||Ice becomes perceptible. Rate of accumulation slightly greater
than sublimation. Deicing/anti-icing equipment is not used unless encountered for
an extended period of time (over 1 hour).
||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. It does not present a problem if this
equipment is used.
||The rate of accumulation is such that even short encounters become
potentially hazardous, and use of deicing/anti-icing equipment or diversion is
||The rate of accumulation is such that deicing/anti-icing equipment
fails to reduce or control the hazard. Immediate diversion is necessary.