Departure Decision with a Marginal Crosswind and a Convective SIGMET
You are the commander of a turboprop on a scheduled sector. Departure aerodrome elevation 1,200 ft AMSL, QNH 1002 hPa, OAT 34 C. Runway 27, 2,400 m long, dry. The latest METAR reads: wind 240/18 gusting 32 kt, visibility 8 km, few CB 1,800 ft, SCT 3,000 ft, temperature 34 C, dewpoint 24 C, QNH 1002. A convective SIGMET is valid for your departure area for embedded CB with tops to FL380 and severe turbulence below FL200. Your performance figures are based on a 2,600 m pressure-altitude runway and a maximum demonstrated crosswind of 25 kt. A line of cells is 25 NM west of the field and moving east-northeast at 20 kt.
1. Assess the crosswind component and the pressure-altitude penalty, and state whether the take-off is within limits.
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Crosswind: Runway 27 (270 magnetic). Wind 240 at mean 18 kt gives an angle of 30 degrees. Crosswind = 18 x sin 30 = 9 kt mean, gusting 32 x sin 30 = 16 kt. Both below the 25 kt demonstrated limit, so crosswind is acceptable but the gust spread (14 kt) will make handling demanding.
Pressure altitude: QNH 1002 is 11 hPa below 1013. At roughly 27 ft/hPa near the surface, that is about +300 ft. Pressure altitude = 1,200 + 300 = 1,500 ft. Temperature ISA deviation: ISA at 1,500 ft is about 13 C, so deviation is +21 C. Density altitude will be substantially higher than 1,500 ft (roughly 1,500 + 120 x 21 = about 4,000 ft). Your performance data were computed for 2,600 m pressure altitude, so the actual 1,500 ft pressure altitude is within the charted range, but you must enter the chart at the correct density altitude, not pressure altitude.
Conclusion: Crosswind and pressure altitude are within limits, but the performance calculation must be redone for density altitude and the gusty, hot conditions.
2. Given the SIGMET and the approaching line of cells, what is the correct go/no-go judgement and what mitigations would you apply?
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The line of cells is 25 NM west, moving east-northeast at 20 kt, so it will reach the field in roughly 1 to 1.5 hours. A convective SIGMET for embedded CB with tops to FL380 and severe turbulence below FL200 means the departure path and climb-out are exposed to severe turbulence and possible hail, lightning and microbursts.
Judgement: a departure is defensible only if you can climb clear of the convective layer before the cells arrive and route away from the line. If your departure route turns east or south, away from the advancing line, and you can be airborne and above FL200 within the window, the risk is manageable. If the route tracks west into the line, or if the delay pushes you into the cell arrival time, the correct decision is to hold on the ground.
Mitigations: (1) obtain an updated forecast and pilot reports; (2) confirm the departure route avoids the line by at least 20 NM from any radar echo; (3) brief the threat of windshear and microburst on final and initial climb; (4) select a take-off alternate and a diversion airfield clear of the system; (5) if departing, use maximum thrust and a reduced flap setting to maximise climb gradient and minimise time in the hazard layer.