Here you can see the barometer set correctly, altitude 6000ft

Screenshot of LNM. Here you can see the elevation profile correctly says 6008ft, the tag on the aircraft says 5379ft. QNH displays correctly as 29.91

I then manually adjusted the QNH in the aircraft to try and get the altitudes to match. This is what it looked like in the PFD:

And in LNM:

FWIW: I checked online real ATIS also and 29.91 was the QNH given for KPDX there as well.

Hope this is OK?

Thanks Alex!

Alwyn

[albar965 1,047]

Thanks a lot for the information, Alwyn! I have to check this in the sim and review the code. Will get back later.

[SlabsFly 1]

Thanks Alex. Just to note that I did have a few mods active when this happened: Bijan's seasons, G36 mod plus a livery. I am tied up for most of the morning but will try again without any mods just in case.

Cheers

Alwyn

[Bob Scott 7,747]

I think this is cold temperature altimetry errors at work, which are a real-world phenomena and are now modelled by both XPlane12 and MSFS.  You are at -10 deg C and 6,000 ft, which produces an indicated altitude error of ~580 ft (see the discussion and spreadsheet tool at https://skybrary.aero/articles/altimeter-temperature-error-correction ).  So at 6,000 ft indicated and -10C, your actual physical altitude is 5,420 ft MSL, which is pretty close to what LNM is depicting.  These errors are why there are cold weather altitude correction tables published for approaches into a number of airports in FAA-regulated airspace, and why there are ICAO PANS OPS procedures for applying cold weather corrections elsewhere.

 

 

[albar965 1,047]

5 minutes ago, Bob Scott said:

I think this is cold temperature altimetry errors at work, which are a real-world phenomena and are now modelled by both XPlane12 and MSFS.  You are at -10 deg C and 6,000 ft, which produces an indicated altitude error of ~680 ft (see the discussion and spreadsheet tool at https://skybrary.aero/articles/altimeter-temperature-error-correction ).  So at 6,000 ft indicated and -10C, your actual physical altitude is 5,320 ft MSL, which is pretty close to what LNM is depicting.

I see. I was not aware that MSFS now also considers this now.

I have to check this in X-Plane as well tomorrow. I suppose this is already covered when reading the actual altitude and indicated altitude datarefs. The latter one should always be equal to the altimeter in the cockpit no matter what sim. Actual altitude should be of course independent of all effects.

Alex

Edit: Yep. Now I see the ISA deviation of -14°C.

Edited February 24, 2023 by albar965
ISA deviation

[Bob Scott 7,747]

3 minutes ago, albar965 said:

I see. I was not aware that MSFS now also considers this now.

I have to check this in X-Plane as well tomorrow. I suppose this is already covered when reading the actual altitude and indicated altitude datarefs. The latter one should always be equal to the altimeter in the cockpit no matter what sim. Actual altitude should be of course independent of all effects.

Alex

Edit: Yep. Now I see the ISA deviation of -14°C.

And to be clear, the altimeter error is a function of actual outside air temp, not ISA deviation, TAT/RAT, etc.  I think the datarefs in XP12 are pretty close to the mark...not sure about the simconnect vars in MSFS.

[albar965 1,047]

Just now, Bob Scott said:

And to be clear, the altimeter error is a function of actual outside air temp, not ISA deviation, TAT/RAT, etc.

Right. Meh. Stupid me. Thanks. 🙂

[Bob Scott 7,747]

7 minutes ago, albar965 said:

Right. Meh. Stupid me. Thanks. 🙂

Stupid?  Oh no, just a tiny bit less brilliant than usual!  😉

[SlabsFly 1]

Oh wow, contrary to Alex, I REALLY am stupid: I was never aware of this! To try and make myself look less bad I will say that all my real world flying happened in rather temperate areas, so this kind of aberrant temperature was never a concern.

Thanks Bob and Alex for the responses and the enlightenment.

Alwyn

[Gason71 128]

1) There are no stupid people, there are only less informed than others  ... 
2) Nobody knows everything 😛

Regards,
Piotr

ps. good solution of the problem finished this nice discussion   🙂
 

[albar965 1,047]

First: Nobody's stupid here (me being only forgetful)🙃. The modeling of the effect is new in X-Plane 12 and MSFS.

I added this to the FAQ: https://albar965.github.io/littlenavmap-faq.html#altitude-correction and updated the user manual with a link as well (user manual not published yet).

I'll see how this can be considered in future LNM versions.

Thanks to all, especially Bob Scott!

Alex

[Puuhbear 0]

-deleted because of error 99: change opperator-

Edited April 27, 2023 by Puuhbear
I was stupid