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Thread: Altimeter off

  1. #1

    Altimeter off

    I'm 700 miles from home base and will be returning on wednesday. Halfway here ATC started telling me my altitUde was 300plus feet higher than indicated. I have to look into it when I return but I have to fly back 700 miles first. Might this be indicative of anything more serious? Or is a flight back okay?
    Thanks.
    rick
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  2. #2
    If it's really a problem for ATC, they'll just aks for position and altimeter reports.

    Sounds to me like it might be a bad encoder. When was it last checked?

  3. #3

    Altimeter off

    Rick:

    If you have GPS, you can use this to confirm altitude until you get your
    altimeter calibrated.

    Don
    ----- Original Message -----
    From: "RKoch" <rick@denverflash.com>
    To: <musketeermail@yahoogroups.com>
    Cc: "BAC Mail" <bac-mail@beechaeroclub.org>
    Sent: Monday, November 20, 2006 11:03 AM
    Subject: [BAC-Mail] Altimeter off


    I'm 700 miles from home base and will be returning on wednesday. Halfway
    here ATC started telling me my altitUde was 300plus feet higher than
    indicated. I have to look into it when I return but I have to fly back 700
    miles first. Might this be indicative of anything more serious? Or is a
    flight back okay?
    Thanks.
    rick
    _______________________________________________
    BAC-Mail mailing list
    BAC-Mail@beechaeroclub.org
    http://www.beechaeroclub.org/mailman/listinfo/bac-mail

    _______________________________________________
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    http://www.beechaeroclub.org/mailman/listinfo/bac-mail

  4. #4

    Altimeter off

    Two key points here.

    One - GPS altitude is substantially less accurate than horizontal position. In the absence of some form of correction (time averaging, WAAS, differential GPS) it is _not_ accurate enough to verify correct or incorrect function of an aircraft grade altimeter.

    Second, and more specific - in 98% of aircraft there is no correlation between the altimeter you see on the panel and what ATC sees. The transponder broadcasts pressure altitude to the nearest 100 feet. The transponder gets this altitude from an encoding altimeter. In most small GA cases this is via a multi-line encoding called Gilman code (or "grey" code). In newer installs or bigger aircraft this could be via an RS-232 serial line. But in most cases this comes from a seperate pressure altimeter. In a GA aircraft it's pretty easy to find - look under the panel and find a box plugged into the static line _besides_ the airspeed, VSI and altimeter, this will be your encoding altimeter.

    In a smaller number of installs the panel altimeter will have a built-in encoder, But regardless the Kollsman window setting has _nothing_ to do with what the transponder reports, it only reports pressure altitude. The ATC displays correct this to actual altitude by calculating from the altimeter correction value input by the controller (or more often these days automatically fed from the appropriate METAR report). Finally the on-air radio and inter-avionics data formats to and from the transponder only report to the nearest 100'.

    So given this 100ft limit of precision at multiple steps and variation between your actual temperature and pressure conditions versus the value the ATC system is using, it's easy to have 200' variation between what you think is your actual and what the controller sees on their display. So 300' is actually the minimum error that ATC will normally note and warn. However, it's certainly not impossible to have these cumulative sources of error add up to 300', especially if you're a little sloppy at setting your Kollsman window. One error that won't be a factor is if you have a static line leak. Your altitude may be inaccurate - but your panel and encoding altimeter will get the same (slighly incorrect) pressure and agree with each other, so this sort of failure will not manifest as an altitude reporting error.

    Now - if you mis-set the Kollsman window you can be seeing the wrong altitude but the transponder will be blindly reporting the "correct" (pressure) altitude. Overall you should be able to come home just fine but consider inquiring your reported altitude at times that it's convenient and likely to be accurate (near the top of the hour near the station who's setting is being used).

    If it doesn't come up again I'd right it off to local condition variations, otherwise repeating a static/transponder check is the sure way to verify correct function and/or isolate the problem component. The encoding altimeter, especially older designs, has a limited field life on the pressure transducers and will become inaccurate eventually. Luckily these are fairly inexpensive items (normally < $300 for a basic single channel unit). One bit of advice - if replacing one of these spend the extra $50-$100 and get a dual channel unit - the 2nd RS-232 channel will come in handy for input to an IFR GPS and/or a Fuel/Air data computer if you ever do that upgrade.

    My original transponder had lots of issues. One thing I like about the new digital units incluing my GTX 327 is they can show what _pressure_ altitude is being reported, so it's easy to do an E6B conversion to true altitude to verify function yourself (or more easily set your panel altimeter to 2992 and cross check).

    Good luck,
    --
    Mark


    ________________________________

    From: bac-mail-bounces@beechaeroclub.org on behalf of Janet and Don Bateman
    Sent: Mon 11/20/2006 1:01 PM
    To: musketeermail@yahoogroups.com
    Cc: BAC Mail
    Subject: Re: [BAC-Mail] Altimeter off



    Rick:

    If you have GPS, you can use this to confirm altitude until you get your
    altimeter calibrated.

    Don
    ----- Original Message -----
    From: "RKoch" <rick@denverflash.com>
    To: <musketeermail@yahoogroups.com>
    Cc: "BAC Mail" <bac-mail@beechaeroclub.org>
    Sent: Monday, November 20, 2006 11:03 AM
    Subject: [BAC-Mail] Altimeter off


    I'm 700 miles from home base and will be returning on wednesday. Halfway
    here ATC started telling me my altitUde was 300plus feet higher than
    indicated. I have to look into it when I return but I have to fly back 700
    miles first. Might this be indicative of anything more serious? Or is a
    flight back okay?
    Thanks.
    rick
    _______________________________________________
    BAC-Mail mailing list
    BAC-Mail@beechaeroclub.org
    http://www.beechaeroclub.org/mailman/listinfo/bac-mail

    _______________________________________________
    BAC-Mail mailing list
    BAC-Mail@beechaeroclub.org
    http://www.beechaeroclub.org/mailman/listinfo/bac-mail


    _______________________________________________
    BAC-Mail mailing list
    BAC-Mail@beechaeroclub.org
    http://www.beechaeroclub.org/mailman/listinfo/bac-mail

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