1 Department of Emergency Medicine, Johns Hopkins School of Medicine, Baltimore, MD.
2 Center for Injury Research and Policy, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD.
3 Department of Mental Health, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD.
Received for publication August 29, 2002; accepted for publication November 27, 2002.
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ABSTRACT |
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accidents, aviation; aging; aviation; cohort studies; safety; wounds and injuries
Abbreviations: Abbreviations: CFR, Code of Federal Regulations; FAA, Federal Aviation Administration; NTSB, National Transportation Safety Board.
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INTRODUCTION |
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The effects of aging on cognitive functions and piloting skills have been studied extensively (9, 10). The literature shows consistently that whereas domain-independent cognitive functions such as sensory, perceptual, and psychomotor skills deteriorate progressively with age, performance in most flight-related tasks such as decision-making, tracking, takeoff, and landing does not differ significantly between older and younger pilots (1113). One of the few flight-related tasks with age-related declines is the ability to respond to communication. Specifically, older pilots are found to be less accurate than younger pilots in reading back and executing tape-recorded air traffic control commands, especially long and fast speech commands (14, 15). The age-related decline in performing communication tasks is attributed to decreased working memory capacity in the elderly (14). It is evident that flight experience can eliminate age-related deficits in the performance of certain communication tasks (e.g., repeating heading commands) and mitigate the adverse effects of aging on the performance of less domain-relevant tasks (1619). Most of these studies, however, were conducted under controlled laboratory conditions using flight simulators and were cross-sectional in essence. Therefore, their value for assessing age-related differences in risk of crash involvement is limited.
The fundamental question concerning the controversy of the "age-60 rule" is whether pilots risk of crash involvement changes significantly in the process of aging. Answering this question requires analyzing prospectively collected exposure and safety record data for a large sample of pilots. Previous studies examining the relation between pilot age and risk of involvement in general aviation crashes have reported contradictory results. Mohler et al. (20) found that the crash rate per capita for pilots aged 60 years or older was similar to rates for younger pilots, whereas Booze (21) reported that the per-capita crash rate for pilots aged 70 years or older was four times the rate for those younger than age 20. Findings from these studies were limited by the cross-sectional design and by the lack of individual-level data on exposure to flight (22). In this study, which was based on longitudinal data, we aimed to assess the associations of age and flight experience with risk of crash involvement in a birth cohort of professional pilots.
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MATERIALS AND METHODS |
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There were approximately 65,000 pilots flying 14 CFR Part 135 operations in 1987, according to the FAAs Airman Information System (24). Eligible for this study were pilots who met the following criteria in 1987: 1) holding a Class I medical certificate; 2) being employed by one of the 14 CFR Part 135 airlines identified from the FAAs list of airline employers; 3) being 4554 years of age; 4) flying for business or both business and pleasure; 5) listing "pilot" or "commercial pilot" as ones occupation; 6) having 20 or more hours of flight time in the previous 6 months; and 7) having 500 or more hours of total flight time. These criteria were used to create a birth cohort of active commuter air carrier and air taxi pilots. A total of 3,306 pilots met these criteria, and all were included in the study.
Data collection
The study subjects were followed up from 1987 to 1997 through several surveillance systems operated by the FAA and the National Transportation Safety Board (NTSB). The Aviaton Medical Examiner System is a computerized database of medical certificates. There are three classes of medical certificates. The Class I certificate has the strictest medical standards; it is primarily for major airline pilots and is renewed every 6 months. Class II has less strict medical standards than Class I; it is for other commercial aviation pilots and is renewed every 12 months. Class III has the least strict medical standards; it is for private and student pilots and is renewed every 2 years. FAA regulations require that an active commuter air carrier or air taxi pilot possess a valid Class I or Class II medical certificate. These certificates can only be issued by aviation medical examiners, who are physicians authorized by the FAA to perform physical examinations for pilots. The medical examination for pilots is intended to detect functional changes and disorders that may increase the risk of sudden incapacitation or that otherwise might have a detrimental effect on flight safety. Biannual medical certification data obtained for these study subjects included information on pilots name, date of birth, gender, occupation, self-reported total flight time, flight time in the previous 6 months, and medical conditions. We merged biannual medical certification data for the 11-year study period for each pilot by matching the data on personal identifying information (pilot certificate number, name, and date of birth).
The NTSB is mandated to report all aviation crashes ("accidents" in NTSB terminology) using a standardized procedure and a set of data forms. The NTSB defines a crash as an event associated with the operation of an aircraft that results in personal death or serious injury or substantial aircraft damage. Central to the NTSB crash investigation documents is the data form called the "Factual Report Aviation," which provides detailed information about the incident, the environment, the pilot, and the aircraft. We scanned the NTSB database to ascertain all crash records that involved the study subjects. A total of 66 crashes involving the study subjects were identified. Investigative data for each of these crashes were coded and merged with medical certification data.
We searched the National Death Index to determine the vital status of the study subjects during the follow-up period. A total of 105 study subjects died during the follow-up period, including 27 who were fatally injured in aviation crashes. Information about the date and cause of death was added to the medical certification and crash records to form a longitudinal database.
Data analysis
The outcome of primary interest was involvement in an aviation crash. Since a pilot is at risk of crash involvement only when he or she flies, a reasonable expression of crash risk is the number of crashes per pilot flight hour, which is derived from the epidemiologic concept of person-time. Self-reported flight hours in the previous 6 months recorded on medical certificates were used to estimate cumulative exposure to flight during the follow-up period. Explanatory variables included in the analysis were age, sex, total flight time at baseline, use of corrective lenses for distant or near vision, and pathology codes indicating specific medical conditions. Total flight time is widely used as a measure of piloting experience (22, 25).
Follow-up data for the study subjects were censored if 1) the subject was involved in an aviation crash; 2) the subjects medical certificate was downgraded from Class I to Class II or Class III; 3) the subject died; or 4) the subject stopped renewing his or her medical certificate because of retirement or another reason. Data analysis proceeded from exploratory inquiry to statistical modeling. At the exploratory stage, crash rates per pilot flight hour were computed according to pilot characteristics. Differences in crash rates were assessed using relative risks and 95 percent confidence intervals. Effects of potential interactions between explanatory variables on crash risk were examined by stratification analysis. At the modeling stage, the accumulated flight time to a crash during the follow-up period was analyzed as a function of the explanatory variables using the proportional hazards model developed by Cox (26). The model is based on the assumption that the accumulated flight time to a crash follows an exponential distribution; therefore, the relative risk associated with a given explanatory variable is constant over time. This assumption, however, can be voided if the explanatory variable is treated as being time-dependent (27). In our analysis, pilot age was centered at 45 years and entered into the model as a time-dependent variable. The effects of total flight time at baseline and other explanatory variables on the outcome measure were also evaluated using the proportional hazards model. The goodness of fit of the model selected was assessed by means of the likelihood ratio test. Data analyses were performed using Statistical Analysis System software, version 8 (SAS Institute, Inc., Cary, North Carolina).
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RESULTS |
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DISCUSSION |
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Theoretically, aging may influence pilots safety performance through two conflicting pathways. On the one hand, in the process of aging pilots may become more vulnerable to various health problems and declines in cognitive function, potentially impairing piloting skills and increasing the propensity for crash involvement. On the other hand, aging may expose pilots to a reduced crash risk because of increased expertise, enhanced safety behavior, and greater job autonomy. Age, a measure of chronologic time, should by itself have little bearing on safety performance if variables indicating age-related changes, such as health status and flight experience, are taken into consideration.
In this study, we found that crash risk was not significantly associated with advancing age. The absence of an association between aging and crash risk was probably due to the "healthy worker effect" resulting from the rigorous medical standards and periodic physical examinations required for professional pilots. That is, pilots who experienced the most age-related declines in health and cognitive function were likely to have been screened out by the medical certification process, leaving the more physically and mentally fit in the workforce. The overwhelming majority of nondisqualifying medical conditions are minor health problems that are unlikely to have any discernible impact on safety performance. The association between aging and safety performance may have been further attenuated by the "safe worker effect." In addition to being healthy, pilots who continue flying at advanced ages are likely to have maintained a favorable safety record. Those with excess risk of crash involvement and violations are more likely to end their careers at younger ages because of disabling injuries and license terminations (6).
Lack of a significant aging effect on crash risk in this study does not necessarily mean that safety performance cannot be affected by aging. Rather, it implies that the current medical standards and physical examination system for professional pilots are adequate in ensuring flight safety. If the pilots who were screened out in the process of aging had remained on active duty, the adverse effect of aging on crash risk might well have emerged as significant.
The finding that flight experience has a protective effect against the risk of crash involvement is consistent with previous studies. In a case-control study conducted among commuter air carrier and air taxi pilots, Li and Baker (25) found that the risk of crash involvement decreases in a nonlinear fashion as total flight time increases. The safety benefit of total flight time is most pronounced in the early, experience-building stage. When total flight time reaches a certain threshold, the marginal benefit diminishes. This nonlinear relation between flight experience and safety performance has been documented in studies involving different pilot groups and using different research designs (25, 2830).
It is known that flight experience can compensate to some degree for age-related declines in cognitive function and that overlearned complex tasks such as piloting are less susceptible to age-related deterioration than abilities to perform in novel situations (14, 16, 18, 19, 31, 32). This study provides empirical evidence that safety performance as measured by the crash rate per million pilot flight hours remains steady until age 60 years. Data for pilots beyond age 60 years were too scarce for results to be conclusive.
Although the target population in this study was commuter air carrier and air taxi pilots, the reported crash rates were based on all crashes experienced by the study subjects before they were censored, including 25 general aviation crashes. It is not uncommon for professional pilots to fly under general aviation regulations for pleasure and recreation while off-duty. The crash rate in general aviation is 4.5 times the rate for commuter air carriers and is about the same as that for air taxis (23). Since information on exposure to flight was unavailable for specific categories of aviation, we were unable to calculate crash rates separately for commuter air carriers and air taxis and for general aviation. It is possible that exposure to general aviation in this study population may increase in the process of aging because of increased leisure time. Given that two out of the three crashes that occurred at age 6064 years involved general aviation flights, the crash rates for older ages reported in this study may have been somewhat overestimated. Additionally, crash risk in this study was assessed without consideration of the probable causes. Previous studies showed that pilot error is a contributing factor in 71 percent of commuter air carrier and air taxi crashes (33). There is little evidence that the prevalence of pilot error is significantly associated with age in professional pilots (33, 34). A study based on general aviation crashes revealed that older male pilots tend to make fewer errors, particularly poor decisions, than younger male pilots (35).
Age and flight experience are intractably interrelated over time. In the presence of longitudinal data, it is tempting yet unrealistic to partition aging effects from effects of other age-related variables, because, in the process of aging, few changes occur in isolation. Results of this study indicate that in the current context of rigorous medical standards and periodic physical examinations, commuter air carrier and air taxi pilots can maintain their safety record at least until age 60. The upper limit of safety age for professional pilots needs to be determined through more confirmatory epidemiologic data. In recent years, Japan and most European countries have revised regulations to allow airline pilots to fly until age 65 (36, 37). To carefully monitor and assess the safety and health of airline pilots over age 60 in these countries, an international surveillance system is warranted.
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ACKNOWLEDGMENTS |
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The authors thank the Civil Aeromedical Institute of the Federal Aviation Administration and the Analysis and Data Division of the National Transportation Safety Board for data assistance.
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NOTES |
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REFERENCES |
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