Cold-Air ILS Approach at a Mountain Aerodrome
Your twin-jet is on a night ILS approach to Runway 27 at a mountain aerodrome. Aerodrome elevation 1,850 ft, touchdown zone elevation 1,820 ft, ILS decision altitude 2,400 ft (550 ft above TDZE). ATIS gives QNH 1013 hPa and outside air temperature −25 °C. The aerodrome has no published cold-temperature altitude correction, and the sector minimum safe altitude is 9,500 ft. The altimeter subscale is set to QNH and the radio altimeter is serviceable. The aircraft is stable at 140 kt IAS. A preceding aircraft reported smooth conditions with no wind shear, but the temperature is far below ISA (ISA at 1,850 ft is about +11 °C). The airport operator's low-temperature procedure contains only one line: pilots to apply cold-temperature corrections at their discretion. You must decide whether a correction is required and by how much.
1. Estimate the true-height error at the decision altitude and state whether the aircraft is higher or lower than indicated. Show your calculation.
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Use the standard approximation: error ≈ 4 ft per 1,000 ft above the altimeter-setting source per 1 °C of ISA deviation. The relevant height is above the aerodrome, not above sea level, because QNH is referenced to the aerodrome: 550 ft. ISA temperature at 1,850 ft = 15 − (1.98 × 1.85) ≈ +11.3 °C. Deviation = −25 − 11.3 = −36.3 °C. Error ≈ 4 × 36.3 × (550/1,000) ≈ 80 ft. Cold air is denser, so pressure falls more quickly with height than ISA assumes; at a given pressure the true height is lower than the altimeter indicates. At an indicated 2,400 ft the true altitude is about 2,320 ft, and true height above TDZE is about 500 ft rather than 550 ft. The correction to add is about 80 ft, giving a corrected DA of about 2,480 ft. Round up for operational margin.
2. The first officer suggests that setting QFE would eliminate the error. Is that correct? Explain the roles of QNH, QFE and QNE here.
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No. QFE sets the aerodrome pressure so the altimeter reads height above the aerodrome; this removes the elevation/pressure-reference part of the problem, but it does not remove the temperature error. The instrument still assumes ISA between the aircraft and the setting source, and the column is about 36 °C colder than ISA. In this cold column, indicated height still exceeds true height by roughly the same 80 ft at 550 ft above the aerodrome. QNH is the correct setting for the approach and for terrain clearance because it is referenced to mean sea level. QNE (1013.25 hPa) makes the altimeter read pressure altitude; it is used above the transition altitude and is not a terrain-clearance setting here. QFE can be convenient for reading height, but it is not a cold-temperature correction and must not be used as a substitute for one.
3. What will you do operationally, and what cross-checks reduce the risk?
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Apply a correction of about 80 ft, rounded to 100 ft, to the DA, giving a personal minimum of about 2,500 ft. Brief the correction and the reason for it. Cross-check the barometric height with the radio altimeter, which measures true height above terrain, and with DME/glidepath distance; a radio altimeter showing about 500 ft when the barometric altimeter shows 550 ft confirms the cold-air effect. If company SOP does not authorise cold-temperature corrections for this approach, do not continue below the corrected altitude; consider diverting to a lower-elevation aerodrome. Do not treat QFE as a fix. Note that the error grows at lower temperatures and at higher terrain elevations, so the correction is approximate and should be rounded upward, never downward. Report the actual temperature and any altimeter discrepancy if the approach is continued.