Embedded Convective Line, Icing, and a Narrow Gap
A Cessna 210 is planned from Nairobi to Kisumu at 9,500 ft, 1 h 40 min. Forecast: isolated CB tops FL300, freezing level 12,000 ft, surface visibility 8 km in showers, 3,000 ft scattered/broken. A PIREP reports moderate icing in cloud at 12,000–14,000 ft and turbulence at 10,000 ft. The aircraft is not certified for flight into known icing. The route ahead shows a line of CBs 20 NM wide, with a gap 15 NM to the south; weather radar shows the gap may close. Fuel is 2 h 30 min. MSA is 11,500 ft. The pilot is at 9,500 ft, 30 NM from the line. The destination is clear but has no ILS. The alternate is 25 NM back, with a 2,000 m runway and VFR conditions. The pilot sees cloud tops at 15,000 ft and a dark base.
1. Assess the hazards and decide whether the pilot should continue at 9,500 ft, attempt the southern gap, or divert.
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The line of CBs presents turbulence, hail, lightning, strong up- and downdrafts, and embedded cells that may not be visible on radar. The aircraft is not FIKI, and the freezing level is 12,000 ft, so any climb into cloud above 12,000 ft risks icing; the PIREP confirms moderate icing there. MSA is 11,500 ft, so the pilot cannot descend freely below the terrain-safe altitude, and cloud tops at 15,000 ft are above the planned cruise. The southern gap is only 15 NM wide and may be closing. Continuing at 9,500 ft toward the line is not acceptable because it leaves no safe escape route once close. The better decision is to divert to the VFR alternate while fuel and options remain, or to hold well clear and reassess only if a safe, confirmed gap and ATC clearance are available. Pressing through the gap without a confirmed escape is not justified.
2. If the pilot decides to use the southern gap, what conditions would make that option acceptable?
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The gap would need to be visually clear of cloud and precipitation, confirmed by ATC and, if available, radar, with a safe lateral distance of at least 10 NM from the nearest CB. The pilot should remain below the freezing level and clear of the dark base, with an altitude that still satisfies MSA and terrain clearance. ATC should be advised of the intended track and given the option to provide traffic information. The pilot should identify an escape route back to clear air and a nearby alternate, and should set a hard decision point: if the gap closes, if turbulence increases, or if icing begins, turn back immediately. Fuel must allow for the diversion plus reserve. Without those conditions, the gap is not a safe option.
3. What should the pilot do if icing is encountered while trying to remain clear of the CBs?
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The pilot should immediately turn back or change heading toward clear air and, if terrain allows, descend to below the freezing level. Pitot heat should be selected on, and the pilot should avoid abrupt control inputs. The pilot should report the icing to ATC, request a lower level or diversion, and declare an urgency condition if the aircraft cannot maintain altitude or speed. The pilot should not climb into the cloud to escape; that would worsen the icing. If the aircraft is accumulating ice faster than it can be shed, the pilot should land at the nearest suitable aerodrome. After landing, the pilot should inspect the airframe and report the encounter.