ATC Baro Pressure setting not possible??

Requesting T/O at various airfields the ATC gives me a pressure setting way below the minimum....like 27.XX. I can't dial it down that low and the altimeter is reading about 2,500 ft

Is this a bug or am I just supposed remember that the airfield is artificially set at 2,500 when I land? This could be problematic at night in sh#tty weather.

Please advise, I prefer this not be my final post :cry:

Its not a bug. DCS ATC reports altimeter setting in QFE. You are looking for a QNH setting.

This thread covered it in depth , I am sure there have been others.

ATC in DCS (for now) only gives you QFE, which might not work unless you're doing a round-robin flight from/to the same field. Here's hope that DCS's ATC overhaul will be much more comprehensive to consider QNH as well.

Here's a nice quick reference between the various types of alitmeter settings:

(sniped).

Everytime i forget it couple of days after reading it but in the a10 there is something about messing with the altimeter before you take off. You shouldnt mess with the altimeter setting cos it screws something in the weapon trajectory calculations or something like that.

@tom_19d...thank you for pointing me to that thread, which after 7 pages and a bottle of wine made exactly no sense whatsoever. However it did clear up the fact my plane is not broke and I have some additional reading and research to do. I now have another little project for the weekend ahead; re-reading the thread and grapsing new concepts.

Thanks & Cheers

AveWhiteGuy -- I can try to sum up.

This is strictly a rule of thumb type calculation, but it can be done in your head, and gets you close enough for about everything we do in the A10...at least until DCS ATC starts issuing QNH some day, which would be great.

It is generally considered that pressure decreases 1"/mg (i'm sure you know but the altimeter setting put in the kollsman window of the altimeter is measured in inches of mercury) per 1000 feet of altitude. Lets assume we set up a mission editor with the plane near Vaziani and leave standard QNH of 29.92.

So you start airborne and head towards Vaziani, field elevation ~1520 MSL. You call Vaziani to land, they issue QFE of 28.34. Divide field elevation by 1000. Moving the decimal leaves you at 1.52. Add this correction factor (1.52) to the issued QFE (28.34) to reach 29.86. Put that in your kollsman window and land.

Here we see why it is a rule of thumb - 29.86 is what we have set. 29.92 is correct. That 0.06 difference in in/mg will present as your altimeter being 60 feet off. It isn't perfect, but barring an ILS at absolute mins it is close enough, and unlike other methods it can be done mentally. It is how I deal with it until we get an ATC update...

Requesting T/O at various airfields the ATC gives me a pressure setting way below the minimum....like 27.XX. I can't dial it down that low and the altimeter is reading about 2,500 ft

This should answer most of your questions:

Here's a nice quick reference between the various types of alitmeter settings:

 

It's a nice illustration, and probably helps to get the idea across, but to the best of my knowledge it's also wrong on several accounts, or at least partly incorrect.

QFE and QNH are actually pressure settings (like 27.95 InHG or 30.08 InHG or whatever). They are *not* altitudes or heights (like 500 ft or 7500 ft or whatever). In fact, the solid line "Sea Level Reference Datum" should be labeled "QNH", and the solid line "Runway" should be labeled "QFE".

QNE, on the other hand, *is* an altitude (like 2560 ft) and is not a pressure setting (like 29.92 InHG). Simply put, QNE is the indicated altitude at standard pressure setting (29.92 InHG). An arrow between the dashed "29.92 Datum" and "Runway" should be labeled QNE.

Yeah, I know, this is not making things any easier, but it is a tiny little bit of a complicated topic.

The above video has it entirely right, to the best of my knowledge. And while it does take almost 20 minutes, I think it does an amazing job of explaining things. :thumbup:

Everytime i forget it couple of days after reading it but in the a10 there is something about messing with the altimeter before you take off. You shouldnt mess with the altimeter setting cos it screws something in the weapon trajectory calculations or something like that.

In reality setting QNH prior to take off is required in order to provide an initial calibration to the IFFCC (which happens at weight off wheels).

However this, along with all the IFFCC altitude source option, are not modelled in DCS. Setting/using QNH is still best practice however for numerous reasons.

@tom_19d....I'm getting the concept but still a little confused about how to set my altimeter to 0.

I'm at the apron Nalchik (Alt:1410) with my altimeter reading 2500ft and Kollsman window reading 29.92 after startup. ATC gives me a QFE 27.33. Doing the rule of thumb calc (1410/1000) + 27.33 =28.74 which dial into Kollsman. Now my Altimeter reads +/-1400....the alt at Nalchik.

So if I take off....fly around and then attempt to land on instraments alone, I run into the ground because I'm 'chasing' 0 on my Altimeter. Where am I getting this wrong?

@Yurgon....I did watch this last night after reading the whole thread and that bottle of wine. Somehow this did make a lot of sense and plan on watching again during the weekend. This is a classic :) I'm thinking of grasping this and produce a more modern video targeted for DCS guys like me.

It just seemed strange to be on the deck at the Nalchik apron and not being able to dial the correct setting to show me at Alt:0 being given by ATC. I've noticed this a lot on NTTR as well.

Time to hit the books again...and thanks to both of you for the explanations

@tom_19d....I'm getting the concept but still a little confused about how to set my altimeter to 0.

You don't!

I run into the ground because I'm 'chasing' 0 on my Altimeter. Where am I getting this wrong?

You're getting it wrong by even thinking of the altimeter reading zero and chasing it in the first place. If you want to confirm height, reference the RADAR altimeter (but don't rely on it). If you can see out of the window, that should be your primary reference. If you can't due to weather/night/both, then you're flying on instruments and they're only their to get you into a position where you can see the runway in order to land.

Instrument flying is done with reference to altitude above mean sea level (MSL), this is what all altitudes marked on instrument charts are with reference to. Set QNH (so your altimeter reads field elevation), and then just get used to it, you need to change you way of building your situational awareness and remember your airfield's elevation above MSL.

QFE works well in some limited situations, such as when you're only flying circuits around an airfield, but that's it. Having your altimeter telling you you're 200 feet above the airfield elevation (which is what you're looking for), won't be much good when you're flying over a hill which is 250 feet above your airfield, or when you're 50 miles away from your airfield.

In reality setting QNH prior to take off is required in order to provide an initial calibration to the IFFCC (which happens at weight off wheels).

 

However this, along with all the IFFCC altitude source option, are not modelled in DCS. Setting/using QNH is still best practice however for numerous reasons.

And here it is, thanks for reminding me what it was eddie. Hope i can keep it in my head this time.

Eddie said it all - when you take off you want the altimeter showing airport elevation and when you return you want the same thing.

The reason you can't get zero to show on the ramp (not that you want to) in some situations is because western aircraft aren't designed to use QFE so mechanically they aren't able roll down that low. The fact that you were able to apply the 1 in/mg to 1000 feet rule and, using the ATC QFE setting, get your altimeter to show field elevation proves that the method works, at least well enough. However, if you are just starting on the ramp, what is actually easiest would be to adjust your kollsman window until the altimeter shows field elevation - then you have QNH. And someone can correct me if I am wrong, but I believe in the DCS world, unless you have dynamic weather on, QNH should be a constant across the map (so you only need to get it once...and it is usually available in the briefing).

In reality setting QNH prior to take off is required in order to provide an initial calibration to the IFFCC (which happens at weight off wheels).

 

However this, along with all the IFFCC altitude source option, are not modelled in DCS. Setting/using QNH is still best practice however for numerous reasons.

Thanks for this Eddie. I have seen this explained in 1A-10A-1 but heard in other threads it wasn't fully implemented. Glad to have this confirmed by a SME.

... if I take off....fly around and then attempt to land on instruments alone, I run into the ground because I'm 'chasing' 0 on my Altimeter. Where am I getting this wrong?

DCS ATC will give QFE (height above the runway) = 0 ft on the runway, so use ATC's value and check/adjust your altimeter to read zero as part of a final take off check.

It's not possible to set DCS ATC's QFE pressure at all airfields i.e. Nevada, as the airfield may be above/below the instruments range of adjustment.

QFE is useful if doing circuits at a 'local' airfield but AFAIK isn't typically used in GA.

A more common reference is QNH (Altitude above mean sea level) i.e. 1411 ft (430 m) @ Nalchik in DCS

Here is part of the real chart for Nalchick (RL height 420 m above MSL):

as can be seen the Decision Altitudes (heights) are given with respect to MSL (QNH),

i.e. for a Category C aircraft approach (Speed >= 121 and less than 141 knots ?) the ILS decision height is 476 m above MSL (56 m above the runway threshold).

Using QNH during takeoff, you would check/adjust your altimeter to read the correct airfield altitude as part of a final take off check.

I'm at the apron Nalchik (Alt:1410) with my altimeter reading 2500ft and Kollsman window reading 29.92 after startup. ATC gives me a QFE 27.33. Doing the rule of thumb calc (1410/1000) + 27.33 =28.74 which dial into Kollsman. Now my Altimeter reads +/-1400....the alt at Nalchik.

You'd use the rule of thumb to convert DCS's ATC QFE, perhaps it isn't usable because you are in Nevada, to QNH which is.

Nalchik ILS Chart: https://opennav.com/pdf/URMN/ad2-rus-urmn-097.pdf

The briefing automatically gives QNH, IIRC (with static weather).

The briefing automatically gives QNH, IIRC (with static weather).

Post 12 this thread

...QNH should be a constant across the map (so you only need to get it once...and it is usually available in the briefing).

Like I said...

It is generally considered that pressure decreases 1"/mg (i'm sure you know but the altimeter setting put in the kollsman window of the altimeter is measured in inches of mercury) per 1000 feet of altitude. Lets assume we set up a mission editor with the plane near Vaziani and leave standard QNH of 29.92.

 

So you start airborne and head towards Vaziani, field elevation ~1520 MSL. You call Vaziani to land, they issue QFE of 28.34. Divide field elevation by 1000. Moving the decimal leaves you at 1.52. Add this correction factor (1.52) to the issued QFE (28.34) to reach 29.86. Put that in your kollsman window and land.

 

Here we see why it is a rule of thumb - 29.86 is what we have set. 29.92 is correct. That 0.06 difference in in/mg will present as your altimeter being 60 feet off. It isn't perfect, but barring an ILS at absolute mins it is close enough, and unlike other methods it can be done mentally. It is how I deal with it until we get an ATC update...

Interesting method. Is there an analogue quick rule for calculations in metric units (meters and mmHg)?

I’m sorry nessuno, I do not know of anything off the top of my head. You could surely work out a proportion of mmMg over meters, but I am not sure if it would be quite as handy as the 1 inch per 1000 foot rule. Hopefully someone around has an answer!

The briefing automatically gives QNH, IIRC (with static weather).

Strictly speaking QNH on the briefing is QFF, not QNH. QNH is defined at each specific location which includes elevation. Two airports at different elevations will have different QNHs even if the weather is completely uniform.

This is why if you set QNH per the briefing at Beslan it might not read the airport elevation but if you go to a sea level airport and set QNH equal to the briefing it will. The magnitude of the error is a result of temperature with standard temperature profile producing zero error.

Similarly QFE only ensures that it reads 0 at the airport. When set QFE and altimeter says 10,000 it doesn't mean you're actually 10,000 above the airport. You're close but it's not exact.

Try it for yourself. Set 0C or 30C in the editor and try Batumi or Beslan QNH, QFE and fly high above with each setting and check your "F2" height.

Strictly speaking QNH on the briefing is QFF, not QNH. QNH is defined at each specific location which includes elevation. Two airports at different elevations will have different QNHs even if the weather is completely....

Fair enough, but until we get an ATC that issues an altimeter setting for each airport, I don't know of a way to compensate (or at least a way that makes any sense to pursue in a cost v benefit sense). How can this knowledge be applied to the OPs issue?

Right now the most accurate way to get QNH is to be on the airport surface and change the pressure knob to whatever is needed to make it display the known airport elevation.

When not at the airport the static QFF is suitable if the airport is near sea level or the sea level temperature is near 15C. But some missions use dynamic weather in which case the briefing number means nothing. You can always, in the A-10C, look up the geometric INS height and calibrate to that the closer to the destination elevation the better.

I used to have a chart that laid all of this out very nicely back in my days of running altimeters and air data computers against an ancient Hass A-1 mercury barometer. It was an important part of doing a daily accuracy check on the barometer. I'll see if I can dig it up again.

[edit] I was hoping I would be able to find a digital representation of the old Hass chart on the internet, but it looks like the paper copy I had back in the days before everything went digital was all that remained, and I lost track of that years ago. I think it had been out of print since the early 60s, so even my copy 20 years ago was in really rough condition. Not sure if its been posted already, but this site seems to work pretty well for getting your baro setting from QFE with a known field elevation.

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

Right now the most accurate way to get QNH is to be on the airport surface and change the pressure knob to whatever is needed to make it display the known airport elevation.

Post number 12 of this thread...

However, if you are just starting on the ramp, what is actually easiest would be to adjust your kollsman window until the altimeter shows field elevation - then you have QNH.

Eagle I ran that conversion on the link against the example I laid out in post #6. Your website was indeed 30 feet closer to correct then my rule of thumb conversion. I am going to look at that site more and try to see how they source their conversions because now I am interested haha.

Frederf, I hadn’t thought of using an EGI derived height, that could be interesting. My memory is that pre-WAAS GPS did a poor job of determining height/altitude but I admittedly don’t have any idea how the A10 handled it or perhaps more importantly how DCS models it, but I think it is worth exploring.

Regardless, as it relates to the OPs question and bearing in mind that civil aircraft only have to have an altimeter that reads +/- 75 feet from a known elevation to be legal for instrument flight, until DCS implements something different I will stick with a method that I can mostly do in my head and only takes a working comm radio to implement...and least until a lot more missions require ILS approaches right to minimums.

DCS ATC will give QFE (height above the runway) = 0 ft on the runway, so use ATC's value and check/adjust your altimeter to read zero as part of a final take off check.

 

It's not possible to set DCS ATC's QFE pressure at all airfields i.e. Nevada, as the airfield may be above/below the instruments range of adjustment.

 

QFE is useful if doing circuits at a 'local' airfield but AFAIK isn't typically used in GA.

 

A more common reference is QNH (Altitude above mean sea level) i.e. 1411 ft (430 m) @ Nalchik in DCS

 

Here is part of the real chart for Nalchick (RL height 420 m above MSL):

 

 

as can be seen the Decision Altitudes (heights) are given with respect to MSL (QNH),

 

i.e. for a Category C aircraft approach (Speed >= 121 and less than 141 knots ?) the ILS decision height is 476 m above MSL (56 m above the runway threshold).

 

Using QNH during takeoff, you would check/adjust your altimeter to read the correct airfield altitude as part of a final take off check.

 

 

 

You'd use the rule of thumb to convert DCS's ATC QFE, perhaps it isn't usable because you are in Nevada, to QNH which is.

 

Nalchik ILS Chart: https://opennav.com/pdf/URMN/ad2-rus-urmn-097.pdf

He only time I’ve ever used QFE in real life was in a seaplane. Other than that the western world usedps QNH. I have no experience with what Russia does, but I have a suspicion based off the coding.

Frederf, I hadn’t thought of using an EGI derived height, that could be interesting. My memory is that pre-WAAS GPS did a poor job of determining height/altitude but I admittedly don’t have any idea how the A10 handled it or perhaps more importantly how DCS models it, but I think it is worth exploring.

Tom, I missed this post before. That's similar to what I do when I'm going to recertify an air data computer that can be calibrated in the field. Before I even connect any equipment to the aircraft, I use an altimeter app on my phone to get GPS altitude at that particular location, and then I call AWOS (which isn't available where I am anymore) or listen to ATIS to get my baro setting. If I dial that baro setting into the ADC and its more than 20 feet off from the GPS altitude, I usually just run through the calibration procedure prior to running everything in the aircraft. It takes out an extra step that ends up being just wasted time if the ADC is out of cal. To get to my point though, I still need current baro from ATIS to do the check. If they gave QFE like DCS does, it wouldn't do me much good. I'd definitely trust your rule of thumb method for figuring that out over just setting the ADC to match the GPS altitude. If you use rule of thumb and your instruments are way off of what they should be, you've got issues with your air data system. Heh, I'm sure DCS doesn't model things like an aircraft that got washed or rained on and got water in the static system, out of cal ADCs, or a bug in the pitot tube though.

I know you know all this stuff already, just wanted to throw out to the community the dangers of setting air data systems to make them match your EGI derived height, or based on the known field elevation of the airport. We both know that wouldn't be a valid method in real life. :D

Got the L-39ZA done today. I know more about that aircraft now than I ever wanted to know. Not particularly difficult to work on, its a really well-built aircraft. Just had to do a lot of exploratory surgery to get the new RG-142 cables run for the WAAS and transponder antennas.