What's "Known" about Icing?

Five months ago Luis Gutierrez, Director of Regulatory and Certification Policy for AOPA, asked that the FAA’s Office of the Chief Counsel rescind a letter of interpretation issued by the FAA’s Office of the Regional Counsel, Eastern Region, defining "flight in known icing conditions". The letter of interpretation, dated June 6, 2006, responded to a citizen's request that the FAA clarify when "known ice" exists for purposes of enforcement action. The concern was that the very broad-brush definition then produced could (and probably would) be applied to just about any meteorological day that precipitation or visible moisture (i.e. cloud) was present. It wasn't only non-meteorological, it was illogical and nothing much more than a lawyer's omnibus description of what he'd once seen in a refrigerator ice-tray. The portents of any such description had the probability of impacting upon warranty, insurance liability and post-accident court-cases as well as FAA regulatory action against individuals. It broke new ice. The definition had effectively factored out all the other variables affecting the icing phenomenon (route, altitude, airspeed, aircraft characteristics, synoptic situation, exposure time, ATC input). If strictly observed, many aircraft would only have been able to takeoff for around eight months of the average year. FAA publications - including the Aeronautical Information Manual - state that visible moisture, along with freezing temperatures, is necessary for structural icing in flight. The factor called "high relative humidity" included in the new definition, is not visible. The AOPA pointed out that relative humidity is not included in FAA or National Weather Service aviation weather reports or forecasts.

"So how are pilots to know when high relative humidity would be a factor to their flight," Gutierrez queried the FAA, "and how are pilots expected to ascertain what constitutes high relative humidity, since this is not defined anywhere?" The FAA is now asking for inputs to clarify its own controversial icing definition. For links to the FAA's request, see the 09 Apr 07 Air Safety Week's "Safety News in Brief" column.