Altitude - What am I missing?

[Pizzicato]

Hey guys,

 

I'm clearly misunderstanding something about how barometric altitude is displayed in the Hornet. I built a simple mission which included a trigger that checked for the player reaching 10,000ft. When the trigger failed to trigger despite the HUD reporting 10,000ft exactly, I looked in the Mission Editor and saw that it was reporting my altitude as ~9,400ft.

Sure enough, I climbed another 600ft to 10,600ft and the trigger fired.

My guess is that this is something to do with air pressure / QNH, but the Mission Editor (using Static Weather) just uses the default of 29.92 inHg.

Can anyone educate me on this, and help me figure out how to set the mission and/or aircraft up more correctly, please?

[RaisedByWolves]

Not sure. But maybe adjust the altimeter to field elevation at start?

[Razor18]

Or maybe to QNH.

[Sharkku]

1 hour ago, RaisedByWolves said:

Not sure. But maybe adjust the altimeter to field elevation at start?

Well, that would work if the field elevation was 600 ft, sure. But the trigger shouldn't check for radar altitude or QFE, it should check absolute altitude. And you shouldn't have to set the altimeter to field elevation anyway, if you set it to correct local atmospheric pressure, it should give you the correct altitude.

 

1 hour ago, Razor18 said:

Or maybe to QNH.

The original post states the weather used was standard atmosphere, 29.92 inHg, which is also the QNH setting. Which is set by default in the Hornet.

 

 

Something else is going on here. Please post your mission and a track.

Edited December 6, 2020 by Sharkku

[Frederf]

QNH is the altimeter setting which produces local airfield elevation reading when at and on that airfield. Different airfields will have different QNHs. This is not exactly the same as the altimeter setting which produces 0 when at sea level, which is QFF which is what DCS calls QNH.

 

It's perfectly normal for barometric altimeters to be wrong. A properly QNH-set barometric altimeter is accurate at and only at the elevation that QNH is for. Generally temperature is the biggest culprit, the more actual temperature is different from standard the quicker the altimeter will be different from true as one leaves the calibrated level.

[Sharkku]

Yes, it must be the difference between the true altitude and the density altitude. Is it very warm in your mission? 

Edited December 6, 2020 by Sharkku

[zildac]

QNH is the altimeter setting which produces local airfield elevation reading when at and on that airfield. Different airfields will have different QNHs. This is not exactly the same as the altimeter setting which produces 0 when at sea level, which is QFF which is what DCS calls QNH.
 
It's perfectly normal for barometric altimeters to be wrong. A properly QNH-set barometric altimeter is accurate at and only at the elevation that QNH is for. Generally temperature is the biggest culprit, the more actual temperature is different from standard the quicker the altimeter will be different from true as one leaves the calibrated level.

That's QFE, no?

[Sharkku]

No, QFE setting causes your altimeter to read 0 when on the ground at the airport for which you have set QFE.

[maxTRX]

I just call it altimeter setting. In DCS, if I feel like playing with it on the local field I set it to whatever I get in the weather brief checking if it jibes with the field elevation. If I don't have the weather brief, I just turn the knob till the field elevation shows up. Then, when passing 18k it goes back to 29.92 Hg.  When doing A/G, it might be good to get the pressure for the specific area from JTAC's or whoever.

Edited December 6, 2020 by Gripes323

[Scaley]

DCS's logic functions (AI, mission editor, etc) all use "true" altitude.  That is, the vertical distance from sea level to the object in question, measured from the sim engine directly.  This is  a number that has no real-life equivalent in aviation because it's normally impossible to measure.  This is the altitude shown in external views in the bottom info bar.  It is also the altitude reported by DCS to external tools like TacView.

 

What you see in the aircraft depends on altimeter pressure setting, air pressure, air temp and anything else the aircraft module designers have modelled (so may or may not include instrument lag, compression effects, etc).  In addition, since you can only correct most aircraft altimeters for pressure and not temperate gradient, your displayed altitude in the cockpit and DCS' "true" altitude will diverge more at higher altitudes and when the air temp is further from ISA standard (15°C)

 

This is a known thing (bug?) and causes lots of odd effects such as AI tankers reporting slightly not-right altitudes, HUD altitudes not matching TacView, etc  

 

If you want to see the effects start at an airfield on a day with temperature far from 15°.  Set you altimeter to exactly match the F2 view reported altitude, then climb to high altitude and observe the difference between what's in the aircraft instruments and the F2 view.  Compare with Tacview and see what was reported during the climb.  

[Harker]

A lot of in-module functions in the F-18 also use the sim altitude at the moment. Things such as the barometric altitude floor warning, FPAS calculations, reporting altitude for radar contacts etc. All of these functions should use the altitude value, as provided by the ADC. The ADC itself takes the pressure setting into account. In the sim, all of these functions ignore ADC altitude and use the sim altitude instead.

Take a look here, especially towards the end.