Cold Air, an Unserviceable Glide Path, and a Thin Fuel Margin

Direction. The altimeter is calibrated to the ISA pressure lapse rate. In air colder than ISA the pressure at a given true altitude is lower than ISA predicts, so for a correctly set subscale the altimeter over-reads: the aircraft is lower than the altimeter shows. The correction is therefore added to every published altitude.

Magnitude

• ISA at 2 400 ft = about +10 C; OAT -25 C, so the air is 35 C below ISA.
• At the DA: 1 060 ft above the source x 35 x 4/1 000 = about 148 ft, say 150 ft.
• At the MDA: 1 320 x 35 x 4/1 000 = about 185 ft.

Practical points

• The correction is based on height above the altimeter-setting source (the aerodrome), not on elevation, and must be applied to all the procedure altitudes with the same reference - initial approach, step-down fixes, MDA/DA and the missed-approach altitudes.
• QNH 1028 does not change the temperature error as long as the correct QNH is set; it is a reminder that pressure altimetry here is a vertical-position tool, not a terrain-clearance guarantee.
• Snow and light icing also mean the approach must be flown on instruments without visual shortcuts. Use the chart table in flight; the rule of thumb is only a cross-check.

Approach. The ILS glide path is not available between 0600 and 0800 UTC, and the ETA of 0645 falls inside that window. The ILS becomes a localizer-only approach, which is a non-precision approach with a published MDA of 3 720 ft and step-down fixes. The ILS DA of 3 460 ft cannot be used.

Vertical profile

• Add the cold correction consistently from the final approach fix onward: about +185 ft at the MDA, giving approximately 3 900 ft indicated as the minimum, and similarly corrected step-down altitudes. The correction is calculated from height above the aerodrome, so each altitude has its own value.
• The trap is correcting one altitude but not the others: correcting the MDA but not the intermediate step-downs produces a profile that is inconsistent, and correcting the step-downs but not the MDA quietly erodes the final margin.
• The crew can still fly a continuous-descent technique using DME distance against corrected altitudes, treating the corrected MDA as a decision altitude, provided the approach is stable by the operator's criteria.
• Because the glide path is out, the missed approach point must be identified by DME or timing, not by glide-slope intercept; the timing must be checked carefully at these temperatures.
• If the required visual reference is not established at the corrected minimum, the missed approach is flown immediately - do not descend further looking for the runway.

Burn 40 kg/min, so 2 400 kg = 60 min total at the destination.

Case 1 - divert without an approach 800 (alternate) + 1 200 (final reserve) = 2 000 kg, leaving 400 kg = 10 min of margin. Comfortable.

Case 2 - one approach, missed approach, then divert 200 (approach and landing) + 800 (alternate) + 1 200 (final reserve) = 2 200 kg, leaving only 200 kg = 5 min of margin. Legal, but thin: any extra vectoring, a second approach or a runway change puts the aircraft below the final reserve.

Resulting plan

• Commit to one localizer approach only, briefed as a one-attempt approach.
• At the final approach fix, cross-check the fuel: it should be at least about 2 200 kg. If it is lower and the weather is at the minima with snow, the correct decision is to divert before flying the approach rather than to fly it and then divert with less.
• Monitor fuel against the 2 200 kg and 2 000 kg figures and be ready to tell ATC early: 'MINIMUM FUEL' if the aircraft becomes committed to this aerodrome, and 'MAYDAY FUEL' if the landing is expected below the final reserve.