Freezing Rain Over the Ridge
You are planning a 120 NM VFR cross-country in a Cessna 172 with no known-icing equipment. The route crosses a ridge with a minimum safe altitude of 7,500 ft MSL. The forecast freezing level is 6,000 ft, with cloud bases at 5,000 ft and tops at 12,000 ft. A pilot report states moderate rime icing at 8,000 ft along your route. The current OAT at 7,500 ft is -3°C, and there is visible moisture in the form of light rain. The destination is 40 minutes ahead and reports VFR conditions, but the only alternate behind you is 25 minutes back with lower terrain. You have 3.0 hours of fuel and no deicing or anti-icing protection.
1. What icing hazard does this scenario present, and why is it more serious than light rime icing in stratiform cloud?
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The presence of visible rain with a freezing level below the planned altitude means supercooled water is likely, and freezing rain can produce clear or mixed icing rapidly. That is more serious than light rime in stratiform cloud because the liquid water content can be high and the accumulation can be fast. Rime ice tends to build forward on leading edges and is rough; clear ice can spread back over the wing and add significant weight and drag while reducing lift and increasing stall speed. Even a small amount of ice on an aircraft with no protection can degrade climb performance.
The pilot report of moderate rime at 8,000 ft confirms that icing is occurring along the route, not just forecast. The aircraft has no ability to shed ice, so the correct response is to avoid the condition entirely rather than test it.
2. Could you climb above the cloud tops or descend below the freezing level to solve the problem?
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Climbing above the tops at 12,000 ft is not a practical solution in this aircraft. The climb would take time through the icing layer, and the aircraft may not have the performance to reach 12,000 ft with ice already accumulating. Cloud tops can also be higher than forecast. Descending below the freezing level is also problematic because the minimum safe altitude over the ridge is 7,500 ft, which is above the 6,000 ft freezing level. Descending below 6,000 ft would not clear the terrain.
A diversion to lower terrain behind you is the more defensible option. If ice has already begun to accumulate, change altitude or route immediately toward warmer air or clear conditions, and avoid a prolonged climb through the layer. Do not wait for the destination’s VFR report to solve an en-route icing problem.
3. What decision and communication should you make now?
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Turn back or divert to the lower-terrain alternate while you still have a clear margin. Advise ATC of your intentions: “N12345 diverting due to icing, request flight following to [alternate] and a lower altitude when able.” If you are not on a service, make a position report and proceed toward the alternate. Do not continue toward the ridge hoping the ice will remain light; the pilot report and freezing rain indicate a real risk of rapid accumulation.
During the diversion, avoid unnecessary maneuvering, keep speed up above the expected stall speed, and monitor engine instruments and control feel. If icing is already present, report it to ATC so other aircraft receive the warning. The decision point is now, not at the ridge: with no anti-ice equipment, the safest option is to stay out of the icing layer entirely.