Icing and embedded thunderstorms on a mountain route
You are planning a VFR flight from Tehran Mehrabad to Tabriz at 11,500 ft in a PA-28 with three persons on board. Fuel endurance is 4 h 00 min and the planned trip is 2 h 10 min. The Tabriz TAF reports wind 240/18G30 kt, visibility 4 km, thunderstorms with rain, scattered cloud at 2,500 ft, broken at 4,000 ft, temporarily 2 km visibility in heavy thunderstorms and broken cloud at 2,000 ft. The freezing level is 9,000 ft. A pilot report from the route gives moderate icing between 10,000 and 12,000 ft and embedded cumulonimbus. Your alternate is 45 min away. Your briefing requires a 45 min reserve. The aircraft is not certified for flight into known icing.
1. Identify the hazards for your planned 11,500 ft VFR flight and explain how each affects the go/no-go decision.
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At 11,500 ft you are above the 9,000 ft freezing level, so any cloud or precipitation contains supercooled water and the pilot report confirms moderate icing between 10,000 and 12,000 ft. The aircraft is not certified for known icing, so this alone is a strong reason not to plan that altitude. Embedded cumulonimbus means severe turbulence, hail, lightning and violent up- and downdraughts may be hidden inside cloud; VFR cannot guarantee avoiding them. The Tabriz TAF shows thunderstorms, gusting to 30 kt, reduced visibility and temporary heavy thunderstorm conditions, so the destination may be below VFR minima on arrival. The safest decision is to delay, change route or change altitude rather than depart as planned. If the flight proceeds, it must remain clear of cloud and CB by a wide margin, which may not be possible in the forecast environment.
2. Using the fuel figures, assess whether the flight can be made legally and safely with the destination forecast. What alternate and fuel plan would you require?
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The basic fuel plan is 2 h 10 min trip + 45 min alternate + 45 min reserve = 3 h 40 min, leaving only 20 min of the 4 h endurance as extra. That is legal on paper but thin for a destination with thunderstorms, gusts and a temporary deterioration to 2 km visibility. With convective weather, holding, diversion or a second approach is likely, so the 20 min contingency is not enough. A safer plan would require either a closer alternate with confirmed VFR conditions, additional fuel for at least 30-45 min of holding, or a delay until the thunderstorms clear. If no alternate has reliable VFR weather, the flight should not depart. The destination forecast makes an alternate mandatory, and the fuel plan must cover the possibility that the alternate itself becomes unavailable.
3. If you are already en route at 11,000 ft and encounter trace icing with a CB ahead, what actions do you take and why?
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Treat trace icing as a warning, not a minor nuisance. Turn at least 20 NM away from the CB and avoid the anvil and any dark, smooth cloud; do not attempt to fly over or under a CB in a light aircraft. If terrain and minimum safe altitude allow, descend toward warmer air below the freezing level, for example 8,000 ft, to stop ice accumulation. If terrain prevents descent, turn back or divert to a route with clear air. Advise ATC of the icing and your deviation. Continue to monitor ice buildup, RPM, airspeed and pitot/static instruments. If the ice is not stopping, or if climb performance and airspeed decay, declare an emergency and land as soon as possible. The key judgement is that known icing and embedded CBs are not conditions to penetrate in a non-certified aircraft.