That some degradation of aircraft performance would be allowed” (Aircraft Icing Handbook, 2000). There are several important points to note about aircraft icing …

Understanding In-Flight Icing

In recent years there has been growing concern over the issue of aircraft structural icing. It is the cause of approximately 30 fatalities and 14 injuries, on average, per year as well as US $96 million annually in personal injuries and damages in the United States alone (Hallet et al, 2002). In 1997 the Federal Aviation Administration (FAA) released its In-flight Aircraft Icing Plan which contained explicit recommendations for a comprehensive redefinition of aircraft icing certification envelopes (Cober and Isaac, 2002). This recommendation stemmed from the October 1994 crash of an ATR-72 near Roselawn Indiana, which was attributed primarily to severe structural icing. Since 1994 there have been several more serious accidents involving aircraft icing which have further increased motivation to better our understanding of icing conditions so that they can more accurately be characterized, detected and predicted.

1995 saw the first Canadian Freezing Drizzle Experiment (CFDE) conducted out of St. John’s Newfoundland. Subsequent studies in 1996/1997, 1997/1998 and 1999/2000, 2002/2003, 2003/2004 referred to as CFDE II/III and AIRS I/1.5/II, respectively, were conducted out of Ottawa, Ontario with flights over the Montreal, Quebec region during AIRS. All studies employed in-situ measurements using research aircraft like the NRC Convair-580, the NASA-Glenn Twin Otter and the SPEC Lear-25 as well as ground-based remote sensing units such as radar and lidar. The main objectives of AIRS, the most recent study, in order of priority were the following: 1) to improve our ability to remotely sense aircraft icing regions using satellite, aircraft or ground-based systems, 2) to obtain additional data to characterize the icing environment which might be used in a revision of “FAR-25 Appendix C”, the criteria used to certify aircraft for icing conditions, 3) to improve our ability to forecast icing conditions and to understand how these conditions develop, and 4) to obtain measurements of aircraft performance within icing conditions and shapes of accretion that might be used on verification of icing model codes or in wind tunnel studies to simulate icing conditions (Isaac et al., 2001). AIRS was the joint effort between many interested parties who contributed both ground and air based measurement equipment as well as funding for the program.

Recent studies have shown that pilot awareness and understanding about in-flight icing needs improvement. Of particular concern are pilots’ understanding of conditions that cause supercooled liquid water (SLW) to form in the atmosphere, the dynamics of icing and the performance degradation associated with icing encounters. In this paper we will attempt to examine some of these aspects of icing. We will begin with the physics of icing, followed by the dynamics of icing and conclude with flight planning and in-flight strategies. At the end of this paper there are included references which are highly recommended for anyone intending to fly into icing conditions.

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