In reference to ENR 1.5 p 4.7.4 (below), both the 2D MDA and the 3D DA must be adjusted when the temp at the QNH source is less than ISA -15.
I realise that temps below ISA -15 are uncommon but it does happen over the winter months at aerodromes like Canberra and Cooma.



Looking at the correction chart below, the MDA/DA (+ procedural altitudes) need to be increased by approx 5.5% at ISA-15 and more in colder conditions. For example, at ISA -15 a DH of 200' would require a correction of 11' - increasing the DA by 11'. In some cases this correction is equal to or greater than the PEC.



With the above in mind, the IREX e-text is unusually silent on the subject of non-standard temperatures as it relates to the MDA/DA - my apologies if I have missed something here.

Can I assume that non-standard temps are outside the IREX syllabus?

Temperature correction is included in the IREX syllabus (MOS Schedule 3). However to date questions have never been asked. However, you have a point, it should be mentioned in the text.

Hi Bob
Thanks for updating the IREX eText to address AIP ENR 1.5 p 4.7.4 - "Correction of Instrument Procedure Minima for non-standard Temperatures"

On reading through DAP 1-1 p 1.5 (see below) the question of compliance now comes to mind. More specifically, when the temp at the QNH source is less than ISA-15, how does one go about correcting the multitude of altitudes/heights that appear on an IAL procedure chart in real time. The task of correcting for temp. become even more onerous if the IAL procedure includes a DME/altitude table for approach path guidance.



Do you (or anyone else) know how one would comply with AIP ENR 1.5 p 4.7.4 and DAP 1-1 p 1.5 in practice?

You have made a good point. I must admit I had assumed that the correction would be applied to MDA and DA. However, the AIP says it applies to all procedure altitudes. Pretty busy time for a single pilot.

Looking at AIP ENR 1.5 p 4.7.4 and DAP 1-1 p 1.5 from a meteorological point of view the surface temperature at the QNH source will not always be representative of the "air column" below the procedure initial approach altitude. This air column starting at the surface may extend to around 3000' AAL.

To make my point, overnight temps a Canberra commonly drop to -5C (approx. ISA-16) in a cold airmass in association with strong radiation cooling over the winter months. Under these circumstances it is a little fanciful to claim that the surface temp at the QNH source is representative of the air column above the radiation inversion.

That being said, the surface temp is a good estimate of the air column at the MDA/DA but above the surface inversion the AIP logic breaks down.

See BoM Surface Radiation Inversion notes for more detail