An Airworthiness Directive (AD) affecting operations in ice by Cessna Caravan operators is getting a cold reception from some. An industry source, who spoke on condition of anonymity, told AVweb Friday that a section of the AD requiring pilots to exit icing conditions if they observe a build-up of greater than a quarter of an inch on the wing strut will be challenged by operators. "That seems to be very conservative," the source said. He noted that a quarter of an inch of ice on a strut wouldn't necessarily translate to a similar amount on lifting surfaces on the plane, which has pneumatic boots and is certified for flight into known icing. The FAA issued its latest AD on Caravan icing last Thursday, a month after the NTSB issued safety recommendations on the subject. The AD, which takes effect Friday, requires pilots to get out of icing conditions considered to be anything greater than "light." The FAA says icing exceeds the "light" definition if there's a quarter-inch buildup on the strut, if the aircraft's speed drops by 20 knots at constant power in level flight, if engine power required to maintain speed increases by more than 400 ft. lbs. or if the plane is unable to maintain 120 knots in level flight. Pilots must maintain a minimum speed of 120 knots (flaps up) in icing conditions. All the limitations must be put in the pilot operating handbook and placarded on the instrument panel.