The NTSB issued a Safety Alert last week reminding pilots to be wary of icing hazards, especially in regard to the operation of de-icing boots. "This Safety Alert, directed to the pilot community, is intended to increase the visibility of airplane icing issues and address procedures taught regarding the accumulation of ice before activating de-ice boots," said NTSB Acting Chairman Mark Rosenker. The new safety alert notes that as little as one-quarter-inch of ice accumulation on the wing leading edge can increase the stall speed by 25 to 40 knots, and cause "sudden departure from controlled flight" at normal approach speeds. Early activation of the de-ice boots limits the effects of leading-edge ice and improves the operating safety margin, the NTSB said. The alert contradicts some of the training that has been standard for 60 years. "Pilots have been taught to wait for a prescribed accumulation of leading-edge ice before activating the de-ice boots because of the believed threat of ice bridging," the NTSB said. However, ice bridging has never been implicated as the cause of an accident, and is extremely rare, and may not exist at all. Yet the delayed activation of boots has been noted in "numerous incidents and accidents" investigated by the NTSB. To read the full text of the Safety Alert (PDF), click here.
Also, NTSB investigator Todd Gunther wrote an article for Professional Pilot about this topic, which is posted online.