Cruise at FL120 over the South China Sea with a Developing Cumulonimbus Line
You are cruising at FL120 in a twin-turboprop from Kota Kinabalu to Brunei, 40 NM from the coast. The weather radar shows a line of echoes 30 NM ahead, tops above FL400, with a gap of 8 NM between two cells. The SIGMET for the area is valid for embedded thunderstorms and severe turbulence below FL200. The aircraft has weather radar but no lightning detector. The outside air temperature is -8°C and the freezing level is at 14,000 ft. Fuel is sufficient for 20 minutes of holding. Your company SOP requires a 10 NM lateral clearance from severe cells.
1. Decide whether to penetrate the 8 NM gap or divert around the line, and justify your choice with reference to the SIGMET, radar, and SOP.
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The SIGMET warns of embedded thunderstorms and severe turbulence below FL200, and the radar shows tops above FL400, so the cells are severe. The 8 NM gap is narrower than the 10 NM lateral clearance required by SOP, and embedded cells may not appear clearly on radar. Penetrating the gap is not acceptable under the SOP. Diverting around the line is the safer option if fuel and airspace permit. The 20 minutes of holding fuel allows a diversion of roughly 60-80 NM at cruise speed, so a lateral diversion around the western end of the line is feasible. Request a deviation from ATC, maintain at least 10 NM from the echoes, and avoid the gap. If the line extends beyond the available fuel range, divert to an alternate airfield.
2. Explain the icing and turbulence risks at FL120 given the temperature and freezing level, and what you would report to ATC.
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At FL120 the OAT is -8°C and the freezing level is 14,000 ft, so the aircraft is in the zone where supercooled water can exist in the updraughts of the cumulonimbus. Icing is possible in the cloud tops and anvil edges, and the aircraft has no de-icing equipment, so avoid cloud penetration. Turbulence is likely to be severe near the cells and in the anvil, and the SIGMET confirms severe turbulence below FL200. Report the position, altitude, and nature of the hazard to ATC so other traffic is warned, and request a deviation. Note the freezing level and temperature in the report. Avoid the anvil, which can extend downwind beyond the visible echo.