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    Retired Pilot Tex's Avatar
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    Altimeter Nerd-ology

    Well, you know your a navigation geek when you wake up on a Saturday morning, find two threads discussing altimeter usage and you grab your coffee and AFPAM 11-216 in order to write up a few points...and your excited about it. Rather than two separate posts to respond to Dojo's discovery with Tacview and add some amplifying info related to Oliver's question on NTTR altimeter settings, I figured I start this which might make it easier to refer back to later...or ignore if you could care less about altimeters.

    First a primer on altitude. There are multiple kinds of altitudes:
    • Indicated - what is displayed on your barometric altimeter
    • Calibrated - indicated altitude corrected for installation error
    • Pressure - The distance above the standard datum plane. In a theoretical "standard" world, at 0 feet, the pressure is 29.92 inHg, temperature is 15 deg C, and both lapse at set rate (see pic below)
    • Density - Pressure altitude corrected for temperature...mostly relevant when computing aircraft performance
    • True - The actual distance above Mean Sea Level (MSL)...derived by correcting density altitude for variations in pressure (Picture the weather man briefing a low pressure system here, a high pressure system there)
    • Absolute - Also known as Above Ground Level (AGL)


    Now in the A-10, and most military aircraft, we have two altimeters...a barometric or pressure altimeter and a radar altimeter. The pressure altimeter has aneroid cells (think little balloons) inside that expand and contract as the pressure exerted on them changes. This same phenomenon is what causes your ears to pop when you fly or dive. Because they expand and contract a constant rate, they can be connected to a spinning needle on a scale (this is part you actually see in the cockpit). But, we need a way to adjust this scale so the numbers around the outside are accurate for where we are flying. This is done through the mechanical knob to set the number in the Kollsman window such as 29.92. We also have a radar altimeter which measures your AGL altitude. It uses a radar pulse to measure directly below the aircraft. With these basics down, lets jump to Dojo's observations with his SAPS effort...

    Quote Originally Posted by Dojo View Post
    ...Now, this was only half of the issue with how the the altimeter readout works in DCS, as altimeter error introduced from temperature/pressure differences at various altitude are *still* not fixed...

    ... reports true MSL, and sure enough, the only deviation currently observed is directly related to temp/pressure affects, and no longer lag...

    ...Perhaps I can work out the math to predict the correct altimeter setting for an anticipated altitude given the QNH and temp known from the departure location...
    Dojo, what you have discovered and are describing, if I understand correctly, is not a DCS bug but correct behavior of the pressure altimeter. As you change altitude in the aircraft, the required setting to have an accurate display of your true altitude will change. If I set my altimeter at 1800 feet (roughly Nellis AFB elevation) to read true altitude (we call this truing out our altimeter), once I climb to 10,000 feet there will be error. This is normal for any aircraft. And you are correct in your assumption that you can predict this effect with math. Someone much smarter than me figured this out years ago and they are called D values. While its possible to derive d values in-flight we normally get them from the weather shop. But in any case, I don't think we need them. We set our altimeter to what would be the local altimeter at Nellis (we are actually truing it out since there is no ATC). This setting is valid on the ground at Nellis (I'll come back to this in a moment as it relates to Oliver's question) and I won't rehash what Eddie laid out here with respect to 476th procedures. But for your SAPS program, my guess is you want the most accurate data. I'd be surprised if the temp/pressure variation was enough to push someone out of tolerance but just in case, here are two ways you can handle it. Option 1 corrects the differences after the fact, option 2 corrects them before the drop.

    1) Set altimeter to Nellis local; near your target, take a reading at your release altitude with the F2 view and compute the difference between what your altimeter says and what your true altitude is; and apply this correction to your results (I think this is what your are doing currently). This keeps your altimeter setting IAW the OI and still yields precise results (although there will still be some error at tip-in/base altitude).

    2) Fly at your release altitude near the target and set your altimeter to read the same as the F2 view (again, truing out your altimeter). This will mean that you are not setting your altimeter IAW the OI (buy maybe CS can approve for specific training DLOs when you have reserved airspace) but you can now fly your profile without having to apply corrections and the error at tip-in/base altitude will be less than option 1.

    Maybe Noodle or Snoopy can jump in here and expand on whether the IFFCC is supposed to correct for this. I suspect it does as its simple math from info the IFFCC has already but I won't pretend to know how it fully works.

    To add to the discussion behind Oliver's question...

    Quote Originally Posted by Oliver View Post
    The new Nellis AFI showed is altimeters remain set in the NTTR, regardless of altitude.
    Quote Originally Posted by Eddie View Post
    ...within the confines of the NTTR we can operate either using the TA/TL or by using a "Force QNH", in the case of the NTTR, Nellis QNH is used as the force QNH....
    Now that we understand how altimeters work and what they display, lets talk about how we use them. Or I should really say why we use them...so we don't hit anything. We don't want to hit the ground and we don't want to hit other aircraft. Both of those are bad things.

    When we are high altitude, we aren't too concerned with terrain, just deconfliction with other aircraft. To ensure all aircraft are on the same reference, we use the standard setting of 29.92 inHg. Now any aircraft flying above the transition layer is on the same setting and can deconflict altitudes. There is no need to change settings as you fly across the country.

    However, when we are low, we have to worry about hitting the ground so True Altitude (above MSL) is most relevant as our charts display obstacles and terrain elevation in MSL. But we still need to deconflict with other aircraft so aircraft in the same airspace need to be on the same setting. We do this with regional altimeter settings (aka QNH). This works by having all aircraft below the transition layer in the vicinity of Nellis (or any other airfield) dial in the local altimeter so they can avoid the ground and each other.

    NTTR allows for the military to have some flexibility. If we look at NTTR as a separate block of airspace, as long as all aircraft in this block of airspace are on the same altimeter setting, they can be deconflicted. But why do we need to change the rules/procedures from what exist all over the rest of North America? Well, we fly in ways that are different than normal civilian traffic. Dojo's SAPS discussion above highlights one reason (though for A-10s its not as prevalent). For example, lets say Eddie is at FL250 in his F/A-18 (with a giant grin on his face) and he is going to conduct an attack run with a release altitude of say 12,000 feet. Its not real convenient for him to have to change altimeter settings on his attack run as he descends through the transition layer. Because we can control who enters and exits the NTTR, we can set a "Force QNH" that allows us to deconflict from the surface to 40k feet without having to change settings. NOTE: This is a main reason why spill outs (leaving your airspace without clearance) are a big deal at Nellis.

    So in a much longer winded explanation that what Eddie provided, outside of the NTTR, normal rules would always apply. Within the NTTR, the real procedures use Nellis QNH as the setting with 476th procedures coming soon.

    NOTE: I'm intentionally ignoring the IFFCC calculations for now as I don't have the tech order to dive into how the real aircraft works and the DCS flight manual doesn't explain the intricacies of how it is replicating it anyway. And it appears from Dojo's latest testing that there is no difference between the baro and the HUD.


    If anyone is not yet ready to commit Hari Kari after reading all this (classic scene from Airplane!), AFPAM 11-216 has a lot of good info. I've included a few files below if you want to play around with an MB-4 flight computer and do some of the calculations.

    Lapse Rate Chart Lapse.PNG


    MB-4 IMG_0939.JPG Link to usable SWF file of MB-4 Computer...should be able to DL and open in IE
    Last edited by Tex; 12Mar16 at 18:08.
    “Rules are made for people who aren't willing to make up their own. " - Chuck Yeager

  2. The Following 15 Users Say Thank You to Tex For This Useful Post:

    Baxter (12Mar16), Dojo (12Mar16), Eddie (12Mar16), Hansolo (12Mar16), Hiccup (13Mar16), InFlames (12Mar16), IronHog (12Mar16), Kimi (12Mar16), Noodle (12Mar16), Odin (12Mar16), Oliver (12Mar16), Recon (13Mar16), Snoopy (12Mar16), Stuka (12Mar16), Wolfman (13Mar16)

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