Icing Behind the Protected Area: Climb Through the Warm Nose or Turn Back?
You are flying a non-FIKI twin turboprop at FL 120 in a uniform stratiform deck with tops reported at FL 200. OAT is −8 °C. The 1200 sounding shows a frontal inversion with a warm nose of +2 °C between FL 160 and FL 180. The destination TAF, valid for your ETA, contains FZRA with a surface temperature of +1 °C and a freezing level at 3,000 ft. The minimum en-route altitude on your route is 9,000 ft, mostly over high ground. The aircraft has de-icing boots, hot props and a heated windscreen, but no bleed-air-heated wing. You have been picking up light rime for ten minutes; it is now building at about 1 mm per minute on the leading edges, and a small amount has appeared behind the protected area. The nearest suitable diversion is 60 nm south, where the surface temperature is +6 °C and the freezing level is above FL 200. Fuel allows 25 minutes of holding.
1. Diagnose the icing: what type is it, what is causing the build-up rate, and why has ice appeared behind the boots?
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Type and severity. The build-up rate is the giveaway. Light rime normally accumulates at a fraction of a millimetre per minute in a stratiform deck; 1 mm/min is a severe rate and is not consistent with small supercooled droplets alone. Ice appearing behind the protected area is the second clue: the boots can shed a thin rime film easily but are overwhelmed by a rapidly accreting, dense deposit that bridges the boot and freezes downstream of it.
Cause. The warm nose at FL 160–180 plus FZRA at the surface is the classic freezing-rain / supercooled large drop (SLD) profile: snow falls from above the warm nose, melts fully in the +2 °C layer, and then falls into the sub-zero air below. The drops remain liquid down to the surface and are large, so on impact they spread and freeze as clear or mixed ice. In the layer immediately below the 0 °C isotherm the drops are only a fraction of a degree below freezing, which is exactly where the largest supercooled drops and the fastest accretion occur.
The essential point is that this is not a boot-compatible icing environment. The aircraft's icing certification and the boot system are designed for Appendix C droplets; SLD can exceed the protection and run back over unprotected surfaces, and boot cycling can make it worse by opening a channel for the water to migrate aft.
2. Explain the consequence of climbing through FL 140–160 into the air above the warm nose.
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To reach the +2 °C layer you would have to fly through the whole depth of the supercooled layer below it - FL 120 to about FL 160 - which in this profile contains the largest drops and the highest liquid water content, and where the temperature is only a degree or two below freezing. That is the worst possible combination: maximum collection efficiency, maximum accretion rate, and maximum runback.
Above the nose the picture flips again: at, say, FL 190 the temperature is back near −10 °C with smaller droplets, which would normally be a better icing environment - but you must survive the transit to get there. For a non-FIKI aircraft carrying a booted wing and 1 mm/min already accreting, a transit of a few minutes through severe clear icing is not an acceptable risk: the drag and weight penalty, the loss of lift and the possibility of asymmetric shedding (with a roll upset) all develop faster than the transit.
Two further points matter for the answer:
- Climbing does not remove the hazard; it relocates you to the far side of the worst layer.
- The descent option is also blocked by the 9,000 ft MEA over high ground, and descending anyway would take you into colder air with freezing rain continuing to the surface - the SLD column extends all the way down.
So neither vertical escape route is clean, which means the decision must be horizontal.
3. Give your decision as commander, with a specific plan and the reasoning that justifies it.
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Decision: turn off the planned route and leave the icing zone horizontally. Do not climb through the warm nose.
Plan:
- Turn south towards the diversion 60 nm away, where the surface is +6 °C and the freezing level is above FL 200. In that airmass the whole column below you is above 0 °C, so any ice already carried will shed and no further accretion occurs.
- Maintain FL 120 initially - the diversion airspace is clear of high ground and descending inside the current cold air would only keep you in the SLD column.
- Apply boot cycles and power as published; do not reduce power sharply, keep speed up for the current ice load, and avoid configuration changes that increase angle of attack and hence the stall speed.
- Advise ATC of the severe icing and request priority; severe icing reports are a safety-of-flight item and ATC will normally clear the airspace.
- Do not attempt the destination. FZRA with a surface temperature of +1 °C means freezing rain on the runway and a contaminated-surface landing with a potentially significant ice load - a bad combination.
Reasoning: the hazard is SLD, the aircraft is not certified for it, the build-up is faster than the boots can manage, and the vertical escape routes are blocked by terrain and by the structure of the profile. The only remaining dimension is lateral, and 60 nm at cruise speed is well inside the 25 minutes of holding fuel even allowing for a diversion and an approach.