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7 Answers

Altimeter reading error due to T being not standard.

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Aircraft Systems

I read somewhere that because the readout of an altimeter is only correct on a standard day, I should be aware of, on a below 15C Temperature day, my altimeter will give me an altitude lower than true altitude ("from high to low, watch out below"). And this error is proportional to the height. The hight I go, the worst it will be. Is that true ? Should I really be aware of that or is the error minimal to really take it into account ?

 

 

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7 Answers



  1. Sam Dawson on Jul 15, 2014

    The error is normally most critical during approach and altitude corrections to DA/MDA should be made in cold weather. My Jepp manual has an altimetry section that gives corrections. As an example, at -10C you would need to add 10% to your altitudes during approach sequences (except for CAT II DA). So on a PT with an altitude of 1500 AGL you would add 150′ to the MSL altitude. With a DH of 200′ AGL you would add 20′ to the DA.
    If I remember correctly the AIM weather section has the same information. Someone please correct me if I am wrong.

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  2. Bidochon on Jul 15, 2014

    Thanks Sam. I had to look for the DA/MDA/DH terms as I’m not instrument rated yet and never heard of those terms before but thanks to you, that’s something else I learned today.

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  3. Wes Beard on Jul 15, 2014

    If you look at the weather station reports in CO airports in the plains versus the mountain pass weather reports you will notice a much higher altimeter setting in the pass to correct for the temperature lapse rate differences. Much bigger difference in the winter than in the summer.

    http://skyvector.com/?ll=39.50719653661028,-105.5315083749321&chart=18&zoom=7&plan=V.K2.OCN

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  4. Best Answer


    Russ Roslewski on Jul 15, 2014

    Cold temperatures will make you think you are HIGHER than you really are – your altimeter will display 5000 feet but you will really be at 4900, 4800, etc., depending on how cold it is. The non-standard temperature variation causes a greater effect the further you get above the altimeter reporting station (typically the airport). So, at 1000 feet above the station there’s not much difference, but at 5000 feet above it, the differences can be significant.

    I posted a more complete explanation of cold temperature effects on instrument approaches on my blog at:
    http://cfiruss.blogspot.com/2014/02/its-cold-are-you-too-low.html, but it applies for VFR as well. Hopefully that should answer your question, let me know if you need anything else.

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  5. Gary on Oct 18, 2014

    What difference does it make what the temperature is if your altimeter is set to the setting of a nearby station?

    High to low, lookout below is fine if you have no broadcast from a nearby station but otherwise, why worry? Just asking.

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  6. John D Collins on Oct 18, 2014

    Gary,

    It makes no difference if you are right next to the location where the altimeter setting is measured and determined and at the same altitude. However, the further you get from that position and the higher above the elevation, the greater the error. Here is an example, the airport is located at sea level and it is very cold, say -20 C at the airport. You are overflying mountains at night nearby at 7000 indicated on your altimeter. Your altimeter is set according to the altimeter setting at the airport. The mountain top is at 6000 MSL. Your true altitude is 6004 feet MS, almost a 1000 feet below your indicated altitude and 4 feet above the top of the mountain.

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  7. Gary on Oct 21, 2014

    Makes sense John. Thanks.

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