Background: This paper examines the use of quantitative incapacitation risk assessment for aeromedical decision-making in determining the medical fitness of multicrew airline pilots, and estimates the effect on flight safety should medical standards be relaxed. The use of the "1% rule" for setting limits for aircrew incapacitation risk is re-examined. Human failure (medical incapacitation) is compared with acceptable failure rates in another safety-critical system, the aircraft engines.
Methods: The expected number of cardiovascular incapacitations occurring in flight was modeled by applying an age-related cardiovascular incapacitation risk to the pilot population. The effect on flight safety of relaxing the maximum acceptable incapacitation risk on estimated incapacitation rates in two-pilot operations was also modeled, taking into account a likely increase in the number of pilots who would be allowed to continue to fly with a known medical condition.
Results: The model overestimates cardiovascular incapacitation risk and, therefore, provides a cautious estimate. If the maximum acceptable cardiovascular risk is increased, the model predicts a disproportionately small increase in the number of such incapacitations in flight.
Conclusions: The evidence suggests that the incapacitation risk limits used by some states, particularly for cardiovascular disease, may be too restrictive when compared with other aircraft systems, and may adversely affect flight safety if experienced pilots are retired on overly stringent medical grounds. States using the 1% rule should consider relaxing the maximum acceptable sudden incapacitation risk to 2% per year.