Maintenance and inspection as risk factors in helicopter accidents: Analysis and recommendations

Abstract

In this work, we establish that maintenance and inspection are a risk factor in helicopter accidents. Between 2005 and 2015, flawed maintenance and inspection were causal factors in 14% to 21% of helicopter accidents in the U.S. civil fleet. For these maintenance-related accidents, we examined the incubation time from when the maintenance error was committed to the time when it resulted in an accident. We found a significant clustering of maintenance accidents within a short number of flight-hours after maintenance was performed. Of these accidents, 31% of these accidents occurred within the first 10 flight-hours. This is reminiscent of infant mortality in reliability engineering, and we characterized it as maintenance error infant mortality. The last quartile of maintenance-related accidents occurred after 60 flight-hours following maintenance and inspection. We then examined the "physics of failures" underlying maintenance-related accidents and analyzed the prevalence of different types of maintenance errors in helicopter accidents. We found, for instance, that the improper or incomplete (re)assembly or installation of a part category accounted for the majority of maintenance errors with 57% of such cases, and within this category, the incorrect torquing of the B-nut and incomplete assembly of critical linkages were the most prevalent maintenance errors. We also found that within the failure to perform a required preventive maintenance and inspection task category, the majority of the maintenance programs were not executed in compliance with federal regulations, nor with the manufacturer maintenance plan. Maintenance-related accidents are particularly hurtful for the rotorcraft community, and they can be eliminated. This is a reachable objective when technical competence meets organizational proficiency and the collective will of all the stakeholders in this community. We conclude with a set of recommendations based on our findings, which borrow from the ideas underlying the defense-in-depth safety principle to address this disquieting problem.

Fig 1. Distribution of flight-hours between helicopter maintenance / inspection and accident (n = 698).

Fig 2. Classification of helicopter accidents by root cause (n = 698).

Fig 3. Flight-hours between maintenance/inspection and maintenance accidents (n = 98).

One case occurred after 200 flight-hours and is not shown for visual clarity.

Fig 4. Type of maintenance / inspection preceding a maintenance accident (n = 98).

Fig 5. Classification and prevalence of helicopter maintenance errors.

Fig 6. Sub-classification and prevalence of helicopter maintenance errors under the “Incorrect torquing or incomplete assembly of nuts, bolts, cutter pins, or safety wires”.

Fig 7. Illustration of the defense-in-depth safety principle, along with a hypothetical accident sequence (the accident occurs as a result of the absence, inadequacy, or breach of various safety barriers).

Fig 8. Flight-hours between maintenance/inspection and accidents (n = 698).