Historically, about two-thirds of all general aviation (GA) accidents that occur in instrument meteorological conditions (IMC) are fatal-a rate much higher than the overall fatality rate for GA accidents. The goal of this National Transportation Safety Board study was to better understand the risk factors associated with accidents that occur in weather conditions characterized by IMC or poor visibility ("weather-related accidents").
The study accomplished this goal using the case control methodology, which compared a group of accident flights to a matching group of nonaccident flights to identify patterns of variables that distinguished the two groups from each other. This methodology expands on previous Safety Board efforts that have typically concentrated on summaries of accident cases. The advantage of the case control methodology is that, instead of focusing on the factors that accidents have in common, and possibly being misled by characteristics common to most pilots/flights, it identifies characteristics that set accidents apart and contribute to their occurrence.
For this study, Safety Board air safety investigators (ASI) collected data from 72 GA accidents that occurred between August 2003 and April 2004. When accidents occurred, study managers also contacted pilots of flights that were operating in the vicinity at the time of those accidents for information about their flight activity. A total of 135 nonaccident flights were included in the study. All nonaccident pilots voluntarily consented to interviews and provided information about their flights, their aircraft, and details about their training, experience, and demographics. That information was compared with data that regional ASIs collected about the accident flights as part of their normal investigations. Additionally, the Federal Aviation Administration provided information about pilots' practical and written test results and their previous accident/ incident involvement.
Statistical analyses were used to determine the relationships between study variables and accident/nonaccident status and to identify variables that could be linked to an increased risk of weather-related GA accident involvement. The analysis revealed several pilot- and flight-related factors associated with increased risk of accident involvement, and the recommended approaches to mitigating those risk factors are discussed in the context of three issue areas:
- Ensuring a minimum level of proficiency for all pilots to recognize and safely respond to hazardous weather situations.
- Identifying and providing additional support for pilots whose performance history indicates an increased risk of weather-related accidents.
- Providing GA pilots with additional guidance regarding sources of preflight weather information.
The Safety Board emphasizes that the conclusions reached in this study are not based on a summary of accident cases, although the merits of such Board studies have proven valuable in the past. Rather, the results are based on a statistical comparison of accident and nonaccident flights that allows for the generalization of findings from this study to the wider population of GA pilots and flights that may be at risk for a weatherrelated accident.
The Safety Board wishes to acknowledge the significant contribution to this study by the GA pilot community. All of the pilots contacted voluntarily provided needed information concerning their flight and aviation experience, providing the control group necessary for this study.
As a result of this study, six recommendations were issued to the Federal Aviation Administration.