Wellington Approach: SPECI Deterioration and Wind Shear
You are the pilot flying a turboprop into Wellington (NZWN). At 15 nm south at 3000 ft, the ATIS is: NZWN 020700Z 18025G38KT 9999 -SHRA SCT015 BKN025 12/10 Q0998, WS RWY 16. A minute later ATC passes a SPECI: NZWN 020715Z 18030G45KT 4000 SHRA BKN010. The aircraft crosswind limit is 25 kt. Runway 16 is in use; company approach minima are 500 ft ceiling and 3000 m visibility. You have 25 minutes fuel to Palmerston North (NZPM) plus reserve. The airport has a low-level windshear warning and the crew has reported moderate turbulence on final. The aircraft has serviceable weather radar and autopilot.
1. Interpret the METAR, SPECI and windshear warning. What has changed and what is the trend?
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The 0700 METAR gives wind 18025G38KT, visibility 10 km, light showers, SCT015 BKN025, QNH 0998, and a windshear warning for runway 16. The 0715 SPECI gives wind 18030G45KT, visibility 4000 m, showers, and BKN010. The trend is deterioration: mean wind and gusts have increased, visibility has fallen from 10 km to 4 km, and the ceiling has lowered from 2500 ft to 1000 ft. The SPECI is a special observation, not a forecast, so it does not tell you what will happen next; it confirms the current trend is worse. Company minima are 500 ft and 3000 m, so the SPECI still reports conditions above minima: ceiling 1000 ft is above 500 ft, and 4000 m is above 3000 m. The windshear warning and reported moderate turbulence are additional hazards not captured by ceiling and visibility alone.
2. Calculate the crosswind component for runway 16 and assess whether it is within limits.
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Runway 16 is oriented 160° magnetic. The wind is 180°, so the angle between runway heading and wind is 20°. Crosswind = wind speed × sin 20°, and sin 20° is approximately 0.342. For the steady wind of 30 kt: 30 × 0.342 = 10.3 kt. For the gust of 45 kt: 45 × 0.342 = 15.4 kt. Both are below the aircraft crosswind limit of 25 kt. The headwind component is also significant: 30 × cos 20° = 28.2 kt, gusting to 45 × cos 20° = 42.3 kt. So crosswind is within limits, but the strong gust spread and low-level windshear mean the aircraft may experience rapid changes in headwind and crosswind on short final. Crosswind alone does not justify diversion; the shear and turbulence do affect the approach stability.
3. Decide whether to continue, hold or divert, and justify with fuel and minima.
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I would continue toward the approach but set a firm diversion trigger. The current SPECI is above company minima, and the calculated crosswind is within the 25 kt limit. However, the windshear warning, moderate turbulence on final and deteriorating trend mean the approach may not be stable. With 25 minutes fuel to NZPM plus reserve, there is enough for one approach and a diversion, but not for prolonged holding. I would brief a stabilized-approach gate, require a go-around for excessive deviation, and divert to NZPM if the next SPECI shows ceiling below 500 ft, visibility below 3000 m, crosswind above 25 kt, or if windshear is encountered on final. Holding is the least attractive option because the trend is worsening and fuel to the alternate is finite.