Question on Pressure Altimeter on a plane when entering an area with higher air temperature.?

Question

an installed altimeter in planes measures atmospheric pressure and translates it into sea level. Why in a plane that flies at a fixed height above the ground will the altimeter show a lower height when the plane enters an area with a higher air temperature?

Answer 1

heat makes things expand, cold cools them down..But I have never seen this phenomenon occur. Sounds like your Alt is busted...NOT good....

Answer 2

In air that is warmer than standard   temperature, the airplane will be higher than the altimeter indicates. Said another way, the altimeter will indicate a lower altitude than actually flown.

Answer 3

All aircraft instruments are calibrated to Standard Atmosphere (SA) which is: 1013.25 hPa pressure and 15 degrees centigrade.

If the temperature is below that, you will fly lower than the indicated altitude. Of course, this assumes that you have first adjusted your altimeter to QNH, which is the pressure at sea level for your location. When flying from one place to another, air traffic control always give you the local QNH since it is important for them that they see your position and altitude sent from your transponder as the altitude they have themselves because they have to separate you from other traffic.

If you fly in uncontrolled (G) zone, and come back to you airfield of departure, you can use QFE, which is to set on the ground the altimeter to 0 feet. But then, you don't have the correct altitude of eventual obstacles on your map because those are always give as above the sea level.

Which means: When you fly QNH during the winter, be careful and give some extra feet if you need to clear an obstacle.

I own a small aircraft that I fly much, here in Norway, and I can swear that, when on the fraffic pattern at 1,000 ft, I feel I am lower during the winter. Other pilots say that it is an illusion since the difference is not that important.

Answer 4

To explain that, you have to understand that the density of air is decrease by increasing the temperature. accordingly the atmospheric pressure at an area with a higher air temperature is less that that atmospheric pressure at an area with a higher air temperature .

Answer 5

What stl_luna_7 says is correct.  The error due to atmospheric pressure can be corrected with an altimeter setting on the altimeter, but temperature error cannot.

A note on temperature correction noted below.  In Canada in approach plates we are required to temperature correct altitudes in approach plates when it is 0°C and below.  For 0°C and below altitudes inside of the final approach fix must be temperature corrected, which is usually just the minimums for the approach.  When the temperature is -30°C and below everything on the plate from the 100 NM safe altitude in must be temperature corrected.  In designated mountainous areas all the altitudes on the approach plate must be corrected starting at 0°C.  This past winter I recall correcting some a 200' AGL minimum on an ILS approach in Winnipeg by 80', 40% of the minimum altitude AGL.