ILS Approach in Freezing Rain with Cold-Temperature Altimeter Error
You are conducting an ILS approach to runway 22 at Astana. The ATIS gives: ceiling 200 ft, visibility 800 m, temperature -2 °C, dew point -3 °C, wind 220°/8 kt, QNH 1024 hPa. Freezing rain is reported at the aerodrome. The ILS glidepath is 3.0°, and the decision height is 200 ft. Your aircraft is certified for flight into known icing and has working anti-ice and de-ice systems. On the approach at 1,000 ft, the localiser is centred but the glideslope is one dot low. You must decide whether to continue the approach, what altimeter correction to apply, and when to go around.
1. What does the freezing rain imply for the approach, and how should it affect your decision to continue?
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Freezing rain means supercooled water droplets that freeze on impact with the airframe. It is one of the most severe icing conditions, and it can overwhelm anti-ice and de-ice systems, especially at -2 °C where the liquid water content may be high. Even with certification and working systems, the presence of freezing rain at the aerodrome should prompt a diversion unless the approach can be completed immediately and the runway is confirmed clear of ice. The ceiling of 200 ft and visibility of 800 m are at the decision height limit, leaving little margin for a missed approach. Continuing is defensible only if you have a confirmed alternate with above-freezing conditions and you brief a missed approach before the final approach fix. If there is any sign of airframe ice accumulation, go around and divert.
2. Explain the glideslope indication and the correction you should make at 1,000 ft.
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One dot low on the glideslope at 1,000 ft means the aircraft is below the nominal ILS path. Do not chase the glideslope by pushing over; correct smoothly by reducing the descent rate or levelling briefly, then re-intercept from below. A one-dot deviation at 1,000 ft is about 0.35° of glidepath error, which at that distance may correspond to roughly 50–100 ft below path depending on range. The localiser is centred, so lateral tracking is good. Because the temperature is -2 °C, the altimeter over-reads in cold air: true altitude is lower than indicated. For an approach at these temperatures, apply a cold-temperature correction to the decision height, typically adding about 10–20 ft for 200 ft DH at -2 °C, or use the operator's table. Do not use the uncorrected DH as the sole go-around trigger; use the corrected DH and visual references.
3. At what point must you go around, and what should you do if the runway is not in sight?
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Go around at the corrected decision height, not below it, if the required visual references are not in sight. For a CAT I ILS, the visual references are the runway threshold, touchdown zone markings or runway lights. With a 200 ft ceiling and 800 m visibility, you may see the approach lights before the runway; if you do, you may continue to land only if the required runway visual range is met. If the glideslope becomes unstable, if airframe ice accumulates, or if the localiser deviates, go around immediately. The missed approach must be briefed before the final approach fix: climb on the published missed approach procedure, set climb power, retract gear and flaps on schedule, and inform ATC. Do not attempt a second approach if freezing rain persists; divert to the alternate with warmer conditions. The key judgement is that the approach is not a 'look and see' exercise in severe icing.