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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

  2. #2

    [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

  3. #3

    Altimeter off

    Rick,

    The ATC report you got was what your encoder was reporting to ATC. That could mean that your altimeter / static system is off. But, it could also be your encoder. A quick check is to set the local altimeter setting and see how far off from the airport elevation the altimeter shows. More than a hundred feet and I'd say you need to look into the altimeter / static system. If it's less than a hundred feet, I'd suspect the encoder. If you suspect the encoder and you are flying IFR or need to transit/land in controlled airspace requiring Mode C, you need to get this resolved now. If you are VFR and confident in your altimeter indication and are only flying through areas of low traffic density away from controlled airspace, I'd disable the encoder (or have it disabled) squawk 1200 and take it home to your favorite shop.

    Marty Vanover
    Phoenix, Az.

  4. #4
    RICK
    DO YOU HAVE A GPS THAT COULD TELL YOU YOUR ALTITUDE CROSS REFERENCE

  5. #5
    Good lead-in for a comment I tried to reply to - but it bounced from Musketeer mail due to some email sender validation issues:

    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).

  6. #6

    [BAC-Mail] Altimeter off

    > Two key points here.
    In most small
    > GA cases this is via a multi-line encoding called Gilman code (or "grey"
    > code).

    *** Actually, "Gray code", named for a Bell labs researcher named Frank Gray
    who patented it. It's main claim to fame is that each adjacent code
    changes by only one bit. I imagine they used it because when the system
    was designed the encoding was mechanical - aneroids driving multi-gang
    switches. And if only one bit changes when you go up and down, you have
    natural resistance to glitches - false codes between positions - you
    just use a make-before-break type of switch. Also, it's what the digital
    guys call "self clocking" - you can put logic on a gray code generator that
    senses when the code has changed, and generate a clock pulse off that.

    GPS is not so good for vertical position - I think it's accurate to
    maybe plus/minus 200 feet. Probably good enough for VFR, no WAY would I
    trust it in IMC.

    - Jerry Kaidor ( jerry@tr2.com )

  7. #7

    Altimeter off

    I was unaware of a MMEL for general aviation airplanes. I do recall something (not often) about the pilot determining if the airplane was safe to fly with missing/inoperative equipment. I haven't looked it up, but it does jog some memory files.

    Marty Vanover
    Phoenix, Az

    rjf <rjf@p339.com> wrote:
    Before doing VFR flight, I'd check to see if your encoder or altimeter
    (whichever has the problem) is required gear for your aircraft. If it
    is, you're not legal to fly VFR. If you're IFR, either being off by
    300' is a big deal.

    Good luck,

    RJF

    Martin Vanover wrote:

    > Rick,
    >
    > The ATC report you got was what your encoder was reporting to ATC.
    > That could mean that your altimeter / static system is off. But, it
    > could also be your encoder. A quick check is to set the local
    > altimeter setting and see how far off from the airport elevation the
    > altimeter shows. More than a hundred feet and I'd say you need to look
    > into the altimeter / static system. If it's less than a hundred feet,
    > I'd suspect the encoder. If you suspect the encoder and you are flying
    > IFR or need to transit/land in controlled airspace requiring Mode C,
    > you need to get this resolved now. If you are VFR and confident in
    > your altimeter indication and are only flying through areas of low
    > traffic density away from controlled airspace, I'd disable the encoder
    > (or have it disabled) squawk 1200 and take it home to your favorite shop.
    >
    > Marty Vanover
    > Phoenix, Az.
    >
    > RKoch <rick@denverflash.com <mailto:rick%40denverflash.com>> wrote:
    > 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

  8. #8
    I was trying to adjust my altimeter to the correct field elevation tonight, I removed the little screw just above the knob on the altimeter, there is nothing behind the screw?

    At one time I would set the correct Baro altimeter setting in the kollsman window, then remove the little screw and move a little locking arm and then adjust the altimeter hands to the field elevation.

    That was quite a few years ago...

    How do you adjust the altimeter? And more importantly, how do you insert the little screw back in the hole?

    Here is a link below.

    http://www.falcongauge.com/PDFs/inst...eteradjust.pdf

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