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2 Answers

Altimeter error

Asked by: 1222 views ,
General Aviation

If on the ground an altimeter reads 75 ft higher than the airport elevation, it will be legal for flying.

But at the decision altitude, on a ILS or LPV approach in IFR conditions down to the limits, such an altimeter will read 200 ft AGL when the airplane is actually only 125 ft AGL. How the pilot should deal with this? Should the pilot go missed, if the runway is not in sight at 275 ft (instead of 200 ft) or, before starting the approach, should the pilot adjust the altimeter to read 75 ft lower, after adjusting the altimeter using the setting provided by ATIS?

Or the 200 ft decision altitude takes into account the possibility of altimeter errors up to 75 ft ?


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2 Answers

  1. Skyfox on Dec 07, 2016

    What I personally would do is use the altimeter settings provided by ATIS or ATC, while keeping that altimeter error in mind during the approach to be fully aware of it as I’m looking at the DH/MDA coming up. I might decide to break off the approach at a higher altitude than indicated in the interest of safety, especially if the weather requires flying right down to minimums, unless I’m very thoroughly familiar with the approach and airport surroundings. Also keep in mind that your transponder has a pressure altitude altimeter for the mode C altitude encoding data, and ATC may inquire if it looks like the aircraft is getting too low (don’t rely on that!).

    There is a certain margin of error factored in to approaches and the airway/airspace system, considering altimeter errors and changing atmospheric conditions, but it’s always best to error on the side of caution.

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  2. Charles22 on Dec 07, 2016

    Thanks for your reply and the good advise.

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