[FIXED] Mission Editor, QNH and Altimeter Indication

Hey guys,

I've been scratching my head about these problem for several hours now, maybe one of you with better DCS experience can help me:

I create a Mission within the Mission Editor, for example on the Nevada map.

The weather settings are:

Temperature 15°C

Wind calm

QNH 29.92 inHG

These settings are equal to the International Standard Atmosphere (ISA).

Now, starting the mission, I jump into the cockpit of, let's say, the A10-C which I parked at Nellis AFB.

I tune my altimeter to QNH 29.92 and it shows me an altitude of round about 1700ft.

Why is this the case? Shouldn't it show Nellis elevation which is around 1850ft?

On the map, i can click on "myself" and it shows an altitude of 1850ft.

Why is there a difference on the altimeter indication?

I suspect it has something to to with the temperature, because changing that changes the indication on the altimeter as well.

Can someone provide me with better information or tell me what I am missing here?

Appreciate your help.

Not that I'm going to be any help to you, GIVME5W, but I thought I can at least let you know that you're not alone. I'm having the same problem. Quite regularly I get different wind directions, wind speeds, and barometric pressures from the ones I set in the mission editor.

I know that weather is basically unpredictable, but that's why we have a simulator, right? :smilewink:

Why is this the case? Shouldn't it show Nellis elevation which is around 1850ft?

On the map, i can click on "myself" and it shows an altitude of 1850ft.

Why is there a difference on the altimeter indication?

I suspect it has something to to with the temperature, because changing that changes the indication on the altimeter as well.

Can someone provide me with better information or tell me what I am missing here?

 

Appreciate your help.

https://forums.eagle.ru/showthread.php?t=76273

Also it`s easy to get confused between QNH and QFE. Divide the airfield altitude in feet by 30 to get the number of millibars above MSL. Add this to the QFE to get QNH or subtract it from QNH to get QFE.

Thanks for your reply rick66.

However, it's not a case of confusion over which Q-code is what.

Staying with Nellis as an example (which means we're talking US units): In the mission editor you can set QNH (default is 29.92). Let's say I leave it at 2992. I jump into the cockpit and my altimeter (set to 2992) shows my altitude at Nellis AFB to be 1600 ft. As GIVME5W pointed out, it should read 1850 ft.

That's what's confusing.

Thanks for your reply rick66.

 

I jump into the cockpit and my altimeter (set to 2992) shows my altitude at Nellis AFB to be 1600 ft. As GIVME5W pointed out, it should read 1850 ft.

That's what's confusing.

Yup i hear you mate, i just tried it myself and the levels are erroneous to say the least. Yeah not sure whats going on, i had to screw my QNH up to 3002 to get QFE of 1850, and the Area QNH was set at 29.95

i`ll let the experts deal with this, sorry i can`t help any more!

There is a difference between the PNEU and ELECT readings:

20° SLT

P- 1630'

E- 1660'

15° SLT

P- 1660'

E- 1695'

Booting up the CADC it shows P ALT 1694.8 B ALT 1694.9, OAT 11.33 (15 minus some temp due to altitude).

Fun fact: If you type "29.9213" into the pressure window you get a different pressure on the briefing 760 vice 759 mmHg. So be aware you can type in more exact XX.XX pressure numbers than the display can show.

In fact the coldest I can make it is -1.3 SL (-4.97 local) which gives a P ALT of 1798.3' which is still well short of the 1849' true.

This has all been the static weather. However when I make it dynamic, 1 system at 0 Pa offset I should get a uniform datum atmosphere.

This gives (15 degrees)

P ALT 1957.1'

B ALT 1857.2'

Elect 1825'

That's much closer. For giggles trying 20 temp (dynamic 0 Pa):

P ALT 1825.4'

B ALT 1825.5'

Elect 1795'

OAT 16.33

Too far down. Maybe it's been calibrated for 15C local instead of 15C SL. Taking 1C off the temp:

P ALT 1831.7'

B ALT 1831.7'

OAT 15.33

What's 18C look like?

B & P ALT 1838.0'

OAT 14.33

16C?

P ALT 1850.7

B ALT 1850.8

OAT 12.33

The actual terrain is 1842' (the airplane F2 is 1849') so best match of P ALT to actual alt comes in the 16-18C SLT range. This requires the dynamic weather mode to be in use. The static weather is way off (~150').

I don't know why 760mmHg static weather is different than a 0 Pascal dynamic weather of the same temperature. The reported local temp is the same in bost cases (11.33C) so it's not a temperature issue. The pressure is just plain different between static and dynamic.

Maybe the "datum" height for the NTTR map isn't at 0m while it is for Caucuses and this is the source of the trouble? What is the minimum terrain height in NTTR?

I'm seeing this too. Setting QNH of 29.92 in ME, but require a vastly different altimeter setting to get an accurate altitude reading.

EDIT: This is happening on Normandy in the AV8B for me. I haven't tested anywhere else.

Sorry guys,

that was my first thread in this forum and I didn't know that you have to subscribe to your own thread to get notifications about answers.

Thank you very much for all of your replies.

Unfortunately I didn't find a solution so far. Further tests confirmed the theory of Frederf. Changing the temperature in the Mission Editor changes the altitude indication on your altimeter.

But what confuses me even more, is the fact that switching to the Map and clicking on your own aircraft as well as switching to the F2-view shows correct altitudes.

So I don't know, maybe it is just a bug on the altimeter indication...

It's normal and correct (real) behavior for altimeter to be different than true if temperature is not standard and you are not at the reference altitude.

QNH is defined in general only for a specific location, usually an airport and is corrected for pressure and temperature at that location only. The relationship between pressure and altitude can be represented as a graphed line. This line connects pressures to altitudes. Pressure offsets are translations of this line without change of slope. Temperature differences from standard are represented as different slopes of line. The adjustable altimeter would ideally have a pressure offset knob and a temperature offset knob for complete calibration. However they only have one, pressure offset, and this adjustment must be used to compensate for both factors.

If the real pressure-altitude relationship and standard instrument pressure-altitude relationship have the same slope (temperature) then it is possible to translate the instrument line to exactly match the real line and the instrument will be calibrated for all altitudes. However if the slopes (temperatures) do not match then it is only possible to make the two lines intersect at a single point. The QNH setting is the sliding of the instrument line such that the crossing point is the altitude of the reference airfield. If the real weather is colder than standard then all altitudes above the airfield will show too high and all altitudes below the airfield will show too low. The effect is proportional to the altitude difference from the reference one (distance on the graph from the intersection) and the temperature offset (difference in slopes).

An altimeter setting which is pressure and temperature corrected for 0 MSL has a distinct name "QFF." An airport at 0 MSL would have a QNH equal to QFF. Using QFF at an airport not at sea level would be pressure corrected but not temperature corrected. QNH shows correct altitude at airfield. QFF shows correct altitude at sea level. This is only possible for QNH = QFF if airfield elevation is 0 or temperature is standard. So what is mentioned in DCS briefing is not QNH, it is QFF! You will notice that for non-standard temperature that briefing "QNH" is identical regardless of airfield where mission starts. In reality this is not true!

But that is not the real problem. DCS has a math error with altimetry. For standard day 15°C SLT 29.92 SLP should be correct everywhere with altimeter set at 29.92 but it doesn't!

At SL A-10 reports 15.00°C at Sukumi (33') and 11.47°C at Beslan (1772'). This is all perfectly normal and correct standard temperature profile for these elevations. Textbook standard would be (11.4754°C) for Beslan's elevation. This difference is -3.53°C. Remember this number.

Altimeter reads too low meaning this is "too hot" for our QNH value of 29.92''. So let's find out what sea-level temperature will make 29.92'' give QNH of 29.92 at Beslan. Turns out it is -7.94°C.

Funny number, right? Why does -7.94°C make the QNH/QFF correct for Beslan? Try taking standard SLT of 15°, add -3.53°C, then multiply by -1. We get 11.47°C. That's the actual temperature at Beslan in our mission. OK, now do that again. (11.47 - 3.53) * (-1). -7.94°C! The operation is being done twice on the sea level temperature to find out what is the local temperature that's correct for that QNH! It should only be done once! It's a simple math/programming mistake!

Appreciate your work Frederf, this could be the answer.

Now... How to convince ED to recheck their code?

Funny number, right? Why does -7.94°C make the QNH/QFF correct for Beslan? Try taking standard SLT of 15°, add -3.53°C, then multiply by -1. We get 11.47°C. That's the actual temperature at Beslan in our mission. OK, now do that again. (11.47 - 3.53) * (-1). -7.94°C! The operation is being done twice on the sea level temperature to find out what is the local temperature that's correct for that QNH! It should only be done once! It's a simple math/programming mistake!

Thanks Frederf for noticing this and writing it down for us.

Weather is really important for me in DCS, good find! :thumbup:

Just checked it in todays OB. Seems to be fixed. Screenshot is Nellis AFB in A-10C with a ambient temp of 15° (as OP wrote).

I'm sorry, I have to dig out that thread again, but I still seem to miss something (or again...):

Spawning at Nellis AFB (Elevation roughly 1850 ft):

[TABLE]Mission Editor QNH: 29.92 Temperature: 15°C F2-View Altitude: 1850 ft Altitude Indicator (when set on QNH value): 1850 ft

Mission Editor QNH: 28.92 Temperature: 15°C F2-View Altitude: 1850 ft Altitude Indicator (when set on QNH value): 1850 ft

Mission Editor QNH: 30.92 Temperature: 15°C F2-View Altitude: 1850 ft Altitude Indicator (when set on QNH value): 1850 ft[/TABLE]==> Changing the QNH within the Mission Editor and then setting it on the altitude indicator within an aircraft shows the same values (which is what I expect!).

However:

[TABLE]Mission Editor QNH: 29.92 Temperature: 15°C F2-View Altitude: 1850 ft Altitude Indicator (when set on QNH value): 1850 ft

Mission Editor QNH: 29.92 Temperature: 0°C F2-View Altitude: 1850 ft Altitude Indicator (when set on QNH value): 1940 ft

Mission Editor QNH: 29.92 Temperature: 30°C F2-View Altitude: 1850 ft Altitude Indicator (when set on QNH value): 1740 ft[/TABLE]==> Changing the temperature but keeping the QNH on the same value gives me different indications within the aircraft while the F2-View altitude stays the same all the time. Why do these values differ? Shouldn't they be the same, because the QNH is the same in these examples? F2-View seems to give the correct values, but the aircraft altimeter does not...?

:dunno:

This is working as advertised. Temperature affects instrument indicated altitude as all altimeters are calibrated to display accurate altitude under ISA (International Standard Atmosphere) conditions.

If the temp is higher than standard the altimeter will under read, of colder, it will over read.

https://www.ifalpa.org/media/2012/15atsbl02-cold-temperature-corrections.pdf

It is odd that at 15 deg the altimeter reads correctly though. Standard temp is around 13 deg at that altitude for accurate altimeter indication. Is the temp setting as you have it in your test at sea level?