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together with a method of reaching .equivalent conditions..
The result was the well-known Appendix C to FAR 25, which defines the icing conditions to which an aircraft must be submitted prior to certification. Two types of ice accretion rates are required: One is called maximum continuous, the other is called maximum intermittent. They are supposed to cover stratiform and cumuliform clouds. Droplet diameter range is considered up to 50μm.
That rule and its interpretative material implicitly acknowledge the variety of ice shapes and requests aircraft to demonstrate that they can sustain 3 inches of ice accretion (on unprotected parts). These 3 inches are a significant amount, derived from a 45-minute exposure. It was chosen to be an acceptable .worst case. or .envelope case.. The flight conditions allowing to determine the location of the ice accretions on the leading edge profile have also been chosen as the worst ones, as they are closer to the wing upper surface than normal flying conditions would allow. It is clear that such a concept penalizes small aircraft more than large ones.
However, recent accident records show that it was insufficiently protective for commuter class aircraft and remained adequate, or even overprotective for large jets.
A number of commuter accident investigations proved that additive risks exist. These risks are associated with droplet diameters outside the FAR 25 Appendix C envelope, i.e. freezing drizzle from 50 to 500μm or freezing rain above. It is intended to extend the certification envelope. However, the accident investigations also show that improving pilot icing awareness would probably better increase the level of safety.
A2.3 Large jet icing certification
As stated above, large jets are less susceptible to icing problems than smaller ones, like commuters. Although the causes are not fully demonstrated, the following facts can be listed:
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Faster flying speeds. This is a very important factor, not only because of the ram effect, which cuts the icing risk at all OATs > -10°C. Atmospheric physics are such that, almost coincidentally, the statistical water content in supercooled droplets dramatically drops off between -10 and -15°C. Therefore, the potential for icing of faster aircraft is largely reduced.

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3 inches of ice accretion is widely accepted as resulting from a severe icing encounter and is taken as an envelope case. It is physically obvious that has less of an impact on large aircraft performance than on smaller ones.

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As previously stated, the mechanism by which ice deteriorates wing performance goes through deterioration of the boundary layer. Most large aircraft, and all Airbus aircraft have slotted slats, designed to generate a very good boundary layer over the main wing.

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In a given icing condition, a leading edge of a larger radius will pick up less ice than a smaller one.

　
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