investigating Altimeter Progressively Incorrect

[Rhayvn]

From a Cold Start, the altimeter, in Electric or Pneumatic, show a progressively larger error the higher you go.  At sea level, it matches.  As you increase altitude it is off by about 600 feet for every 10,000 feet up you go.  It reads too low.  External views, tacview, F10 map, and other airframes all the show the correct altitude (Flying in formation to test).

This was tested on multiple clients by multiple users on different maps.

[Tholozor]

Altimeters aren't perfect, much like airspeed indicators, or fuel gauges in cars. Because the altitude is derived from atmospheric pressure, there are inherent errors as it varies with the air density and temperature. However, as long as aircraft in the general vicinity utilize the same settings, they're still able to deconflict or reference their altitude differences. In the case of general aviation, aircraft switch to standard pressure (29.92inHg) once they pass the transition layer (for North America, this is at 18,000ft) in order to maintain a fixed flight level reference with everyone else, since atmospheric pressures change from location to location.

https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_1.7.html

Edited August 16, 2023 by Tholozor

[Rhayvn]

I've been a pilot for years, I'm well aware of how they work, which is why I am reporting this.  Because it appears to be due to differences in temperature set in the mission editor but also is different between modules.
1.  It's consistent between those of us that tested it.
2.  The error increases at a steady rate from zero on the ground.
3.  Changing temperature causes the amount of error to change but the error is still pretty much linear.  It is not Density altitude, which would be a consistent deviation throughout the altitudes tested.
4.  The F15 was tested in formation and did not have the error.  The number the 15 shows is consistent with the number reported by other views in DCS.

If the 16 altimeter has a real life difference between normal indicated altitude and other types such as density or pressure altitude, it would be good to know that.  Perhaps it is modeled correctly and the 15E is not (The 15 automatically adjusting for temperature deviation).  That would mean the 16 is showing real Indicated altitude and the 15 is showing True altitude.  Perhaps that is even correct.

If it's meant to model variations in temperature, why is it zero on the ground (seemingly regardless of how high up the airfield is)?

Just one example from the Caucasus free flight instant action.  Which is an air start and is relatively close compared to the other tests my group has done.  I have seen this altitude be off by as much as 400 ft at 8000 ft ASL and 1500 or more above 20,000.  OAT is 10C in this mission.
4000 indicated.  3922 actual.
8000 indicated.  7842 actual.
18000 indicated. 17650 actual.
30000 indicated. 29430 actual.

 

[Nealius]

5 hours ago, Tholozor said:

Altimeters aren't perfect, much like airspeed indicators, or fuel gauges in cars. Because the altitude is derived from atmospheric pressure, there are inherent errors as it varies with the air density and temperature.

Tolerance listed in the AIM you linked is +/- 75ft. OP's readings in testing exceed those tolerances. 

[WHOGX5]

On 8/17/2023 at 4:09 AM, Rhayvn said:

I've been a pilot for years, I'm well aware of how they work, which is why I am reporting this.  Because it appears to be due to differences in temperature set in the mission editor but also is different between modules.
1.  It's consistent between those of us that tested it.
2.  The error increases at a steady rate from zero on the ground.
3.  Changing temperature causes the amount of error to change but the error is still pretty much linear.  It is not Density altitude, which would be a consistent deviation throughout the altitudes tested.
4.  The F15 was tested in formation and did not have the error.  The number the 15 shows is consistent with the number reported by other views in DCS.

If the 16 altimeter has a real life difference between normal indicated altitude and other types such as density or pressure altitude, it would be good to know that.  Perhaps it is modeled correctly and the 15E is not (The 15 automatically adjusting for temperature deviation).  That would mean the 16 is showing real Indicated altitude and the 15 is showing True altitude.  Perhaps that is even correct.

If it's meant to model variations in temperature, why is it zero on the ground (seemingly regardless of how high up the airfield is)?

Just one example from the Caucasus free flight instant action.  Which is an air start and is relatively close compared to the other tests my group has done.  I have seen this altitude be off by as much as 400 ft at 8000 ft ASL and 1500 or more above 20,000.  OAT is 10C in this mission.
4000 indicated.  3922 actual.
8000 indicated.  7842 actual.
18000 indicated. 17650 actual.
30000 indicated. 29430 actual.

I can confirm this, and it is indeed different between modules. This has been an issue for a very long time and even though most aircraft in DCS have a small amount of error (which is to be expected), for some reason the DCS F-16C's error is extremely large. The size of the error me and my community has encountered lines up perfectly with the values in your post.

[The_real_slim_Sshep]

Altimeter test.trk this thread got marked to need a trk, so here you go

[Lord Vader]

Hi @The_real_slim_Sshep

Thank you for the track, but it seems it doesn't depict the OP's deviations. Running your track, I only see a slight error that appears induced by air density. We are analysing if this is within the tolerances for these systems but like someone said above, altimeters are prone to errors and you should expect some deviations from digital readings. 

[ShuRugal]

On 8/16/2023 at 6:07 PM, Tholozor said:

Altimeters aren't perfect, much like airspeed indicators, or fuel gauges in cars. Because the altitude is derived from atmospheric pressure, there are inherent errors as it varies with the air density and temperature. However, as long as aircraft in the general vicinity utilize the same settings, they're still able to deconflict or reference their altitude differences. In the case of general aviation, aircraft switch to standard pressure (29.92inHg) once they pass the transition layer (for North America, this is at 18,000ft) in order to maintain a fixed flight level reference with everyone else, since atmospheric pressures change from location to location.

https://www.faa.gov/air_traffic/publications/atpubs/aip_html/part2_enr_section_1.7.html

 

https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C/part-43#Appendix-E-to-Part-43

 

600 feet of error at 10,000 is nearly eight times the acceptable error, as spelled out in the Part 43 maintenance regulations.

[WHOGX5]

On 2/5/2024 at 3:31 PM, Lord Vader said:

Hi @The_real_slim_Sshep

Thank you for the track, but it seems it doesn't depict the OP's deviations. Running your track, I only see a slight error that appears induced by air density. We are analysing if this is within the tolerances for these systems but like someone said above, altimeters are prone to errors and you should expect some deviations from digital readings. 

It's also worth noting that this error does not apply to all airframes. I've flown the F-16C in formation with F-15E's in DCS, with the same barometric pressure set on our altimeters, and we had somewhere between 500-1000 feet of difference between our indicated barometric altitude at FL200-ish while flying wingtip.

[Lord Vader]

Like I said, I've asked for the tolerances to be reviewed and if any change is required, it will be added in a future update.