Mountains grow taller in the Cold of Winter. (Continued)
The FAA was slow to stress altimeter inadequacies in the far North. What works down below, often does not up here. Planners brought remote automated weather reporting stations to Alaska and soon found that the pressure readings did not go high enough, nor the temperatures low enough, but never mind, the instruments were soon covered in so much snow that they did not work anyway.
Only recently have the meteorology notes carried any charts depicting the necessary altitude corrections for the kind of cold weather that dominates Alaska, Canada and sometimes Minnesota. Even now, there seems equivocation in saying that these differences "may exist", that the differences are due to altimeter error. I wonder how many aircraft have flown into the side of a mountain because these design inadequacies were not publicized. The altimeter gives the information for which it was designed. Temperature was not built into the instrument, nor was the altimeter setting designed to accommodate pressures above 31” or below 28”. Altimeter settings above 31 are not uncommon in the cold dry high pressure of the Alaskan interior. Read section 7-2-2 and 7-2-3!
The so-called error can be a killer: top row, height above airport in feet, to the left, temperature reported degrees C. The intersecting value is the number of feet you will be below that which the altimeter indicates, from Jeppesen IFR plates, Meteorology Safety of Flight, US 7-2-3, Table 7-2-3 ICAO Cold Temperature Error Table. (I am not sure this chart gives the whole story)
200 500 1,000 2,000 5,000
+10 10 10 20 40 90
+-0 20 30 60 120 280
-10 20 50 100 200 490
-20 30 70 140 280 710
-30 40 100 190 380 950
-40 50 120 240 480 1,210
-50 60 150 300 590 1,500
The FAA was slow to stress altimeter inadequacies in the far North. What works down below, often does not up here. Planners brought remote automated weather reporting stations to Alaska and soon found that the pressure readings did not go high enough, nor the temperatures low enough, but never mind, the instruments were soon covered in so much snow that they did not work anyway.
Only recently have the meteorology notes carried any charts depicting the necessary altitude corrections for the kind of cold weather that dominates Alaska, Canada and sometimes Minnesota. Even now, there seems equivocation in saying that these differences "may exist", that the differences are due to altimeter error. I wonder how many aircraft have flown into the side of a mountain because these design inadequacies were not publicized. The altimeter gives the information for which it was designed. Temperature was not built into the instrument, nor was the altimeter setting designed to accommodate pressures above 31” or below 28”. Altimeter settings above 31 are not uncommon in the cold dry high pressure of the Alaskan interior. Read section 7-2-2 and 7-2-3!
The so-called error can be a killer: top row, height above airport in feet, to the left, temperature reported degrees C. The intersecting value is the number of feet you will be below that which the altimeter indicates, from Jeppesen IFR plates, Meteorology Safety of Flight, US 7-2-3, Table 7-2-3 ICAO Cold Temperature Error Table. (I am not sure this chart gives the whole story)
200 500 1,000 2,000 5,000
+10 10 10 20 40 90
+-0 20 30 60 120 280
-10 20 50 100 200 490
-20 30 70 140 280 710
-30 40 100 190 380 950
-40 50 120 240 480 1,210
-50 60 150 300 590 1,500
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