Aviation is commonly promoted as the safest form of transport. US airlines caused the deaths of 285 people in 1988 whereas in contrast, 46,730 persons died on US roads that same year (Cosgrove and Condit, 1990). In the period 1980 to 1990 scheduled US air carriers experienced an average of one fatal accident per million departures (a failure frequency of 10-6). However, the machine aspects of the system were performing at a failure rate of around 10-7, since less than 12% of accidents were the result of mechanical systems failures. Failure of the human elements of the aviation system accounted for 88% of fatal accidents (Boeing, 1990).