Surely the categorisation of icing severity HAS to be based on the rate of accumulation and not the ability of a specific aircraft to deal with ice accumulations? I appreciate a pilot report on icing is subjective as in turbulence reports.
Absolutely not.
Icing severity is very much aircraft dependent. Icing severity in flight is a Pilot Report (PiRep) function, and is always subjective.
The amount of ice which accumulates depends on the aircraft structure, type of protection, etc. One airplane may be accumulating very little ice, while an airplane flying along side it may be experiencing severe icing.
Severe icing is that icing which is beyond the capability of the airplane to handle the conditions or the ice. Trace ice to one airplane may be severe to another.
I've operated during weather research flights in conditions which produced 3 inches of ice buildup. If you experienced this in many aircraft you would be unable to remove it, you would experience significant weight and aerodynamic penalties, and might very well consider it severe icing. So long as we could operate in those conditions and our ice protection kept our critical surfaces clean, we kept flying, and in fact sought out the maximum icing rate we could find.
By most accounts, that level of icing would be severe. For us, it was not.
Icing is always subjective. Furthermore, a change in the capability of the airplane may change a moderate icing condition to a severe one with no change in the amount of ice or the rate of accumulation.
I experienced a very significant buildup of ice in a large four engine radial powered airplane some years ago, with large horns or walls of ice extending off the top and bottom of the leading edge, into the slip stream, some eight inches or more. Strange looking twisted tentacles built forward into the slipstream off the propeller domes. The aircraft was controllable, but we evaluated our options. We were in the process of requesting a change and making that change when I began to experience aileron snatch with the controls attempting to jerk to the right. We descended only about 3,000' before much of the ice shed from the wings and the control issue immediately ended.
One could easily say that such a level of ice constitutes a "severe" buildup. It was definitely severe icing, and rapidly exceeded our ability to remove it, and then immediately thereafter, to control the airplane. This took place in a very short period of time. However, it's noteworthy that up until the aileron snatch was experienced, no adverse control effects were observed...quite likely this would have been severe ice much earlier for some airplanes, but this airplane remained controllable much farther into the icing process. We had no intention of seeing how far it would go; the ice buildup occured while we were maneuvering to get clear of the icing conditions. Ice, the rate of buildup, how it builds up, the type of icing, etc, affects different airplanes in different ways.
Ice that is melted off the leading edge of a wing but runs back and refreezes may represent a severe icing condition to that airplane, but not to other airplanes in the same area, experiencing the same conditions.
Icing is very much relative. One cannot make an assumption that any report of icing as trace, light, moderate, etc, represents the true icing conditions that one can expect to experience. Simply because that's what the other guy got...should never be taken on assumption to indicate what you might experience. One man's ceiling is another man's floor, and the same applies to aircraft. What's light to one can easily be severe to another.