As
M(F)S indicates, the certification requirements are very limited.
The details are outlined in the following link with some very good general advice; 'begin the course', skip the intro and see
Aircraft design for certification.
Some of the assumptions surrounding the certification and operation may be in other documents; the following extract I believe came from the UK AIP:
All pilots encountering un-forecast icing are requested to report time, location, level, intensity, icing type and aircraft type to the ATS unit with whom they are in radio contact with. It should be noted that the following icing intensity criteria are reporting definitions; they are not necessarily the same as forecasting definitions because reporting definitions are related to aircraft type and to the ice protection equipment installed and do not involve cloud characteristics. For similar reason, aircraft icing certification criteria might differ from reporting and/or forecasting criteria.
Icing Types
‘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.
Icing Intensity (accumulation)
‘Trace Ice’ - becomes perceptible. Rate of accumulation slightly greater than rate of sublimation. It is not hazardous even though de-icing/anti-icing equipment is not utilized, unless encountered for more than one hour.”
‘Light’ - The rate of accumulation might create a problem if flight in this environment exceeds 1 hour. Occasional use of de-icing/anti-icing equipment removes/prevents accumulation. It does not present a problem if de-icing/anti-icing equipment is used.
‘Moderate’ - The rate of accumulation is such that even short encounters become potentially hazardous and use of de-icing/anti-icing equipment, or diversion is necessary.
‘Severe’ - The rate of accumulation is such that de-icing/anti-icing equipment fails to reduce or control the hazard. Immediate diversion is necessary.
The aircraft AFM icing clearance generally relates to the certification definition. There are few if any absolute regulations or limitations relating to icing conditions, these are deemed operational issues and thus the source of many misinterpretations and errors.
Note that the ice accumulation definitions relate to the capability of aircraft’s de/anti-icing system; thus in-flight reports of icing conditions are most unreliable unless you know the capabilities of the other aircraft and how the crew are using their systems.
Edit: Also see
In Flight Icing Encounter