Icing band on descent into Dunedin
In July you are flying a Cessna 172 from Christchurch to Dunedin, descending in VMC beneath a solid stratiform deck in continuous moderate rain. At 8,000 ft the outside air temperature is -3 C; at 6,000 ft it is 0 C; the surface at Dunedin is +5 C. The area forecast puts the freezing level at 7,000 ft and warns of moderate icing in cloud and precipitation between 6,000 and 9,000 ft. You have carburettor heat and a carburettor temperature gauge, but no airframe de-icing and no ice-detection equipment. The Dunedin METAR reads 220/15KT 9999 -RA BKN020 05/04 Q1004. Your nominated alternate, Alexandra, 35 nm inland, is clear with a surface temperature of -1 C. You have 1 h 30 min of fuel and Dunedin is 18 minutes ahead.
1. Identify where the icing risk is greatest on this descent and explain why level flight would be worse than continuing down.
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The risk band is where the temperature is between about 0 and -5 C in visible moisture. Here that is roughly 7,000 ft down to 5,500 ft: the freezing level is 7,000 ft and the 6,000 ft temperature is 0 C, and you are flying through continuous moderate rain, so supercooled water is present. Continuous rain from a stable layer is the signature of supercooled large droplets, which produce clear ice that can build quickly on the leading edges and tailplane and is far more dangerous than rime.
Level flight at, say, 6,500 ft would hold you in the middle of that band in the rain indefinitely, allowing ice to accumulate until you cannot maintain height. Descending through the band is the escape: the exposure is limited to the few minutes it takes to reach 5,000 ft, where the temperature is about +3 C and any ice will melt. The correct plan is a deliberate, continuous descent in VMC, keeping clear of cloud, at a speed that preserves margin (VNO or below), with carburettor heat applied and the descent briefed before entering the band.
The one thing you must not do is level off in the rain to think about it, or climb back into the colder air above.
2. What indications would tell you that airframe icing or carburettor icing is occurring, and what would you do?
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Airframe icing: a white or clear line building on the leading edges, ice on the wing struts, windscreen and pitot; a change in the sound of the aircraft; a gradual loss of airspeed for a fixed power setting; reduced climb performance; and, most importantly, a creeping increase in the stall speed with a possible tailplane stall if ice collects behind the wing. Vibration or a rumbling noise can mean ice is shedding from the propeller.
Carburettor icing: an unexplained RPM drop on a fixed-pitch engine, rough running, and a carburettor temperature gauge sitting in the caution range. It can occur at ambient temperatures well above freezing because the venturi and throttle produce a 20-30 C temperature drop, and a descent at reduced power is the worst case.
Actions: apply full carburettor heat and expect a further RPM drop and roughness as the ice melts before recovery; do not reduce power further; enrich the mixture. For airframe icing, change altitude - here, continue the descent out of the band - reduce angle of attack, avoid abrupt manoeuvres, and land as soon as practicable. If the ice does not stop accumulating, the correct decision is to land rather than to press on.
3. Assess the alternate, Alexandra, and decide whether it is a genuine option.
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Alexandra looks attractive on paper because it is clear, but it is a trap. A surface temperature of -1 C in a Central Otago basin after recent precipitation means freezing conditions at the surface: freezing fog, ice on the runway and taxiway, and a cold pool that persists well into the day. Any residual moisture aloft will fall as freezing rain into that basin, so the alternate carries the very hazard you are trying to escape.
There is a second problem. Alexandra is 35 nm inland, which means climbing back into the 6,000-9,000 ft band where the forecast warns of moderate icing in cloud and precipitation, in an aircraft with no de-icing. You would be trading a descent out of the icing band for a climb into it, followed by an arrival at a sub-zero, possibly contaminated airfield.
Decision: continue to Dunedin, which is at sea level with a temperature of +5 C, above freezing, with 10 km visibility and a 2,000 ft broken layer - a straightforward VFR arrival. Land there and wait for conditions to improve. Alexandra is only a valid alternate if Dunedin goes below minima while Alexandra is reporting a positive temperature and clear conditions, and even then it must be flown with a plan to avoid the icing band.