This paper examines the quantitative reduction in risk associated with use of a weather briefing. It examines fatal weather accidents (accidents where weather is cited as a cause or a factor) that occurred during the 1964 to 1981 time period and documents statistics showing that pilots of these flights had a lower incidence of use of weather briefings than the pilot population overall. The study also notes that weather accidents represent almost 40 percent of all fatal accidents. They are characterized as being related most often to flight in low ceilings or when fog or rain is present. The types of pilot error in fatal weather accidents include continued visual flight into adverse weather conditions, improper preflight planning, and improper inftight decision making. The study uses Bayesian decision theory to estimate the probability of an accident with and without weather briefings from observable parameters such as the probability of an accident, the probability of use of weather briefings, and the probability that an accident flight had a weather briefing. The results show that a fatal weather accident is about 21/2 to 3 times as likely if a flight did not have a weather briefing. The study also shows how increasing the incidence of use of weather briefings can reduce fatal weather accidents.

Although the safety value of obtaining weather briefing information prior to flying is well recognized by most aviators (1, 2), there has been almost no empirical research into the reduction in risk associated with the presence of such information for a flight. There is a history of interest in improving the dissemination of weather information in the United States (3, 4). But, even though a weather briefing is a regulatory requirement for cross-country flights in the United States (5), some pilots elect to fly without one. The question of how aircraft accident rates would change if more or fewer flights had access to weather briefing information is difficult to examine directly because little information is available about the use of weather briefings for aircraft flights that did not result in an accident. Thus, it has been difficult to develop exposure-based measures of the increased risk of flying without weather information, in conditions where it could have made a difference in the outcome of the flight. Moreover, the absence or presence of weather briefing information is often unknown or not recorded during accident investigations by the National Transportation Safety Board (NTSB) or the Federal Aviation Administration (FAA). This paper explores the value of a weather briefing in general aviation flying in the United States. It first develops data for weather-related accidents. It shows that fatal weather accidents occur under conditions that relate primarily to degraded ceil-