If the aircraft altimeter indicates a higher altitude than the check altitude, the difference between the two altitudes must be added to the approach minima
While I accept that is what it says in the AIP its horse****.
What you guys are discussing is the difference between
Indicated Altitude and
True Altitude
The formula to correct for this is;
TA = IA +/- 4' per Deg of ISA Devn(Ht/1000). 'HT' in the formula refers to Height above the Temperature datum (the airfield)
Or there are correction tables available - but probably not in the Oz AIP
You should correct for TA for
ALL IAL altitudes, LSALTs etc when temperature is significantly
COLDER than ISA. No need when temp is warmer than ISA...which is what they are alluding to in that AIP reference.
Example: You're conducting an ILS at a MSL airport with 4000' MSA and its ISA - 20. So -5C
To correct the MSA;
TA = IA -(4x20)(4)
So if you descended to 4000' IA you would actually be at 3680' TA...leaving you only 680' above the highest obstacle. Therefore you would descend to 4320' IA and you would be 4000' TA therefore preserving your 1000' MSA protection.
If the platform altitude was 2000' you would correct that in the same way adding TA=IA-(4x20)(2) 160'. An IA of 2000 = TA 1840' - So platform should be raised to 2160'.
If the GS check height is 1000' you correct that the same way TA = IA -(4x20)(1) 80'
BUT now you're following the G/S so you are passing over a geographical point following a fixed beam so with the G/S centered you expect the IA to say 1080' (your TA is 1000') and that is PERFECTLY correct and proper.
If the minima is 200' you correct in the same way adding (-4x20)(.2) - 16'.
Now you're descending towards the ground and MUST be visual at 200' - if you descend to an uncorrected IA of 200' you will actually be at 184'...so you add 16' and at an indicated altitude of 216' your TA will be 200'. If visual land if not Go Around.
As you can see the magnitude of the ISA Devn
altimeter error reduces the closer you get to the height of the temperature datum - the airfield. If you just add the difference at the OM you may not get visual and end up flying a missed approach for no good reason.
Now you guys do the correction of all the above for ISA + 20 and you will soon see why there is no need to correct IA in the ISA + case.