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Thread: Position Error Correction Altimeter and ILS minima

  1. #1
    L18C-95's Avatar
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    Position Error Correction Altimeter and ILS minima

    I realise somewhat removed from the usual Super Cub topic.

    In UK Australia if the POH does not publish a PEC for the pitot/static system we have the practice to add 50 foot to a precision ILS minima. If the POH publishes a PEC, typically either zero or 30 feet, then minima is adjusted with this figure.

    The FAA has a 75 foot tolerance on the ground ramp altimeter check, and has a duty of care on altimeter errors. But does either the AIM or the Instrument Handbook define position error correction and discuss adjusting minima?

    Obviously if the altimeter over reads by 75 feet there is a duty of care to consider adjusting minima?

    This is not pressure error correction related to low temperature or high pressure outside the calibration (31 inches) of the altimeter.


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    14CFR43 Appendix E covers altimeter calibration. Every time you get your 91.411 and 91.413 check done, you should get a copy of the altimeter calibration error table. I canít say Iíve ever seen an altimeter calibration error table added to the flight manual of any airplane Iíve ever flown!

    I donít see any limitations or other regulatory requirements related to altimetry beyond the 24 month 91.411 check.

    Chapter 7 section 2 Paragraph 7-2-3 of the Aeronautical Information Manual Talks about errors of +- 75í indicates questionable accuracy and it should be repaired.


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    i guess I donít understand the question. Are you asking if the minimums for a Cat1 ILS is different for a C172 vs a B747 in the US?

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    L18C-95's Avatar
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    Quote Originally Posted by md11freighter View Post
    i guess I donít understand the question. Are you asking if the minimums for a Cat1 ILS is different for a C172 vs a B747 in the US?
    In effect yes. In addition to a superior pitot static system a transport category aircraft will also have a radar altimeter which will allow the use of CAT1 system minima, ie 200 feet OCH.

    Recall that the OCH for CAT1 ILS is only 130 feet, so assuming OCH and system minima are the same for a given runway, and your Skyhawk 50 year old altimeter is over reading by 70 feet, you may only have 60 feet OCH. Now letís suggest you are a bit rusty and you have a half fly up at minima, your OCH is now close to, well, zero. Without a PEC correction you might still be legal, but not exactly in a good place.

    Hence some countries adding 50 feet to CAT1 minima to take into account position error in the static system. This could be due to the altimeter, old static system plumbing, paint/dirt disturbing the static ports, etc, etc


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    In my experience Cat 1 minimums are typically barometric altimeter minimums (decision altitude). Cat 2 or 3 minimums are with the radar altimeter (decision height). I’ve never increased minimums for a Cat 1 ILS for a possible error of an altimeter.
    That’s also what the outer marker altitude check is for....to see if you are in the right place at the right altitude.
    I’ve flown the 747, DC10, and MD11 aircraft and we’ve used the same minimums we would use in my fathers Cessna if we were shooting just a cat1 ILS.
    Granted; if the aircraft was capable, not degraded for some reason we would probably set up an approach for Cat 3 minimums if the weather was that low and the aircraft can do it.
    As far as the OCH...the requirements for that change with aircraft. The DC10 needed an OCH of 150’. That was always on the brief for an approach, and was never a problem. That far down the ILS things are pretty tight as far as the signal beam width, so even with a dot or two off it shouldn’t be a factor.
    The RNAV stuff....that’s another story and we do add 50’ to that.
    Last edited by md11freighter; 07-11-2020 at 08:32 AM.

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    L18C-95's Avatar
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    We also have a glide slope check, usually at a DME range identified on the approach plate - but it is treated as a gross error check that we have not captured a false lobe on the glide slope, or the glide slope is not giving an incorrect signal.

    It has been suggested that position error correction is a UK/Australia concept and am starting to believe that is the case.


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