Re: low temperature correction
I am a long way away from active involvement, but maany years ago we were asked to make temp corrections to decision heights/altitudes and to any step-down fixes inside the final app fix if the temp was ISA minus 15 or below For simplicity we were allowed to use airfield temp 0ºC or below for airfields up to 1000ft amsl.
QFE is the actual airfield surface pressure and is not affected by air temperature. The error is engendered in the height gap between the airfield and the aircraft, and varies according to the size of this gap. If you used a false QFE to adjust for temp error at a 500ft decision height your altimeter would be still give false readings at heights above and below 500ft. I don't see how ATC could do this Of course, once you have made your decision at the correct point, with the calculated temp correction incorporated, your indicated altitude is still wrong, but with decreasing error down to touchdown. Your RADALT will be right.
QNH is calculated from QFE, the measured airfield pressure, down to an assumed msl on the assumption that ISA temperatures apply. You set QNH, and the altimeter then back-calculates, again using ISA, and displays field elevation accurately when you are on the airfield, regardless of actual temps. At any height above or below the airfield the altimeter is then subject to temp errors if ISA temperatures do not apply. As with QFE and decision heights, so with QNH and decision altitudes. The size of the temp error depends on the height gap between the airfield and the aircraft (and the temp, of course).
How would the aircraft systems generate a changing temp correction as the aircraft descended, keeping the indicated height correct? Is it possible?
Dick W