How would the calculation run if say the airfield was 6000ft above MSL, the OAT was 35*C and the ILS check altitude was 7200ft. What should the altimeter read.?
First step: Work out your ISA deviation.
Second step: multiply that by 4 (double it, then double it again)
Third step: multiply the second step number by the
height above the field (in thousands of feet).
Fourth step: temperature deviation LOW, look out beLOW (i.e. LOW temperatures mean you are closer to the ground than indicated, conversely high temperatures are "safer"). If the ISA deviation was negative, ADD the number to the true height off the chart to find your altimeter reading at that height.
For the above particular problem:
- At 6000' AMSL the ISA temp should be 15ºC-(6000' times 2º/1000') = 15-12 = 3ºC. Reported temp is 35ºC, so you are ISA+32º
- 32 doubled is 64, doubled is 128
- Check altitude is 7,200' so check height is 1,200'. 128 times 1.2 (thousands of feet) = 153.6 (154 feet rounded up)
- Temp is high, so subtract the 154 from the crossing height :~ 7,200' - 154 = 7,046 feet. That is what your altimeter will read passing the ILS check height.
So, if the altitude of 7,200 was your MDA, and you went around at 7,200
indicated - you would have gone around 154 feet too high - safer, because it's hotter.