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Altimeter setting exceeding 31.00 in Hg and raising approach minimums

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FAA Regulations, Flight Instructor, General Aviation

. In the rare event that the local altimeter setting is over 31.00 in Hg, why is the decision made (by the FAA) to raise the minimums on instrument approaches if aircraft are operating in a region with this high of a setting? Assuming the typical kollsman window maxes out at 31.00 and the local altimeter is 31.05, you will still be flying at an altitude higher than whats indicated on your altimeter(assuming you have 31.00 set in your window). Therefore, regardless of what your approach mins are, you would still be above them, in this case approximately 50 feet. My logic may be wrong here, but I’ve just been having a tough time grasping the concept when the altimeter setting is above this 31.00 range. My only other thought would be that if the altimeter setting was greater than 31.00 in Hg, there is a chance that it could also be extremely cold and therefore the FAA is assuming worst case scenario and adding this buffer due to errors in the altimeter because of non standard temperature.

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1 Answers

  1. John D Collins on Mar 06, 2014

    You are correct that the altimeter will read higher than MSL altitude under these circumstances. However, since you can’t adjust your altimeter to the higher pressure, you have to still use the same minimum altitudes as indicated on your altimeter. Since you will be higher at the DH/MDH, you will need a higher ceiling and in some cases a higher visibility to see the runway. For example, if the altimeter setting is 31.20, on a category 1 ILS, you will reach your DH 200 feet higher than you would if you were able to adjust your altimeter. At the regular 200 feet DH, you will be about 2800 (1/2 SM) feet from the threshold. At the point where you indicate 200 feet under these conditions, you will be 6600 (1.25 SM) feet from the threshold. So if you expect to land under these conditions, you will need greater visibility. Also if the reported ceiling is 200 feet, you will still be in the clouds at 400 feet, so to expect to complete the landing the actual ceiling will have to be higher to match your additional height since you can’t adjust your altimeter.

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