Answers for Pilots: Icing

Cold, hard challenge

December 1, 2011

Winter flying

Each winter, AOPA members call the Pilot Information Center with questions about icing, asking, what, exactly, is “known icing”? When can I fly and when am I grounded?

The answer depends to an extent on the kind of deicing equipment an aircraft has. But, since the majority of general aviation aircraft are not equipped for FIKI (flight into known icing), we’ll focus on non-equipped aircraft flying in cold weather.

AOPA has had continuing conversations with the FAA on this issue, and there’s some regulatory history involved that is a source of confusion for many pilots. Here’s a brief summary.

In June, 2006, the FAA issued a letter of interpretation defining the term “known ice” as it relates to flight operations. The letter stated, “Reduced to basic terms, known icing conditions exist when visible moisture or high relative humidity combines with temperatures near or below freezing.” This definition — which added high relative humidity as a new weather condition that constituted known icing — could have grounded general aviation aircraft for the winter season.

This restrictive interpretation was not consistent with other FAA publications, which state that just two of those three conditions — visible moisture and freezing temperatures — are necessary for structural icing in flight. High relative humidity is not visible moisture. AOPA responded with a strongly-worded letter to the FAA stating that high relative humidity was a subjective condition that had no defined value and lacked credible scientific evidence that it contributed to structural icing. AOPA asked FAA to rescind the 2006 interpretation, which FAA did finally do on September 22, 2008.

So, where does that leave us now? In January, 2009, the FAA issued another Letter of Interpretation (currently in effect), which reverted back to the two weather conditions — visible moisture and freezing temperatures — that the AIM and other FAA publications point out as conditions for possible icing. It also expands the interpretation as it applies to an enforcement action, stating, “The FAA does not necessarily consider the mere presence of clouds…at or below freezing temperatures…to constitute known icing.” And further, that ice on an aircraft is not the only factor FAA will consider in an enforcement proceeding. Cases will be considered individually based on pilot preparation and action.

What does this mean to us as we are planning a winter flight? FAA squarely places responsibility on the pilot to evaluate all weather information and decide whether in-flight icing is a possibility on any given day. If an unfortunate pilot does find himself flying an aircraft that’s accruing ice, and it’s witnessed by an FAA inspector on landing, FAA will, in addition to evaluating preflight planning, assess what steps were taken at altitude to fly out of those conditions.

So, for any flight, regardless of the forecast, a well thought out icing escape plan is essential.

The Air Safety Institute has a couple of excellent resources: an online course, “ Weather Wise, Precipitation and Icing,” and a Safety Advisor, “ Aircraft Icing.”

Questions? Give AOPA’s Pilot Information Center a call, 800-USA-AOPA (872-2672). We’ll be glad to talk further with you about this or any other aviation issue.

Kathy Dondzila

Kathy Dondzila | Manager, Technical Communications, Pilot Information Center

Technical Communications Manager, Kathy Dondzila, joined AOPA in 1990 and is an instrument-rated private pilot.