Some additional information.
NASA has an icing lab whose homepage can be found here:
http://icebox.grc.nasa.gov/
From the ATR Brochure on icing: Even light ice accretion in the thickness of 1 mm (1/25 th of an inch) may reduce the CL (coefficient of lift) by 0.5. From an aerodynamic side this is half the lift. So keeping up the speeds is the first thing to do.
In the ATR turboprop (22 tons MTOM) this means up to around +40 kt for flap retraction speeds and up to +18 kt on APCH speeds with working de-icing eqipment. And the boots on the ATR are doing a terrific job.
SSG's considerations - nothing to add.
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