The 1997 rule change, published in 1995, was designed to improve the safety of middle-sized aircraft (10–30 passenger seats) by placing them under the stricter Part 121 regulations governing larger aircraft. Published FAA data and showed increases in the rates of Part 135 and Part 121 crashes after 1997, but did not reveal whether there had been any change in the safety of scheduled flights with 10–30 seats, the subject of the 1997 rule change. Results of these analyses illustrate the big difference between published rates based upon the applicable regulatory Part and rates based upon the number of passenger seats. Since the regulations differentiate on the basis of number of seats, the latter calculations are necessary to discern any actual effect of a rule change that was based upon passenger seats.
The 1997 regulatory change expanded the applicability of Part 121 regulations to include scheduled passenger operations involving medium-sized (10–30 passenger seat) “commuter” aircraft, previously governed by scheduled Part 135 regulations. The immediate effects on published crash rates were a sudden tripling of the Part 135 rate (in part because of shifting safer operations to Part 121) and a slight increase in Part 121 rates (which now included a group of operations with higher rates than traditional Part 121 operations). Overall, the rate for all scheduled operations combined also increased. Only by distinguishing among aircraft of various sizes, however, was it possible to examine trends in crash rates of aircraft with 10–30 seats.
When scheduled flights were categorized by the number of passenger seats rather than by FAA operational requirements (Part 135 vs. Part 121), it became evident that beginning in 1994—3 yr before the rule took effect—crash rates of flights with 10–30 seats (the subject of the 1997 rule change) were even lower than crash rates of flights with more than 30 seats. Nor did these midsized commuters experience a change in crash rates after 1997: their average crash rate for the first 5 yr after the rule change was the same as for the 4 yr prior to the change. Crash rates of all three sizes of aircraft were lower in 2003–07 than in 1998–2002; the 10–30-seat commuters showed the least change between these two periods.
Reasons for the improved safety prior to 1997 of scheduled commuters with 10–30 seats probably include the 1992 requirement that all turbine-powered aircraft with 10 or more seats must be equipped with approved ground proximity warning systems to prevent their all-too-prevalent controlled-flight-into-terrain crashes (13
). Subsequent to 1997, the rule’s requirements for improved training may have contributed to these midsize commuters maintaining a low crash rate. Although other aspects of the rule may also have contributed, the initial intent to impose stricter requirements for flight and duty time has never been implemented (14
The reduction in scheduled Part 135 crash rates both before and after the rule change is in contrast to the lack of a consistent reduction in mishap rates of Part 121 flights, which did not change markedly during 1983–2003 (4
) [we generally use the term ‘mishap’ rather than ‘crash’ in connection with Part 121 because a large proportion of Part 121 adverse events are injuries resulting from turbulence, rather than crashes (4
)]. Other research, however, has shown that certain types of Part 121 mishaps did change in their incidence rates: in particular, the rates of mishaps on the ground increased, especially those in the ramp area or while taxiing (2
After 2000, crash rates of scheduled Part 135 operations involving the smallest scheduled aircraft—those with < 10 passenger seats—declined dramatically (, ). This decline was not a result of the rule change, which did not affect the smaller aircraft now comprising all of the Part 135 operations. Post-1997 rates for these smallest commuters are actual (not estimated) because the numbers of both crashes and departures were known. Since 1997, 91% of these small-commuter crashes have occurred in Alaska, where major efforts have been made recently to reduce crashes. Among such efforts by the FAA is the Alaska Capstone project, which provides pilots with information on weather, terrain, and air traffic and permits trained pilots to fly safely at lower altitudes using GPS (6
). An additional measure was the installation of live remote cameras in mountain pass locations to provide weather information (5
). The very high crash rates of scheduled commuters, which prior to 1997 combined small with midsize (10–30-seat) commuters, may have contributed to the FAA decision to subject midsize commuters to Part 121 regulations.
Limitations of this study include the small numbers of crashes, which make the changes in trends unstable. To counteract this problem, data for 5-yr periods were combined. Another limitation is the need to estimate the frequency of departures in relation to the number of seats, which was accomplished by applying percentages based upon changes in the fleet of in-service aircraft with 10–30 seats, and changes in the number of departures in 1998 vs. 1996, which presumably represented departures of 10–30-seat commuters. Estimates further from 1997—in the 1980s, for example—may be less valid than those closer to 1997.
As illustrated in , the fleet size of 10–30-seat commuters changed considerably during the study period. Prior to 1997, commuter service using this size of aircraft was increasing. The decline in the number of in-service aircraft with 10–30 seats after the rule change is worthy of note because it reflects airline decisions to operate with larger aircraft once the incentive of less-strict Part 135 regulations was no longer pertinent. Researchers should be alert for this type of phenomenon, which may influence the effect of other regulatory changes.
The practice of combining published crash rates of scheduled commuters having 10–30 seats with the very high rates of commuters with < 10 seats contributed to the appearance that these midsize aircraft had high crash rates prior to the regulatory change that took effect in 1997. The major strength of the study is the use of data on the number of passenger seats to divide the aircraft into three meaningful categories based on the number of seats, thereby making it feasible to evaluate the possible safety effect of a rule that otherwise could not be analyzed.
A major implication of this research is that the safety of Part 135 commuters (i.e., the smallest commuters) deserves continued attention. The recent attention to air safety in Alaska is promising. For all three size categories of scheduled flights, the downward trend in recent years is encouraging. While the trend may not specifically reflect changes due to the 1997 rule change, the continued attention to aviation safety by the NTSB, the FAA, and the aviation community appears to be bearing fruit.