Icing can pose a significant risk to aircraft, affecting the structure, induction system, or instruments. Structural icing on the wings disrupts the airflow and adds weight, reducing lift and raising the stall speed. Icing in the induction system blocks airflow to the engine and may lead to failure. Ice covering the pitot tube or static ports will affect airspeed indicator, the altimeter, or both.   

Icing can occur whenever an aircraft flies through visible moisture (clouds or precipitation) at temperatures 0 degrees C (32 degrees F) or below. Pilots can avoid icing hazards by flying below the freezing level (the lowest altitude in the atmosphere over a given location at which the air temperature reaches 0 degrees C) or flying clear of visible moisture. It is possible to have multiple freezing layers when a temperature inversion occurs above the defined freezing level. Pilots should utilize the Winds and Temperatures Aloft Forecast (FB) as well as graphical Freezing Level charts to determine the lowest freezing level. Icing Forecast charts, Low-Level Significant Weather charts, AIRMETS, SIGMETS, and PIREPs can help pilots avoid areas where icing is likely to occur.

The three types of ice that can accumulate on aircraft during flight are:

Clear Ice — large drops that strike the aircraft surface and freeze slowly

Rime Ice — small drops that strike the aircraft surface and freeze rapidly

Mixed Ice — a combination of the above (supercooled water drops varying in size, intermingled with snow or ice particles, building a rough accumulation)

The intensity of aircraft icing is categorized in the following manner: 

Trace — ice becomes perceptible and the rate of accumulation is slightly greater than the rate of sublimation; de-icing/anti-icing equipment is not used unless encountered for an extended period of time (over 1 hour)

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 or prevents accumulation

Moderate — the rate of accumulation is such that even short encounters become potentially hazardous; use of deicing/anti-icing equipment or diversion is necessary

Severe — the rate of accumulation is such that deicing/anti-icing equipment fails to reduce or control the hazard; immediate diversion is necessary