From the
Tech Log Archive, in the
Altimeter Correction Cold WX OPS
Variations from ISA temperature start becomming really applicable at or below ISA-15°. If it is not represented graphically in your Ops manual, you can use the 'rule of thumb':
+4 feet for each -1°C temperature variation from ISA, per 1000' altitude above the airport (QNH datum)
The airport QNH is adjusted so that it is accurate at the field, it is only the airspace above the field elevation that needs to be corrected for density variation.
e.g. For a 200' Cat I minima at, say ISA-20°, this produces a correction of (4 x 20 x 0.2) 16ft. Round it up to 20ft. and, remembering the rhyme: "ISA low? Watch out below!", set the published Decision Altitude PLUS 20ft. on the minima bug.
At an OM crossing height of say 1300ft AGL the variation is much larger (4 x 20 x 1.3 =)104ft. so your crossing height, on glide slope, would be more like 1404ft. indicated above the field elevation.
This assumes that the ISA-20° is constant throughout the atmosphere, and not affected by an inversion, which would mess up the calculations.
quid, the outer marker is just a warning to check the altimeter as the signal does occur over a 150 feet or so. When the marker beacon sounds, you perform the altimeter check at the
DME distance shown on the chart - acurate to 10 or 20 feet.
As the QNH is rounded to the
nearest hpa, then it will be acurate to within 15 feet.
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