Crash rates of scheduled commuter and air carrier flights before and after a regulatory change

Introduction: In 1997,in an effort to reduce the crash rate of scheduled commuter flights,the FAA required aircraft with 10-30 passenger seats to operate under stricter rules. Training and other requirements of 14 CFR Part 121 rules were applied to these midsize commuters,which previously had operated under the less strict Part 135 rules. Published crash rates obscured changes related to aircraft size. This research was undertaken to determine whether the rule change affected crash rates of aircraft with 10-30 passenger seats. Method: We determined the number of passenger seats on each Part 135 or Part 121 aircraft that crashed between 1983 and 2007. For aircraft with < 10,10-30,and > 30 seats,we estimated the numbers of departures and crash rates,adjusting for changes in total departures and numbers of in-service aircraft. Results: The Part 135 crash rate tripled in 1997 when commuters with 10-30 seats were excluded,reflecting the administrative change. However,the crash rate of aircraft with 10-30 passenger seats began to decline 4 yr before the rule change; thereafter,their rate was lower than for larger aircraft. The fleet size of aircraft with 10-30 passenger seats increased from 1983 to 1997,then declined as they were replaced with larger aircraft in response to the rule change. Discussion: No effect of the rule change on crash rates of 10-30-seat aircraft was apparent. The decline in their crash rates began before the rule change and may have been related to the 1992 requirement for ground proximity warning devices.