11.2 A STATISTICAL REVIEW OF AVIATION AIRFRAME ICING ACCIDENTS IN THE U.S. Kevin R. Petty* and Carol D. J. Floyd National Transportation Safety Board, Washington, DC 1. INTRODUCTION For years, airframe icing has been recognized as a Past research has documented the hazards of significant aviation hazard. Icing encounters can lead to aircraft icing by identifying icing related accidents in the increased aerodynamic drag and weight, along with a late 1970s and 80s (Cole and Sand 1991). The study reduction in lift and thrust. Together, these factors result herein attempts to provide contemporary statistics on in a higher stall speed and degradation in overall aircraft airframe icing accidents by examining events that took performance. To maintain altitude and counter the place from 1982 to 2000. Although airframe icing effects of drag during flight in icing conditions, the angle accidents only accounted for a small percentage of the of attack is generally increased and power is applied to total aviation accidents, they resulted in 583 accidents the engine(s). This can further expose unprotected and more than 800 fatalities during the 19-year period. regions of the aircraft to ice accretions. If exposure is prolonged, the aircraft will lose the ability to continue 2. DATA AND METHODOLOGY stable flight. Of equal importance is ice that accumulates on The National Transportation Safety Board maintains aircraft surfaces prior to takeoff. One of the first jet air a database of civil aviation accidents and incidents. An transport category accidents linked to airframe icing occurrence is defined as an accident when the occurred on December 27, 1968. A Douglas DC-9, operation of an aircraft, with the intent of flight, results in operated by Ozark Air Lines, Inc., crashed shortly after substantial damage to the aircraft or death or serious takeoff. In this case, the aircraft suffered substantial injury to any person. In contrast, an incident is an performance penalties when it was subjected to freezing occurrence that influences the safety of an aircraft’s drizzle before takeoff. operation, but does not meet the criteria for an accident Considerable progress has been made in (49 Code of Federal Regulations (CFR) 830.2). The understanding the meteorological conditions associated NTSB database is composed mainly of accidents and with airframe icing (Sand et al. 1984; Cober et al. 1995; contains over 650 fields for each accident record, Bernstein and McDonough 2000; Politovich and including information regarding the aircraft, Bernstein 2001). A substantial amount of interest and environment, crew, injuries, and phase of flight. The research into icing, with attention to supercooled large database was used to identify accidents in which droplets (SLD), was generated when an ATR-72 was airframe icing was either a cause or factor during the destroyed after it experienced an uncommanded 19-year period from 1982-2000, with a factor defined as departure from controlled flight and crashed near any condition or situation that played a role in the cause Roselawn, Indiana (1994). A ridge of ice that accreted of the accident. This period was chosen to exclude the behind the deice boots contributed to an unanticipated influence of September 11 and to ensure data aileron hinge moment reversal and an abrupt loss of continuity. control. The accident raised awareness about the For portions of this study, accidents were stratified hazards of operating in SLD conditions, which are not into their respective segments of operation, which accounted for in 14 Code of Federal Regulations (CFR) include general aviation (GA) and 14 CFR Parts 135 Part 25, Appendix C. Because supercooled large and 121. The majority of general aviation flights are droplets can run back and freeze on surfaces behind an personal and recreational in nature; however, some airplane’s deicing boots, it is extremely hazardous. flights are conducted with the intent of generating In recent years, icing research has translated into revenue. In general, GA constitutes civil aviation applied technologies aimed at diagnosing and operations not covered under 14 CFR Parts 135 and forecasting icing hazards for both ground and in-flight 121. Part 135 generally refers to commuter airlines (i.e. aviation operations (McDonough and Bernstein 1999; scheduled) and air taxis (i.e. nonscheduled), and Part Rasmussen et al. 2001; McDonough et al. 2004). 121 normally references major airlines and cargo Continued development and improvement of such carriers1. Because of distinct operating characteristics technologies, along with training initiatives, will aid in within the Part 135 segment of operations, further reducing the number of icing related accidents. segregation into schedule and nonscheduled operations was performed. The regulatory differences between *Corresponding author address: Kevin R. Petty, 1 Prior to March 1997, scheduled aircraft with 30 or more seats fell National Transportation Safety Board, Office of Aviation under Part 121, while those with less than 30 were considered Part Safety (AS-30), 490 L’Enfant Plaza East, S.W., 135. Because of regulatory changes, Part 121 now includes all aircraft Washington, DC 20594-2000; email: [email protected] with 10 or more seats; thus, some commuters once regarded as Part 135 are now considered Part 121. scheduled and nonscheduled Part 135 operations is beyond the scope of this paper, but detailed definitions Airframe Icing Fatal Accidents (1982-2000) 20 pertaining to these sectors of operation can be found in 14 CFR 119.3. 18 GA 16 Par t135 s Par t121 3. FINDINGS nt 14 de i c 12 c A 3.1 Annual Accidents, Fatal Accidents, and 10 of r Fatalities e 8 b m 6 u N Figure 1 displays all airframe icing accidents from 4 1982 to 2000. The accidents have been grouped into 2 the three predominant segments of aviation operations: 0 GA, Part 135, and Part 121. It is clearly evident that GA accidents dominate the total number of accidents during 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Year the period. GA accidents were responsible for 80.6% of all airframe icing accidents, while Part 135 and Part 121 Figure 2. Fatal airframe icing accidents from 1982- accounted for 17.6% and 1.7%, respectively. However, 2000. the annual GA accident rate significantly declined during the same period. The number of general aviation accidents dropped from a high of 49 in 1982 to 17 in Airframe Icing Accident Fatalities (1982-2000) 2000. 90 . 80 GA Part135 70 Airframe Icing Accidents (1982-2000) s Part121 e 60 ti i 60 l ta GA a 50 F 50 Part135 f o r s 40 t Part121 e n b e 40 m d 30 i u c N c 20 A f 30 o r 10 e 20 mb 0 u N 10 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 Year 0 Figure 3. Fatalities resulting from airframe icing 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 accidents for the period 1982-2000. 198 198 198 198 198 198 198 198 199 199 199 199 199 199 199 199 199 199 200 Year Figure 1. Aviation accidents associated with 3.2 Seasonal Distribution airframe icing for the period 1982-2000. Eighty-one percent of all airframe icing accidents took A graph depicting the number of fatal airframe icing place between the beginning of October and the end of accidents is provided in Figure 2. There were one or March. Figure 4 presents the percentage of icing more fatalities in 47% of the GA accidents, while only accidents by month, and it shows that the largest 26% of Part 135 accidents were fatal. Six out of the ten percentage of accidents happened in January. The Part 121 accidents during the period were fatal monthly distribution of accidents is well-correlated with accidents. the frequency of freezing precipitation and ice pellets in Airframe icing accidents led to 819 deaths spanning the U.S. Cortinas et al. (2004) found freezing the 19-year period reviewed in this study. As one might precipitation and ice pellets occur most frequently in the anticipate, GA accidents were responsible for the U.S. and Canada during December, January, and largest number of fatalities (522). The observed annual February. However, they noted that Arctic coastal decrease in fatalities within the GA segment of regions experience these precipitation types mostly operation is directly correlated with the decline in the between May and October. Utilizing pilot reports and overall number of GA accidents (Figure 3). Peaks in the measurements from research aircraft, Bernstein and number of fatalities linked to Part 135 and 121 McDonough (2000) found a significant relationship operations are related to some very notable U.S. icing between lower atmospheric SLD conditions and surface accidents, including Air Florida flight 90 (Washington, observations of freezing precipitation and ice pellets. DC 1982), USAir flight 405 (Flushing, NY 1992), Comair General aviation and Part 135 aircraft traditionally fly at flight 3272 (Monroe, MI 1997), and American Eagle lower altitudes and at slower speeds than air transport flight 4184 (Roselawn, IN 1994). category aircraft; as a result, they are more likely to encounter icing conditions, including SLD environments. Although it is evident that icing accidents are more approach, adjustments are made to the configuration of likely to occur in the winter months, it should be noted the airplane (increase flap setting, reduce speed, etc.). that icing accidents do occur throughout the year. When ice is present, configuration changes can cause Contrary to what might be expected, none of the an airplane to stall without warning. summer accidents were located in Alaska.