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7 Answers

What does adjusting the kollsman window do inside the instrument?

Asked by: 8996 views Aircraft Systems

Does it merely move the card/indication or does it physically change the pressure from 29.92 inHG to match the altimeter setting pressure by compressing/expanding the aneroid wafer?

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7 Answers



  1. Kris Kortokrax on May 20, 2015

    There is a cutaway picture at this address:

    http://en.wikipedia.org/wiki/Altimeter#/media/File:Sens_alt_components.PNG

    The knob operates through gears to change the position of the hands.

    I’m intrigued by the question. Unless one is attempting to invent a better mousetrap, who would care? I know when I set the barometric pressure in the window, it indicates my altitude above sea level. The same function is accomplished by a computer in a Garmin G1000. I can’t draw the electrical schematic, nor could I decipher it if provided with one. But I know my altitude will be displayed if I set the correct pressure. Same thing with a transponder. I don’t know whether it uses AM or FM or PM, but I know it displays the code entered and my altitude on the screen.

    Not chastising you for asking a question. I just don’t know why anyone needs to get this deep in the weeds.

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  2. Russ Roslewski on May 20, 2015

    Now Kris, I doubt this was a question on a pre-solo test or something like that. Some people just have an innate curiosity about how things work. I’m one of them. Drew sounds like the kind of guy who would take apart an old attitude indicator to see how it’s all connected, like I did.

    I’m always wondering how things work. Heck, when I was 6 my parents got me a book titled “How Things Work” which is still on my bookshelf. it had cutaways of jet engines, car transmissions, rockets, all kinds of stuff. I was and still am fascinated.

    It’s an interesting and reasonable question and thank you for the link to the picture. To Drew I would say, see if you can find an old one and take it apart. I might just do the same!

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  3. Kris Kortokrax on May 20, 2015

    I understand the need to know how some things work. I’ve overhauled car engines, aircraft engines, lawnmower engines. Taken apart and reassembled fishing reels, guns etc. for cleaning. When I was a kid, I built a radio from an oatmeal box, some wire and a crystal. Repaired our TV many times.

    I would be interested in the difference in orientation of the gyro in a turn coordinator vs. a turn and bank.

    When turning the knob on an altimeter, it was always apparent that it moved the hands. As the aneroid is a sealed unit, I always just assumed that a set of gears was employed to make the change. In 45 years of flying, I never had any curiosity beyond that.

    While we might have curiosity about how things work, life is too short to find out how everything works. If I have a need to know how something works, (like a fuel system, electrical system, hydraulic system, etc.), that’s worth pursuing. Concentrating on minutiae takes time that can be better spent learning essential things.

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  4. Drew on May 20, 2015

    Kris and Russ, yes, the reason for wanting to know is my curiosity. First, because I’m not someone can simply learn/memorize something because “that’s just the way it is” unless I understand it from my POV, and second, because I’m just a curious person. I’ve posted difficult questions and scenarios on this site that served almost no practical purpose except to quell my curiosity haha.

    Kris, funny you should mention the gryo differences between in a turn coordinator. I was itching to ask how (not why) a TC displays RoR information and RoT information in my next batch of questions. It should be posted here in the near future if I can’t find the answer from my research.

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  5. Kris Kortokrax on May 20, 2015

    There is a nice discussion on TC vs T&B starting on page 7-17 of the Pilot’s Handbook of Aeronautical Knowledge. I’m not so sure that the Turn Coordinator was an improvement over the Turn & Bank.

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  6. Greg Taylor on Jul 09, 2015

    “While we might have curiosity about how things work, life is too short to find out how everything works.”

    Wha? The day I stop trying to find out how everything works, you can start shoveling dirt on me….

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  7. crossaint on Sep 12, 2016

    The altimeter’s bellows push or pull at a sector gear wheel when they “feel” pressure changes. The sector wheel meshes with a very small pinion to move the 100ft needle, which in turn, much like a clock’s gear, is rotating the other pointers.

    Paul Kollsman’s invention was to add a fraction of the angle that the barometric scale is turned to the rotation of the 100ft pointer. This is done by letting the sector gear not directly drive the 100ft pointer but a driving a gear wheel combination whose second gear is then meshing with the 100ft pointer’s pinion.

    This additional gear stage is mounted, guess what, on the barometric scale rotates when the scale is turned around the first pinion, which makes it itself rotate around its axis. This extra rotation is then transfered to extra rotation of the 100ft pointer.

    Regarding the turn coodinator vs. turn and bank indicator:
    the turn coordinator gives a quicker indication that you are about to turn, as it already reacts on the banking of the aircraft. Again, two values are added to give a reading, here it is the roll rate and the turn rate. You will quickly see the improvement when your attitude indicator fails.

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