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Asked by: 1817 views Aircraft Systems

I know that it is good operating practice to compare the altimeter reading to the published altitude on the approach chart when passing over the outer marker while on the glideslope of a ILS. When making this comparison, what exactly does the comparison indicate? NOTE: Assume the altimeter is properly calibrated and there is no nonstandard temperature effects and no extreme atmospheric pressure effects.   Thank you for the feedback.  

3 Answers

  1. John D Collins on Sep 05, 2013

    It is simply a cross check of the GS indication and your altimeter. It can pick up gross errors caused by an improperly set altimeter or a GS Indicator that doesn’t function properly. As you point out, the correspondence is not absolute because the altimeter is affected by temperature, but they should correlate within a hundred feet or so. It is just a test of reasonableness and is the last chance you have for such a check in the approach.

    If you think about it, the VOR is supposed to be checked every 30 days, but the GS or localizer doesn’t have to be. It is worthwhile to check their operation on good VFR days when you can verify proper operation visually. Other than this, cross checking against a second ILS indicator-receiver is another way of cross checking as is checking altitude loss of approximately 300 feet per NM.

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  2. Dan Chitty on Sep 05, 2013

    Great points. Thank you John.

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  3. Ron Klutts on Sep 05, 2013

    It’s your last chance to make sure that the device your depending on to tell you when you arrive at DA is in fact going to be accurate and not lead you into the ground.

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