"WARNING"

Severe icing may result from environmental conditions outside of those for which the aircraft is certificated.

Flight in freezing rain, freezing drizzle, or mixed icing conditions (super cooled liquid water and ice crystals) may result in:

ice build-up on protected surfaces and exceed the capability of the ice protection system, or
ice forming aft of the protected surfaces.

This ice may not be shed using the ice protection systems, and may seriously degrade the performance and controllability of the aircraft.

During flight, severe icing conditions that exceed those for which the aircraft is certificated shall be determined by the visual cues described below. If one or more of these visual cues exists, immediately request priority handling from Air Traffic Control to facilitate a route or an altitude change to exit the icing conditions. The cues are:

unusually extensive ice accumulation on the airframe and windscreen in areas not normally observed to collect ice, and/or
accumulation of ice on the upper surface of the wing aft of the protected area, and/or
accumulation of ice on the engine nacelles and propeller spinners farther aft than normally observed.