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Why should an Altimeter agree within 75ft of airport elevation?

Asked by: 7573 views Commercial Pilot, Flight Instructor, Private Pilot, Student Pilot, Weather

I learned from my instrument training, and while doing my instrument cockpit check, the altimeter is to read within 75ft of airport elevation.

Aviation weather book recommends 20ft, otherwise must be removed from service.

 

Does anyone have any idea why they mandate such numbers on flight ?

 

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2 Answers



  1. John D Collins on Mar 31, 2016

    The 75 foot is a reasonable cross check limit. 20 feet is too tight for this purpose in my opinion. The altimeter IFR check mandated by 91.411 and specified in Appendix E of part 43 has several tolerances. The basic scale error starts out at 20 feet, but by the time the pressure altitude is 6000 feet, the tolerance is 40 feet. There are several other tolerances including the Kollsman setting which is within 25 feet. Then the pilot setting can introduce some error as can the altimeter setting source. In addition, the altitude at one place on the airport can be different than the altitude of the airport elevation. Add all this together, and 20 feet is just too tight and 75 feet will catch any major errors.

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  2. Charles22 on Nov 15, 2016

    Let’s assume for a moment that an altimeter reads 75 ft higher than the airport elevation. It will be legal to fly. For the purpose of landing in IFR conditions down to the limits, should that altimeter be adjusted to read 75 ft lower than with the ATIS provided altimeter setting?
    Thanks

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