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[FIXED] Altimeter does not take temperature into account


Rakuzard

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Hey everyone,

 

I'm sorry if this is already reported, the search function didn't yield any results.

 

The altimeter readout shows a wrong altitude with the correct(/real) QNH set in the Kollsman window and vice versa.

More specifically: the altimeters in the C-101CC and C-101EB only respect the pressure setting in the mission and not the temperature.

 

I placed a C-101CC on the ground at Nellis. The F10 map showed the underlying terrain at 1.860 ft MSL.

Pressure was set to 29.92 inHg, temperature to +15 °C at sealevel.

The altimeter in the C-101CC (and EB) shows the correct altitude:

C101CC_Std.jpg

(ME settings: "QNH" 29.92 inHg, temperature +15 °C (ISA Standard Day))

 

With the same pressure and a temperature of 25 °C at sealevel, the QNH can be calculated to be 29.99 inHG at 1.860 ft MSL.

The C-101CC (and EB) incorrectly report an altitude of 1.860 ft MSL at 29.92 inHG:

C101CC_StdPlus10.jpg

(ME settings: "QNH" 29.92, temperature +25 °C (ISA Standard Day + 10 °C))

 

Just for comparison and to check for correctness I jumped into an A-10C with the same environment settings:

A10C_StdPlus10.jpg


Edited by Vibora
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I would be surprised if any general purpose "steam gauge" style altimeter takes temperature into account.  I don't have experience with real A-10's so I don't know if they have  temperature compensation or its just modeled that way. 

 

Deviations between actual altitude and indicated altitude are quite normal and the general adage is "when going from high to low, look out below".  This means at colder temperatures or moving to a lower pressure (without a change in the Kohlsman window) then the indicated altitude will be higher than actual and you will be closer to the ground than you think.  For a 10 degree difference such as between 15 and 25 degrees the change would be neglible from an operating point of view.  When you compare -25 and 15 then you might have to think about it and compensate for it.

 

[Edit]  Just thought I should add that here in North America, a barometric altimeter is considered "safe for operation" if it reads within 75 feet of airfield elevation when the pressure window is set correctly.   These types of altimeters are not as precise as computer simulators tend to make them appear.


Edited by Andurula
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I may have misworded my post. 🙂

 

The underlying problem is that the value one enters in the mission editors "QNH" field is just the pressure at sea level, independent of the set temperature at sea level. It's more like a QFF.
This does make sense because, assuming the same pressure at sea level, an airfield with an elevation of 0 ft has a different QNH than an airfield with an elevation of 1,000 ft.
But the C-101 treats it as QNH.

 

Let's stay with the ISA standard day + 10 °C scenario:
if I place a C-101 at Nellis it shows 1,860 ft with 29.92 inHg in the window. Now I place one at Tonopah Test Range and it shows 5,540 ft with 29.92 inHg in the window. The altitude is correct in both cases, but the pressure setting in the Kollsman window is not.
This is what I meant with "it doesn't take temperature into account". 🙂

 

Now when I treat the pressure value set in the mission editor as the QFF, the QNH can be calculated to be 29.99 inHg at an altitude of 1,860 ft and 30.12 inHg at an altitude of 5.540 ft.

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Sorry, I am not following you.  That is not saying you are wrong but it doesn't match what I understand about altimeters and measures of atmospheric pressure.

 

"The underlying problem is that the value one enters in the mission editors "QNH" field is just the pressure at sea level, independent of the set temperature at sea level. It's more like a QFF."

 

As far as I understand, "QNH" is the barometric pressure adjusted to sea level.  There is no temperature component to the QNH.

 

"This does make sense because, assuming the same pressure at sea level, an airfield with an elevation of 0 ft has a different QNH than an airfield with an elevation of 1,000 ft."

 

The actual measured pressure will be different at the two airfields because they are at different altitudes relative to sea level but the QNH will be the same because it always corrected to sea level.

 

 

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Unfortunately the entire altimetery system in DCS is currently unrealistic.

 

It is built around an “absolute true altitude” concept, where AI aircraft, aircraft systems & sensors, mission editor features, and basically everything else related to altitude at all, only use the exact true altitude, regardless of the air pressure, air temperature or altimeter setting.

 

I wrote the below in another thread, but I’ll past it here as it’s relevant too.

 

My hunch is that DCS has basically been "fudging" correct altimetry for some time. Seems the underlying code all uses "DCS true" altitude, and only in some specific cases, such as player/client aircraft altimeters, does it appear as though any additional pressure correction is able to be applied. I guess 10+ years ago, when the entire ecosystem was no where near as complex or realistic as it is now, this wasn't really noticeable. Unfortunately with the plethora of additional data sources and emphasis on realism that DCS has today, it is starting to become a notable issue.

 

All aircraft. Including AI, should have the ability to correct their altimeter for changes in pressure, and all systems that use altitude as an input such as radar, datalink etc, should be using this corrected figure.

 

Unfortunately, I picture this as a hugely complex issue to solve though. The weather system, AI logic, and individual avionics of almost every aircraft in the game would need to be updated. 

 

As I said, this is a tough one for ED. There is most likely so much code, both new and old built on top of the original "true alt' concept, that it would be a massive task to change this effectively.

 

 

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I also don't follow.

 

If I remember correctly, the altimeter is just a barometer in which pressure is showed as altitude, according to a modelled ISA (International Standard Atmosphere) and the pressure at sea level (and thus the zero value for altitude) is set with the Kollsman knob.

 

If you get the QNH value for the airport and you set that value in the altimeter, it should read the airport elevation when you are at the appropiate place (airport elevation is defined as the highest point on the runway). If you are at any other place in the airport (apron, taxiways or any other point in the airport other than that particular spot) the altimeter will show that altitude (=elevation of that point since you're still on the ground).

 

This is true for an airport at any elevation and in fact, allows you to set your altimeter properly even if you don't have QNH information provided by a met office. This happens on uncontrolled airstrips for ULM, etc... You know the field elevation, then you move the Kollsman knob until the altimeter reads it correctly. The pressure showing on the Kollsman window is the QNH.

 

The altimeter doesn't take into account the real temperature and in fact has errors, but since all of them (altimeters) are calibrated the same errors are the same for everybody. Those errors are not that big to pose a real problem for aviation and aircraft don't smash into terrain or other aircraft.

 

To me seems that the behaviour of the module is correct and in fact, I can see in your A-10 picture that your altimeter shows a tad more than 1860 but the pressure is set differently to those pictures on the C101. Go to the A-10 and check altitude with 29.92 and it should read exactly like the C101. And viceversa, the C101's altimeter should match the A-10's if both Kollsman windows are set equally.

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On 4/22/2021 at 11:54 AM, Scrofa said:

Go to the A-10 and check altitude with 29.92 and it should read exactly like the C101. And viceversa, the C101's altimeter should match the A-10's if both Kollsman windows are set equally.

 

They don't match.

This is the very core of the problem.

 

Settings in the mission editor: 29.92 inHg, 35 °C, both aircraft placed at Tonopah Test Range.

 

A-10:   Alt_A-10C_1.jpg 

C-101: Alt_C-101CC_1.jpg

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Ok, if both are at the same place that's wrong.

 

According to wikipedia, the elevation at Tonopah Test Range airport is 5550 ft, so the C101 would be right and the A-10 wrong. I'm pretty sure the temperature is not taken into account by the altimeter.

 

I don't have that map so I'm not sure the airport has the ground mesh correctly modelled so that different parts of the airport have different elevations according to reality (the "elevation" figure is, as I stated above "the highest point in the runway").

 

Have you checked any other aircraft in the same spot? Any other map? I'm really curious now...

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On 4/19/2021 at 9:13 PM, Andurula said:

As far as I understand, "QNH" is the barometric pressure adjusted to sea level.  There is no temperature component to the QNH.

Not exactly.Yes QNH is temperature-dependent in the sense that it is an altimeter setting which would not be correct for a different temperature all else being the same. The process of discovering QNH when you know the local air pressure doesn't require knowing temperature because you have the local air pressure. You just have to find the altimeter setting that produces the field elevation when at the local air pressure. However if you know air pressure somewhere else (like SLP) then you need temperature to convert pressures in different places.

 

RE: QFF. There's no thing as "the QNH." QNH is a calibration value such that the altimeter displays an airfield elevation when at an airfield surface. QNH is always for a particular place. Everywhere else it is wrong. Denver, Atlanta, and Moscow will have different QNH even if the atmosphere is uniform around the Earth because temperature squishes or stretches the height-pressure relationship such that calibrating altimeter to read true everywhere is impossible. QFF is QNH for a particular place called sea level and that's what the QNH is that you read off the DCS briefing: QNH for a mythical airport at 0 meters MSL. For DCS to give you true QNH it would have to give you different numbers for every airfield.

 

An altimeter isn't something which "takes temperature into account" per se. The issue is that the C-101 altimeter is behaving as if it can correct for temperature when it's not a feature of the design of the real instrument. An altimeter set to the same Kollsman setting at the same place with the same sea level pressure but with different temperatures will show different altitudes. This is because the real air pressure at the instrument is different with different temperatures even though all other factors are fixed.

 

Take a flight from airport A at 0 MSL to airport B at 5,000 MSL on a non-standard temperature day. Whatever altimeter setting was accurate at airport A cannot be accurate at airport B. The calibrations are different. If the C-101 altimeter is accurate everywhere with a singular setting in non-standard temperature then it isn't simulating a real instrument.

 

 

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I'm going back to the first post to look closely to the initial situation. I add in red some observations.

 

Quote

I placed a C-101CC on the ground at Nellis. The F10 map showed the underlying terrain at 1.860 ft MSL.

Pressure was set to 29.92 inHg, temperature to +15 °C at sealevel.

The altimeter in the C-101CC (and EB) shows the correct altitude:

C101CC_Std.jpg

(ME settings: "QNH" 29.92 inHg, temperature +15 °C (ISA Standard Day))

 

With the same pressure and a temperature of 25 °C at sealevel, the QNH can be calculated to be 29.99 inHG at 1.860 ft MSL. Have you calculated this with a formula or is it the setting you input in the ME?

The C-101CC (and EB) incorrectly report an altitude of 1.860 ft MSL at 29.92 inHG:

C101CC_StdPlus10.jpg

(ME settings: "QNH" 29.92, temperature +25 °C (ISA Standard Day + 10 °C)) This doesn't appear to be wrong, if the ME has the QNH set at 29.92, regardless of the temperature, you're setting the pressure conditions in the simulation. It agrees with the principle "when you set the QNH you get airport elevation in the altimeter". If you try another mission with QNH 29.92 and temperature, let's say, +35, it should read the same, because you are imposing the conditions.

 

Just for comparison and to check for correctness I jumped into an A-10C with the same environment settings: Please make sure what environment settings are those, 29.92 as stated in the picture above or 29.99 on the ME as stated further up.

A10C_StdPlus10.jpg

 

Then going to your second post,

 

On 4/23/2021 at 9:25 PM, Rakuzard said:

 

They don't match.

This is the very core of the problem.

 

Settings in the mission editor: 29.92 inHg, 35 °C, both aircraft placed at Tonopah Test Range. According to this

elevation at airports in the Nevada map are better than the Caucasus map. Could you provide more information about the captions below? Are they over the runway? parking spots far away from the runway? (the 300 ft diference between the spot the A-10 is located and the field elevation is quite high, that would be a really hilly airport).

 

A-10:   Alt_A-10C_1.jpg 

C-101: Alt_C-101CC_1.jpg

 

Regardless of why this discrepancy is happening, an altimeter is just a barometer and as such doesn't take into account temperature. The scale is related solely with the ISA atmosphere model and thus it will never display real altitude (true altitude) when has moved out of the calibration point, but an approximation. The error is not that high that compromises operations as ground clearing is achieved with visual references, radio altimeter or with altitude diferences that take into account possible inaccuracies due to temperature, different pressure conditions than the departure/arrival airport, etc...

 

Now I have to say that I'm very curious as to why is happening and I'll run some tests on the caucasus map in some aircraft to see the altimeter behaviour.

 

Could you please tell us if you're using the dinamic weather?

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  • Vibora changed the title to [FIXED] Altimeter does not take temperature into account
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