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High to Low QNH Effect on Indicated Altitude

Asked by: 12648 views ,
Student Pilot

Could someone please help me with this question. I have the 'correct' answer but I get a different answer, I simply can't agree with it/calculate it. Keen to see peoples answer without me posting the correct answer so peoples calculations aren't influenced by it..

Q: You take off from Aerodrome X (elevation 17 ft) with QNH 1029 set and you fly to Aerodrome Y. If you forget to set the Aerodrome Y (elevation 13 ft) QNH of 1018, what would your altimeter read upon landing?

If you could briefly explain your calculation that would be much appreciated.

Thanks

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2 Answers



  1. Kris Kortokrax on Sep 25, 2014

    I know that air pressure decreases as we travel higher in the atmosphere at a rate of about 1 inch per 1000 feet (in the lower altitudes). Standard pressure is 29.92″ or 1013.25 mb.
    1″ would equal about 34 mb (1013.25 / 29.92)

    The difference in your QNH values is 11 mb. This would be roughly a 333′ difference in the elevation at which the pressure levels would exist.

    The pressure at airport Y is lower. This lower static pressure would make the altimeter think it is at a higher altitude than it really is.

    13′ + 333′ would mean the altimeter would read in the neighborhood of 346 feet if you were on the ground at airport Y.

    This is a dangerous situation if one is in the clouds shooting an instrument approach, because you will think you are higher (safer) than you really are.

    If you are visual, one would hope that you would notice a 300 foot disparity.

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  2. hcourtney on Sep 25, 2014

    Thanks, I get a similar answer to you, 343ft

    However the supposed correct answer is -317 (minus 317)

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