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Discrepancy in altitude / QFE / QNH settings


earnil

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Hello!

I have noticed a ... let's call it a glitch ... and I'm not really sure if I'm just doing something wrong or this is actual problem.

 

Situation - Mission 3 of Red Flag Campaign - starting position (this is just an example, I have noticed similar behavior in other settings as well)

 

On F10 map, the altitude of Nelis AFB is noted as 561m / 1840ft

QNH as per mission editor is 1012 hPa / 760 mmHg

Pressure setting on main altimeter (which shows 0 altitude) is 955 hPa

Pressure setting on backup altimeter - 1012 hPa and it shows altitude 480m

 

I did a cross check with some online tools that can calculate QNH / QFE / h like this one:

http://www.hochwarth.com/misc/AviationCalculator.html#QNHQFE

 

Altitude of 561m with QNH 1012 hPa is consistent with QFE of 946 hPa (not 955 that is shown on main altimeter)

QFE 955 hPa is consistent with altitude of 480m as shown on backup altimeter rather then in game 'true' altitude of 561m of Nelis AFB.

 

I'm not sure if my explanation makes sense so I'm attaching a picture as well (hopefully worth 1000 words)

 

Same issue happens with Caucasus map as well and the error appears to grow with altitude, i.e. if the plane spawns at sea level, there is full consistency, higher the altitude more the setting seem to differ.

 

I'm taking my chances and reporting this as a bug, apologies if this is just me not understanding something properly.

696019909_QHNQFEaltitudediscrepancy.thumb.PNG.9cfe4574fec12beab21e78997fa9ee55.PNG

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Ok, thanks a lot! I guess that's it.

 

Question specifically for Viggen though. How does this impact deployment of weapons that rely on correct QFE settings?

 

For example, in this particular case, does the plane 'know' it's in fact at altitude 561m and will it calculate the impact point correctly?

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QNH as per mission editor is 1012 hPa / 760 mmHg

First, this is not correct. If it is ME that got it wrong or you I can't tell. But:

1012 hPa = 759 mmHg

760 mmHg = 1013.25 hPa

Enough of nitpicking...

In real life QNH, as used by the ATC, is not the same as you get from/in converters like the one you linked to. The one you get from ATC and in ATIS is a QNH that will provide you with a safety margin vertically to geographical features and structures (including man made) in a preset radius from the airfield, and for a set time-frame, usually valid for around 30 minutes.

If you get a QNH from a nearby airfield, set the altimeter to it and approaching a high radio tower, say 150m high above ground and say 711m above sea level (as if it was standing on 561m elevation). If you then level yourself at altimeter reading 771m exactly and fly over the tower, you will notice you're not actually at 771m but quite much higher, the vertical safety margin.

I don't know for sure if this is implemented in DCS, but your example point in that direction. Then using the given QNH in a calculator like you did will not give the results you expect. The backup altimeter shows 480m not because something is wrong, but because due to the safety margin if it had been showing 561m you would have been flying 81m above the airfield.

Hope this helps.

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1012 hPa = 759 mmHg

760 mmHg = 1013.25 hPa

 

I know but that's what it shows (see screenshot).

 

In real life QNH, as used by the ATC, is not the same as you get from/in converters like the one you linked to.

 

Well, I don't think it's exactly like that either. Airport QNH is based on pressure reading at that airport (QNH is calculated from QFE) and therefore those converters would work (I actually think that by definition QNH has to show airport's true altitude above MSL).

 

What you probably mean is regional QNH which is the lowest QNH from certain region.

511711591_QNHHG.thumb.PNG.7ffbb1ea3fe1cf7581bae0c3c84bd11b.PNG

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I know but that's what it shows (see screenshot).

 

 

 

Well, I don't think it's exactly like that either. Airport QNH is based on pressure reading at that airport (QNH is calculated from QFE) and therefore those converters would work (I actually think that by definition QNH has to show airport's true altitude above MSL).

 

What you probably mean is regional QNH which is the lowest QNH from certain region.

 

The briefing in the screenshot say 760/29.92 which equals 1013.25 hPa, so for some reason the backup altimeter get the wrong setting.

 

Yes indeed I mean regional QNH, and that's what is used at airports, not the local one. It's actually not only the regional one either, but one that has been corrected for forecasted pressure changes during a set time-frame, usually 30min. That is what I have been told during my CPL training a few years ago anyhow.

Helicopters and Viggen

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Discrepancy in altitude / QFE / QNH settings

 

Yes indeed I mean regional QNH, and that's what is used at airports, not the local one.

 

 

 

That’s interesting. I checked our AIP and it’s quite specific that in controlled airspace you need to use local QNH and regional only outside controlled space (well as you can imagine it’s not that straightforward but that’s the gist:-)

Probably must be defined differently for your country. Didn’t realize that even things like that differ from country to country.

 

edit: grammar

 

Sent from my iPad using Tapatalk

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I checked our AIP and it’s quite specific that in controlled airspace you need to use local QNH and regional only outside controlled space

I must agree it's quite interesting. I tried to find out in our AIP (Sweden) but could not find anything specific about this, only referring to QNH, no regional/local definitions. They referred to ICAO DOC 8168 and 4444, but I could not find anything there either. Maybe it's too late for this kind of brain activity (3am, can't sleep).

Where are you from and in what AIP section did you find your information? Can you provide a PDF?

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I must agree it's quite interesting. I tried to find out in our AIP (Sweden) but could not find anything specific about this, only referring to QNH, no regional/local definitions. They referred to ICAO DOC 8168 and 4444, but I could not find anything there either. Maybe it's too late for this kind of brain activity (3am, can't sleep).

Where are you from and in what AIP section did you find your information? Can you provide a PDF?

 

 

 

Here it is!

https://lis.rlp.cz/vfrmanual/actual/enr_3_en.html

 

Section 3.4.1

 

Also, I just noticed it’s no longer called AIP but rather VFR manual (but it’s apparently same thing). Just to be completely transparent, my knowledge about this comes from the fact that at one point I was doing a sail plane license which I couldn’t finish due to work commitments (I completed the training but never went through the examination) so I’m by no means expert and i’m well out of the loop so I don’t claim any particular expertise on this. This is just based on what I remember from my training and what I was able to check in the relevant documents so it may be possible that I’m misremembering / misinterpreting this.

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Good morning,

in Germany the actual local QNH is used for altimeter setting at an aerodrome and a so called Area QNH is used for enroute flights (i.e. IFR Flights or VFR cross country). These Area QNH values are calculated with a certain safety margin as mentioned above.

The difference in altimeter reading on ground, even with correct QNH values, maybe the result of the difference between the published Field Elevation (measured at the Aerodrome Reference Point) and the elevation at the present parking spot. These differences could be quite significant.

I don‘t own the Viggen nor the Red Flag Campaign, so I‘m not able to check this item myself.

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I must admit I'm starting to question my own statement about QNH above. European countries in general should have similar rules, even if differences can exist. Either the information I got during training was wrong (teachers was former students), I interpreted correct info wrong or I just have bad memory. I have contacted our CAA for a clarification and awaiting answers.

Anyhow, WhiskyDelta is correct about elevation difference between Airdrome Reference Point and parking, even if 71m/233ft (81m at altimeter setting 1012 corrected to 1013.25) sounds pretty much, especially for Nelis.

Could it be that ED for simplicity went for regional QNH consistently even at airfields? Most of the time playing we are airborne away from any airfield anyhow.

 

Edit:

The CAA got back to me and yes, I was wrong. We do have different QNH for airfields and outside. My apologies.


Edited by Holton181

Helicopters and Viggen

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even if 71m/233ft (81m at altimeter setting 1012 corrected to 1013.25) sounds pretty much, especially for Nelis.

 

I actually checked this before submitting here and the altitude differences for Nelis are within couple of meters so this shouldn't be a case.

 

It appears to be caused by a (probably DCS wide) bug that Frederf mentioned, which affects (if I understand it correctly) the way how altimeter displays the QNH in cockpit

 

https://forums.eagle.ru/showthread.php?t=201641

 

My followup question was basically whether this can have specific impact on Viggen since it needs correct pressure setting to calculate the release point for certain weapons (ARAK, AKAN, the BOMBS:)(I don't think any other DCS aircraft uses it quite like that).

 

The question is, whether Viggen's computer 'knows' correct altitudes / pressures and just displays them incorrectly on backup altimeter or whether this bug also affects the release point calculations. Especially in a light of many people reporting issues with ARAK which relies heavily on correct QFE settings.

 

Although I did couple of tests yesterday and it seems to me that in fact, it's only about the display of altitude / QNH on backup altimeter. The QFE settings seem to be unaffected. I still don't know though.

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When it comes to Viggen specific stuff I'm out of ideas. I own it but basically only fly around with it sometimes, not really into airplanes.

Hopefully some of the devs can chime in, maybe they know more about the pressure handling in DCS too.

Helicopters and Viggen

DCS 1.5.7 and OpenBeta

Win7 Pro 64bit

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  • 3 years later...

Update:

 

Static weather, ISA+0, QNH 760mmHg.

BESLAN airfield

RWY 10

Elevation: 525m (according to F-10 map)

Altimeter showing ~520m when Kollsman window is set to 1013mbar.

Seems OK

525m ~1720ft

rule of thumb: 27ft per mbar pressure (in lower pressure areas) equals 63,7mbar

1013-63mbar = 950mbar

Altimeter showing 951mbar, which is 1 mbar more than required, but satisfactorily

 

BTW: Standby altimeter still not showing the same values as the main altimeter.

Deviations from ISA indeed HAVE influence on the standby altimeter. See attached TRK files (ISA+0, and ISA+10)
 

2021-03-19_Viggen_ISA_plus_0_Altitude_Check.trk 2021-03-19_Viggen_ISA_plus_10_Altitude_Check.trk


Edited by TOViper
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