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Patrick Smith has an interesting post about icing, how it affects the airplane and the airlines, and a bit about accidents due to icing.

Snow will not stick to an airplane during flight. Ice, however, is another story. Owing to aerodynamic forces, it tends to adhere to the thinner, lower-profile areas, and not to larger expanses. It will build on the forward edges of the wings and tail, for example, around engine inlets and on various antennae and probes. Left unchecked it can damage engines, throw propeller assemblies off balance, and rob the wings of precious lift. In a worst-case scenario it can induce a full-on aerodynamic stall — the point when a wing essentially ceases to fly.

Airframe ice comes in three basic flavors: rime, clear and mixed. Rime is the common one, appearing as a sort of white fuzz. The rate at which ice accretes is graded from “trace” to “severe.” Severe icing, most commonly encountered when flying through freezing rain, is a killer. But it’s also rare, and tends to exist in thin bands that are easy to avoid or fly out of. On the whole, in-flight icing is considerably more of a threat to smaller noncommercial planes than it is to airliners — jets especially. Even in the heaviest precipitation it is uncommon to see more than a trace amount of rime on a jetliner.

Read the whole thing at Ask the Pilot on Salon.

PIREPs Relating to Airframe Icing AIM 7−1−20 has some good information as well.

b. A pilot can expect icing when flying in visible precipitation, such as rain or cloud droplets, and the temperature is between +02 and −10 degrees Celsius. When icing is detected, a pilot should do one of two things, particularly if the aircraft is not equipped with deicing equipment; get out of the area of precipitation; or go to an altitude where the temperature is above freezing. This “warmer” altitude may not always be a lower altitude. Proper preflight action includes obtaining information on the freezing level and the above freezing levels in precipitation areas. Report icing to ATC, and if operating IFR, request new routing or altitude if icing will be a hazard. Be sure to give the type of aircraft to ATC when reporting icing.

1. Trace. Ice becomes perceptible. Rate of accumulation slightly greater than sublimation. Deicing/anti-icing equipment is not utilized unless encountered for an extended period of time (over 1 hour).

2. Light. The rate of accumulation may create a problem if flight is prolonged in this environment (over 1 hour). Occasional use of deicing/anti-icing equipment removes/prevents accumulation. It does not present a problem if the deicing/anti-icing equipment is used.

3. Moderate. The rate of accumulation is such that even short encounters become potentially hazardous and use of deicing/anti-icing equipment or flight diversion is necessary.

4. Severe. The rate of accumulation is such that ice protection systems fail to remove the accumulation of ice, or ice accumulates in locations not normally prone to icing, such as areas aft of protected surfaces and any other areas identified by the manufacturer. Immediate exit from the condition is necessary.

Rime ice. Rough, milky, opaque ice formed by the instantaneous freezing of small supercooled water droplets.

Clear ice. A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets.

Mixed Ice. Simultaneous appearance or a combination of rime and glaze ice characteristics.

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