Known icing conditions means either:
- Icing is forecast at that level
- The temperature is in the icing band (0 to -15C) and there is visible moisture.
Icing is divided for certification purposes into light, moderate and severe depending mainly upon rate of acretion.
Light icing forms slowly and can be broken off critical parts of the airframe (leading edges and propellors) using boots. TKS can be used to prevent it forming and is more effective when used to prevent than cure. Any ice that forms on unprotected parts of the airframe will just sit there until conditions change (typically in the decscent) and can have a considerable impact on performance.
De-iced types can be flown indefinitely in light icing, though typically it will cost 10-25kts of airspeed.
Some non de-iced types will fly quite happily for some time in light icing conditions, others (those with more critical aerofoil design) suffer quite quickly. I will not specify which types fly quite happily with ice on or I will be crucified, burned and eaten alive by those who accuse me of sending others to their certain deaths.
Moderate icing forms more quickly and is very threatening to any non de-iced type and is hard work even with de-icing. I have had medium twins (eg C404, Chieftain) running at maximum continuous power and only just over the stall when exposed to moderate icing for more than a few minutes. Moderate icing may eventually flow back to freeze control surfaces, and is not great.
I have only (thank God) encountered severe icing once in thirty years and thousands of hours. It is quite petrifying. It happens mostly inside CBs and under warm fronts (freezing rain) and has to be seen to be believed. I executed a steep 180 when I encountered it, and was probably only exposed for 20 secs. I reckon another 20 secs and the aircraft would have been unflyable and uncontrollable. In those 20 secs there must have been two inches of ice on the leading edges and it had flowed back to a depth of about an inch to about half the chord of the wing. The windscreen went opaque instantaneously. De-icing equipment would make little difference when faced with those conditions. The only solution is to clear the conditions very quickly.
Timothy