Freezing rain ahead of a warm front
You are flying a non-FIKI, turbocharged single-engine aeroplane at 6,500 ft in stratiform cloud. The OAT is −2°C, and you have accumulated a thin trace of rime ice on the leading edges; pitot heat is on. The area forecast ahead shows a warm front with the 0°C isotherm at 8,000 ft, a +4°C layer at 11,000 ft, and cloud tops at 13,000 ft. Freezing rain is reported below 8,000 ft in the area 30 nm ahead. The aircraft has no deicing boots, no heated windshield, and no alcohol anti-ice; climb performance at 6,500 ft is 250 ft/min. The MEA is 6,000 ft over terrain rising to 4,500 ft. The destination is 45 minutes ahead; the alternate behind is 30 minutes and has a surface temperature of +3°C. A pilot report 20 nm ahead reports severe icing in freezing rain at 7,500 ft.
1. Explain how the warm-front profile produces freezing rain and why it is more hazardous than the trace of rime ice already on the aircraft.
Read the worked answer
The warm layer at 11,000 ft is above 0°C, so snow falling from the cloud tops melts as it passes through it. The drops then fall through the 0°C isotherm at 8,000 ft into subfreezing air at 6,500 ft, where the OAT is −2°C. They remain liquid because they are small and do not have time to freeze, but they freeze on impact with the airframe; that is freezing rain. The trace of rime ice comes from small supercooled droplets in stratiform cloud and is a warning, but freezing rain produces clear ice that can accumulate rapidly, add weight, and destroy lift. The pilot report of severe icing at 7,500 ft confirms that the layer ahead is not a marginal trace. For a non-FIKI aeroplane, freezing rain is a no-go hazard, not something to test for a few minutes.
2. Evaluate the options: climb above 8,000 ft, descend, continue, or turn back.
Read the worked answer
Climbing above 8,000 ft would put the aircraft into the warm layer, but it requires passing through the freezing-rain layer between 6,500 and 8,000 ft. At 250 ft/min that is about six minutes in the worst icing, with no deicing equipment, so the climb is not a safe escape. Descending does not help: below 8,000 ft the air is subfreezing all the way to the surface in this profile, and the MEA is 6,000 ft over terrain rising to 4,500 ft, so there is little room and no warm layer below. Continuing for 45 minutes through reported severe icing is unacceptable. Turning back to the alternate, where the surface temperature is +3°C and the freezing rain is behind, is the only option that removes the hazard rather than trying to outclimb it.
3. State your decision and the key actions you would take immediately.
Read the worked answer
I would turn back to the alternate now, before entering the area of reported freezing rain, and advise ATC of the trace rime and the severe icing report ahead. I would keep pitot heat on, avoid abrupt control inputs, and monitor the leading edges and airspeed; if ice is already present, I would not climb into the freezing rain to 'get above it'. I would request a route that remains in above-freezing air or clear air, and I would be prepared for a change in trim and stall speed if the trace increases. The key judgement is that the warm layer aloft is a trap, not an escape: the safe decision is to reverse course while the aircraft is still west of the freezing-rain area.