Pilots should “activate boots as soon as the airplane enters icing conditions,” according to a safety alert released in December by the NTSB. The alert (SA-014) is yet another attempt by the Board to persuade pilots that there is no such thing as ice bridging and that pilots should not wait for ice to build to one-quarter to one-half-inch thickness before inflating boots in icing conditions. The NTSB has even added the subject to its Most Wanted List of Transportation Safety Improvements, specifically recommending that for currently certified aircraft, the FAA “require that airplanes with pneumatic de-ice boots activate boots as soon as the airplane enters icing conditions.”

The term ice bridging is used to describe a phenomenon in which ice forms a hollow shell over the de-icer boot. Conventional wisdom suggests that if pilots inflate the boots too early, before enough ice builds, instead of breaking off cleanly the ice can create a hollow shell beyond the extension of the inflated boot and the boot can’t break the shell of ice. The FAA and NTSB do not believe that ice bridging exists, and research conducted in a NASA icing wind tunnel has failed to substantiate the claim that ice bridging is a real phenomenon. The FAA held a conference on the subject in November 1997 to address the issue, and the consensus of NASA, FAA, NTSB, boot manufacturers and aircraft operators was that ice bridging is not real. According to the NTSB alert, “The Safety Board has no known cases where ice bridging has caused an incident or accident, and has investigated numerous incidents and accidents involving delayed activation of de-ice boots.” The NTSB does hedge its statements slightly, however, adding, “Ice bridging is extremely rare, if it exists at all.”