ILS Approach with Altimeter and Wind Shear Concerns
You are flying a Category C aircraft on an ILS to runway 27. QNH at the aerodrome is 1002 hPa, elevation 200 ft. The outer marker is at 4.0 NM from touchdown on a 3.0° glidepath. Reported surface wind is 240°/25 kt gusting 40 kt; the tower reports a 15 kt loss of airspeed at 500 ft AGL on short final. Your minima are DH 200 ft and RVR 550 m. The local altimeter setting is not available; you set QNH 1002 hPa. OAT at the OM is +8 °C.
1. Calculate the indicated altitude at the outer marker and compare it with the published crossing altitude, accounting for temperature error.
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Glidepath height at 4.0 NM on 3.0° ≈ 4.0 × 6076 × tan3° ≈ 1274 ft above touchdown. Add aerodrome elevation 200 ft → about 1474 ft AMSL. With QNH 1002 hPa (below standard 1013), true altitude is lower than indicated by roughly 30 ft per hPa × 11 hPa ≈ 330 ft if you set 1013; but you set the local QNH, so the altimeter reads aerodrome-relative height correctly at the threshold. Temperature: ISA at 1474 ft is about 15 − 2 × 1.47 ≈ 12 °C; actual +8 °C is 4 °C colder, so true height is lower than indicated by about 4/273 × 1474 ≈ 22 ft. The indicated crossing altitude should be about 1470 ft; expect to be slightly low in true terms. Confirm with DME/OM and do not descend below the glidepath.
2. Assess the wind shear and decide the go-around criteria and configuration.
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Surface wind 240/25G40 gives a crosswind component of 25 sin30° ≈ 12.5 kt gusting 20 kt, within most Category C limits but near the gust limit. The reported 15 kt loss at 500 ft AGL is significant: on a 3° path, an immediate go-around at the first sign of airspeed loss below VREF is mandatory. Set go-around thrust at the decision height if the airspeed trend is negative, and do not attempt to salvage the approach. Use full flaps only if the crosswind and shear allow; otherwise land with a reduced flap setting to improve go-around performance. Brief the missed approach before the OM, and expect a possible wind shear alert on the approach.