Is DCS Altimeter Broken?

[MstrCmdr]

Don't know if this is correct behavior however ive experienced this 2x in multiplayer.

Mission began and ended in Gaziantep (Syria map) in the A10C2.

QNH was 30.16, set on altimeter, and runway elevation and altimeter elevation matched at 2240ft.

On the overhead break i stabilized at a baro alt of 3740 (runway elevation plus 1500ft).

This should have given me a radar height of 1500ft however i was reading 1700ft on the radar. 200 ft off.

What am I doing wrong?

Edited March 1, 2022 by MstrCmdr
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[Wayno]

Can confirm the issue, I was part of that flight.

[BIGNEWY]

please include a short track replay example showing the problem, you can save the track replay when you exit a single player mission or if multiplayer check your saved games dcs tracks multiplayer folder. 

If you can reproduce in a single player mission it would be better and the track will be smaller. 

thanks

[Frederf]

3 hours ago, MstrCmdr said:

Don't know if this is correct behavior however ive experienced this 2x in multiplayer.

Mission began and ended in Gaziantep (Syria map) in the A10C2.

QNH was 30.16, set on altimeter, and runway elevation and altimeter elevation matched at 2240ft.

On the overhead break i stabilized at a baro alt of 3740 (runway elevation plus 1500ft).

This should have given me a radar height of 1500ft however i was reading 1700ft on the radar. 200 ft off.

What am I doing wrong?

 

DCS does not give QNH. The ATC gives QFE and the briefing gives QFF which it labels as QNH. You will find that at airport surface setting value from briefing "QNH" (QFF) will not equal airport elevation. The difference depends proportionally on vertical distance from sea level and temperature difference from standard. A high altitude airport and hot temperature will indicate you are much below known airport elevation. Remember that altimeters have a calibration point where they are correct. Everywhere else they may not be.

• QFE reads zero at airport surface
• QNH reads airport elevation at airport surface
• QFF reads zero at mean sea level

Because QFE and QNH are calibrated to a specific airport it is impossible to have a universal value for all airports. There is no such thing as "the QNH" only "a QNH for this particular airport."

[=475FG= Dawger]

4 hours ago, MstrCmdr said:

Don't know if this is correct behavior however ive experienced this 2x in multiplayer.

Mission began and ended in Gaziantep (Syria map) in the A10C2.

QNH was 30.16, set on altimeter, and runway elevation and altimeter elevation matched at 2240ft.

On the overhead break i stabilized at a baro alt of 3740 (runway elevation plus 1500ft).

This should have given me a radar height of 1500ft however i was reading 1700ft on the radar. 200 ft off.

What am I doing wrong?

 

Were you over precisely the same spot on the airport as that is all that is relevant for comparison?

[MstrCmdr]

57 minutes ago, Frederf said:

DCS does not give QNH. The ATC gives QFE and the briefing gives QFF which it labels as QNH. You will find that at airport surface setting value from briefing "QNH" (QFF) will not equal airport elevation. The difference depends proportionally on vertical distance from sea level and temperature difference from standard. A high altitude airport and hot temperature will indicate you are much below known airport elevation. Remember that altimeters have a calibration point where they are correct. Everywhere else they may not be.

• QFE reads zero at airport surface
• QNH reads airport elevation at airport surface
• QFF reads zero at mean sea level

Because QFE and QNH are calibrated to a specific airport it is impossible to have a universal value for all airports. There is no such thing as "the QNH" only "a QNH for this particular airport."

Thanks for the information.  The briefing labeled QNH as 30.16 and the baro reading on the hud matched the airport elevation and I returned to the same airport.  I stay away from the ATC...

So if they match at airport elevation why would I be 200ft off just 1500ft up?  I'm not a real pilot...

6 minutes ago, =475FG= Dawger said:

Were you over precisely the same spot on the airport as that is all that is relevant for comparison?

Gaziantep is like 10ft difference from the main apron to runway 28.  All I know is I was 200ft off in radar elevation to the baro on the HUD when overflying the runway in the break which is like what happened????  At first I thought well I mathed wrong.  Then the same thing happened on the next mission so I said ok what am I doing wrong?

[=475FG= Dawger]

15 minutes ago, MstrCmdr said:

Thanks for the information.  The briefing labeled QNH as 30.16 and the baro reading on the hud matched the airport elevation and I returned to the same airport.  I stay away from the ATC...

So if they match at airport elevation why would I be 200ft off just 1500ft up?  I'm not a real pilot...

Gaziantep is like 10ft difference from the main apron to runway 28.  All I know is I was 200ft off in radar elevation to the baro on the HUD when overflying the runway in the break which is like what happened????  At first I thought well I mathed wrong.  Then the same thing happened on the next mission so I said ok what am I doing wrong?

There are any number of things that will cause a difference between radar altimeter and a barometric altimeter. You have to start from a point on the airport with a KNOWN elevation.

The entire airport is not at the published elevation, only the designated point where the measurement was taken.

Good luck finding that published point in DCS and you cannot assume what is on a real world chart will correspond to DCS.

Once you find a known point elevation, park your airplane there and set the altimeter to match that elevation exactly. I would suggest pulling the center of the runway elevation from the F10 map. And then you would have to fly, straight and level over that point exactly to see if they match. If you are not straight and level, your radar altimeter will read higher than actual. 

Real airplanes are allowed 75 feet of error in the barometric altimeter and still be certified for instrument flight so I think you are engaging in an exercise in frustration in expecting to match the baro to the radar altimeter.

There are a ton of environmental factors that introduce errors into the barometric altimeter and it is unknown how any of that is simulated in DCS.

Edited March 1, 2022 by =475FG= Dawger

[Frederf]

8 hours ago, MstrCmdr said:

 The briefing labeled QNH as 30.16 and the baro reading on the hud matched the airport elevation and I returned to the same airport.  I stay away from the ATC...

So if they match at airport elevation why would I be 200ft off just 1500ft up?  I'm not a real pilot...

 

Biggest effect was probably simply the terrain is not flat. It's perfectly possible to be 1500' above the airport but only 1300' above a hill below you on approach. The terrain to the NW of the field is commonly in the 2400-2500s elevation even within 3 miles.

But I still want to impress upon you that barometric altimeters do not show true altitude in general, even when calibrated. A calibrated altimeter only reads correct at the level it was calibrated for. Everywhere else it's wrong. If you set QNH at a high elevation airport on a cold or hot day and fly up to 20,000' instrument reading you will be no where near 20,000' true altitude.

Here's an example of QNH at two different airports with "QNH 29.92" in the briefing in all six cases and the only difference being temperature and airport elevation. Notice at Beslan when hot/cold the real QNH for that airport is wildly different than the briefing value.

• -5°C SLT
• Beslan (elevation 1729') QNH is 29.77''
• Batumi (elevation    33') QNH is 29.92''
• 15°C SLT
• Beslan (elevation 1729') QNH is 29.92''
• Batumi (elevation    33') QNH is 29.92''
• 35°C SLT
• Beslan (elevation 1729') QNH is 30.04''
• Batumi (elevation    33') QNH is 29.92''

Notice that with standard SLP and various temperatures the QNH at a sea level airport is effectively unchanged. But the farther away you get from the calibration point temperature matters a great deal.

I'll try to replicate the circumstance. Gaziantep runway surface actually varies considerably in elevation from 2289' at the RWY10 numbers to 2196' at the RWY28 numbers. The airdrome data of 2287' elevation must be indexed off the RWY10 end. At the default runway spawn location the A-10C2 is altitude 2204' on a spot of terrain 2197'. The airplane's sensor port is 6-7' above the tire contact surface. I have chosen the default +20°C SLT and 30.16''Hg SLP. QNH to achieve altimeter reading for surface under airplane and airplane reported true altitude are 30.18'' and 30.19'' respectively. If I set QNH from the briefing blindly as 30.16'' I get a reading of 2180' or so on the ground.

The terrain is anything but flat not only on the approach to the runway but even the runway itself. Two hundred feet of discrepancies could happen easily just by valleys and hills. But just to illustrate the altimeter/radalt exactly I'll fly over the same takeoff spot at 3697' true altitude and note the readings.

At 3695' true alt over a point on the ground 2197' elevation I have baro reading 3675' (QNH 3019'') and 1500R radalt. Adjusting Kollsman to 30.16 baro reads 3650'.

[Blindman]

The Landing procedure gives you 1500ft AGL, so that is a height you should read from the radar altimeter, not an altitude you calculate from the baro altimeter.

[=475FG= Dawger]

43 minutes ago, Blindman said:

The Landing procedure gives you 1500ft AGL, so that is a height you should read from the radar altimeter, not an altitude you calculate from the baro altimeter.

Nope. You just add 1500 (or 1000 or 800 or whatever you like) ft to field elevation, fly that on the baro and call it good. 

[Frederf]

The procedure is most likely 1500' AAL or above aerodrome level. Traffic pattern height is often stipulated in a way that it's a certain value, possibly rounded, and referenced baro QNH.

The actual legal height should be issued by the controlling agency. Usually it's the airport elevation (highest point on the field regardless of elevations of any bit of runway or approach area) plus a number. In any case it's read from a document. How picky anyone is about a few feet in practice is another issue.