Appropriate Calculation of Risk per Flight Hour for Rotorcraft Safety Risk Management

Quantitative Risk Assessment has become essential in rotorcraft safety risk management. Measures of risk include Cumulative Fleet Risk (also called Risk Factor), Risk per Flight, and Risk per Flight Hour. Each measure applies to a different situation and can produce the same or different predictions of future risk. Risk for a large fleet of aircraft might be accurately predicted by Cumulative Fleet Risk, whereas Risk per Flight or Risk per Flight Hour might be best for a small fleet of rotorcraft, a flight test program, or a fleet with low flight hours. Calculating risk per flight hour seems as simple as dividing the number of previous occurrences by the flight hours for the total fleet, but this is appropriate only in the case of random distribution. Most failures that lead to hazards are not random because the failure mechanism has a specific cause. A more appropriate method is to develop the future event forecast using Quantitative Risk Assessment, then divide that by the future fleet hours. The simple division process requires only two numbers and can be completed quickly, but with a possibly inappropriate or misleading result for anything but a random distribution. The approach presented here results in a risk prediction that is appropriate for hazard rates that are increasing, decreasing, or constant, and for non-random distributions, which could prevent misleading or unconservative risk management decisions.