Subchapter 9.5 – Weather Deviations and Route Changes

You are the pilot in command. If ATC clears you to proceed through weather you judge to be hazardous, you have the authority (and the duty) to refuse or to request a deviation. ATC is not in the aircraft and cannot assess the actual conditions you are experiencing.

Example: ATC says, "HB-CDA, continue descent to Flight Level 80." But you are in moderate icing and already have visible ice on the windscreen. You should respond: "HB-CDA, unable Flight Level 80 due to moderate icing, request Flight Level 120 to exit icing layer."

ATC will approve your request. They would rather you deviate than you have an accident.