Cold weather altimetry & seasonal changes to minimum altitudes
In the low altitude approach environment how does ATC take into account the differences in height above the ground caused by flying the same indicated altitude when the temperature gets very cold (-33C and colder)?
I understand the phenomenon of how cold weather affects uncompensated barometric aircraft systems. What I want to know is how you men & women at ATC factor this in when handling aircraft or whether you correct for this at all?
The first time I came across this phenomenon I was a FO flying a CAT III approach to an airport where the surface temperature was -46C. Everything was “right” with the jet and the way we had the approach set up. However going by the published glideslope crossing height point on the approach we were some 360’ higher than the published figure. At the time I didn’t understand why.
If we have a sea level island airport and it is ISA (+15C & 29.92) and you (ATC) assign me 5,000’ - I will be at 5,000 indicated altitude and 5,000’ above the sea below me. If the temperature drops to -50C and you assign me 5,000’ and I fly 5,000’ indicated altitude (altimeters correctly set and no onboard system failures) I will be only 3,500’ above the surface of the sea.
The vast majority of the people I fly with do not understand how significantly cold surface temperatures affect the distance/room/clearance the jet has over the underlying terrain when they fly assigned or published altitudes that are not compensated for the existing low temperatures. Surprisingly: when the altimeter indicates several hundred feet higher at the FAF than the published values (cold temperatures at the aerodrome: -33C and colder) and they go through the normal mantra: “outer marker (or equivalent) altitude checks no flags” I have yet to have a FO question the discrepancy between the indicated altitude and the published altitude on the chart. Nor have I had a FO brief the expected change in indicated altitude.
Are the following views of ATC fairly accurate (low altitude approach airspace)? In Canada ATC assigned altitudes are compensated for temperatures. In the United States at some military airports altitudes assigned are compensated for temperature, at civilian airports they are not. I don’t know about Europe-never flown there. Africa/Middle East/SE Asia temperatures never get low enough to have any significant operational consequence.
When you handle us, what do you do different operationally when the temperature is much colder than ISA?
Best wishes to all for a safe and wonderful New Year! Thanks for all the work you do keeping us safe, apart, and away from the ground!!!!