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Flight Into Known Icing Conditions: An Update

By Gregory J. Reigel

© March, 2009 All rights reserved.

Although we are, hopefully, finished with the worst of the winter here in the north woods, this also signifies a time of year when airman must exercise additional vigilance regarding the dangers presented by icing conditions. A recent Legal Interpretation issued by the FAA's Office of Chief Counsel attempts to clarify some of the past uncertainty regarding "known icing conditions" and discloses the FAA's current position on the issue.


Before discussing the Legal Interpretation, it is helpful to understand why the FAA issued the Legal Interpretation. Back in 2006, the FAA's Counsel for the Eastern Region issued a legal interpretation defined "known icing conditions" in a manner that would have resulted in severe constraints on when individuals in aircraft without deicing equipment could fly. The Eastern Region stated that "high relative humidity" constitutes known icing conditions, which meant that in high relative humidity conditions when the temperature is near or below freezing, pilots would have to fly an aircraft with deicing equipment, or not fly at all.

Recognizing the problems with this legal interpretation, both in application and in the impact upon general aviation operations, the Aircraft Owners and Pilots Association demanded that the Office of Chief Counsel rescind the Eastern Region's letter of interpretation. Rather than rescinding the letter, on April 7, 2007 the Chief Counsel published a Notice of Draft Letter of Interpretation on Known Icing Conditions in the Federal Register. Later, on September 22, 2008, the Chief Counsel withdrew the Eastern Region's letter of interpretation in its entirety. Subsequently, after considering the comments submitted in response to the Eastern Region's letter and the draft letter of interpretation, on January 16, 2009 the Chief Counsel issued the current Legal Interpretation.

The Legal Interpretation

The Legal Interpretation begins by observing that flight into known ice is not directly referenced in FAR Part 91. (Although FAR 91.527 discusses operating in icing conditions, it only applies to large and turbine multi-engine aircraft.) However, several Part 91 regulations require a pilot to consider the consequences of flying in such conditions. For example,

FAR 91.9(a) states that "no person may operate a civil aircraft without complying with the operating limitations specified in the approved Airplane or Rotorcraft Flight Manual" and many aircraft manuals state that a particular aircraft type is not approved for flight in known icing conditions:

FAR 91.13(a) states that "[n]o person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another" and;

FAR 91.103 specifies that "each pilot in command shall, before beginning a night become familiar with all available information concerning that flight: including weather reports and forecasts for flights under IFR or not in the vicinity of the airport.

The Legal Interpretation then states that "known ice" is the same as "known or observed or detected ice accretion" which is defined in paragraph 7-1-22 of the Aeronautical Information Manual as "[a]ctual ice observed visually to be on the aircraft by the flight crew or identified by on-board sensors." That is, the ice must actually stick to the surface of the aircraft.

Next, the Legal Interpretation distinguishes "known ice" from the term "known icing conditions" that is used in most flight manuals. "Known icing conditions" involve "circumstances where a reasonable pilot would expect a substantial likelihood of ice formation on the aircraft based upon all information available to that pilot." Absent a specific regulation defining "known icing conditions", over the years, National Transportation Safety Board (NTSB) decisions have held that such conditions exist "when a pilot knows or reasonably should know about weather reports in which icing conditions arc reported or forecast." The Chief Counsel observes that the NTSB decisions are "consistent with the FAA's long-held position in enforcement actions that a pilot must consider the reasonable likelihood or encountering ice when operating an aircraft."

As has always been the case, assessments of "known icing conditions" are fact-specific and will depend upon the circumstances in each case. The Legal Interpretation states that "[w]hether a pilot has operated into known icing condition…will depend upon the total information available to the pilot, and his or her proper analysis of that information in evaluating the risk of encountering known icing conditions during a particular operation."

What The Legal Interpretation Means For Pilots

Simply because ice forms on an aircraft during a flight, that fact in and of itself will not necessarily result in enforcement action by the FAA. The FAA will evaluate the pilot's actions, both pre-flight and during the flight, to determine whether they were reasonable in light of FARs 91.9(a), 91.13(a}, and 91.103. The evaluation will specifically include a review of all weather information available to the pilot and will determine whether the pilot's pre-flight planning took into account the possibility of ice formation, alternative courses of action to avoid known icing conditions and, what steps were taken by the pilot to exit known icing conditions once they were encountered.

As a result, pilots must carefully evaluate all of the available meteorological information relevant to a proposed flight. This should include applicable surface observations, temperatures aloft, terminal and area forecasts, AIRMETs. SIGMETs, and pilot reports (PIREPs). If a reasonable and prudent evaluation of all of the information together indicates that the proposed flight will occur under conditions that will cause ice to adhere to the aircraft along the proposed route and altitude of flight, then it is a good bet that "known icing conditions" probably exist and that the FAA would take that position in a subsequent enforcement proceeding.


At the end of the day, this Legal Interpretation does not give pilots a "bright-line" test to use in answering the "known icing conditions" question. However, it makes clear that pilots will still need to evaluate all available information both before and during a flight in order to make a reasonable determination as to whether "known icing conditions" are present along the proposed route of flight. Pilots will also need to be able to prove that they performed this evaluation if they find themselves defending against an alleged violation arising from an icing situation.

The information contained in this web-site is intended for the education and benefit of those visiting the Aero Legal Services site. The information should not be relied upon as advice to help you with your specific issue. Each case is unique and must be analyzed by an attorney licensed to practice in your area with respect to the particular facts and applicable current law before any advice can be given. Sending an e-mail to Aero Legal Services or Gregory J. Reigel does not create an attorney-client relationship. Advice will not be given by e-mail until an attorney-client relationship has been established.

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