Icing in a stratocumulus deck: route around, not through
Late January, 09:00Z. You plan LIRF (Roma Fiumicino) to LIPE (Bologna Borgo Panigale), 170 NM, crossing the central Apennines near Perugia. Aircraft: C172, VFR, endurance 3 h 40 min, no de-icing or anti-icing equipment, fixed-pitch propeller.
The briefing gives: a stratocumulus/stratus layer between 7000 and 10500 ft over the Apennines, with moderate rime icing reported inside it by two aircraft; freezing level at 4000 ft; temperature at 9500 ft about -8°C; ridge tops 6000–7000 ft; MSA in the Apennine sector 8500 ft. Visibility below the deck is 10 km or more, and the deck tops are forecast to remain near 10500 ft all morning.
Alternative routing: south of the mountains, over the Tyrrhenian coast at 6500 ft, clear of cloud, adding 45 NM and about 25 minutes. Fuel allows the deviation. On the ground at Roma it is +9°C with a dewpoint of +7°C and showers in the area.
1. Explain where the icing risk sits on this route and what type of ice you would expect, and why neither climbing nor descending is a solution.
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The hazard is the layer itself, not the mountains. Between 7000 and 10500 ft the air is below freezing (freezing level 4000 ft, -8°C at 9500 ft) and contains visible moisture in the form of supercooled droplets. That is the definition of airframe icing, and the -0 to -10°C band is where it accumulates fastest. Two reports of moderate rime inside the same layer raise this from a forecast to an observation.
Rime is the expected type in a stratiform deck: small droplets freeze on impact and build a rough, milky deposit that eats into lift and adds weight. If freezing drizzle were present the droplets would be larger and you would get clear (glaze) ice, which is heavier and faster-acting – the worst version of this scenario.
Why not change level? MSA in the sector is 8500 ft, so you cannot legally go low enough to get below the freezing level (4000 ft) while crossing the ridge. And you cannot go over: the tops are 10500 ft and forecast to stay there, above MSA and above a comfortable VFR altitude for an unpressurised, non-oxygen aeroplane. Climbing into a deck you know is icing is not an escape; it is the hazard.
2. Give your routing decision and explain the residual risks you must manage, including anything that is not airframe icing.
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Take the coastal deviation. The extra 45 NM and 25 minutes cost roughly 25 litres of fuel against 3 h 40 min of endurance, which is nothing; crossing an observed icing layer at MSA in a C172 with no anti-icing equipment is a risk of losing lift, and possibly control, with no way out except a descent that the terrain does not allow. A 25-minute detour for a 170 NM trip is the cheapest decision of the day.
Residual risks. First, the coastal route keeps you at 6500 ft, above the freezing level of 4000 ft, but clear of cloud – so no airframe icing, yet the air is cold and humid. Second, the showers reported at Roma and the +9/+7 spread mean high relative humidity at low level: on descent into Bologna or during any prolonged low-power descent, carburettor icing is a live risk even at well above 0°C, so use carb heat as the flight manual requires and do not lean aggressively. Third, arrival weather at Bologna still has to be checked against VFR minima; the deck may extend onto the plain. And if the coastal route later turns out to be cloud-covered too, the correct action is to turn back rather than accept ice.