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3-5
accidents per mile, departure, flight hour, or year to be the most appropriate metric. A
passenger probably would consider an appropriate metric to be the number of fatalities per
passenger (origin-to-destination) trip to be most appropriate. Thus, for a passenger, the
number of stops en route is of interest, as one’s risk of an accident in a terminal
environment is roughly proportional to the total number of takeoffs and landings during
the trip. For these types of accidents, as the number of intermediate landings increases,
the risk measured in terms of accidents per departure might remain relatively constant, but
the risk measured in fatal accidents per flight hour, passenger fatalities per flight hour, or
passenger fatalities per trip would increase.
The average seating capacity of large air carriers might also affect the value of a risk
metric for some types of accidents and types of metrics. To understand this, consider the
following, hypothetical situation. Suppose, in the future, all large aircraft were replaced
by ones with twice the seating capacity and, as a consequence, half as many flights occur.
Then, as explained below, one would expect changes in risk metrics to be approximately
as shown in Table 3-3. (It is assumed that the percent of fatalities in a fatal accident is
constant for any chosen load factor. In other words, the percent of fatalities in a fatal
accident does not change if the seating capacity and the number of occupied seats both
double.)
For any single aircraft, the risk of being in a single-aircraft accident would be about the
same, but the number of accidents per year would be half as great because only half as
many aircraft are flying. Passenger fatalities per year would remain about the same,
because while there would be half as many fatal accidents per year, there would be twice
as many passengers involved in each accident that occurs. Because there would be the
same number of passenger trips per year, the number of fatalities per passenger trip would
also remain the same.
While the number of passenger fatalities per year would remain the same, the number of
flight hours and departures would halve. Hence the number of passenger fatalities per
flight hour and per departure would double!
The situation for midair collisions is different. Most analyses suggest that the probability
of a midair collision is roughly proportional to the square of the number of aircraft in a
given airspace region. With half as many aircraft flying, one expects the number of midair
collisions per year to be about 1/4 the present number. Since there would be about 1/4 as
many collisions and half as many departures and flight hours, the number of midair
collisions per flight hour or per departure would be about half. Since there would be
twice as many passengers per airplane, but 1/4 as many midair collisions, the number of
passenger fatalities per year in midair collisions would be about half the present number.
The number of passenger trips per year would remain the same, so the number of fatalities
in midair collisions per passenger trip would be about half. With half as many fatalities per
year and half as many flight hours and departures per year, the number of fatalities per
flight hour or per departure would remain about the same.
SEPARATION SAFETY MODELING
3-6
Table 3-3
Changes in Risk Metrics if Aircraft Capacities Double but Number of Flights Halve
Metric Change
Single-aircraft fatal accidents per
flight hour or per departure
No change (assumed)
Single-aircraft fatal accidents per
year
Half of current rate
Passenger fatalities in singleaircraft
fatal accidents per
passenger trip or per year
No change
Passenger fatalities in singleaircraft
fatal accidents per flight
hour or per departure
Double the current rate
Midair collisions per year Approximately 1/4 current rate
Midair collisions per flight hour or
per departure
Approximately half of current rate
Fatalities in midair collisions per
year or per passenger trip
Approximately half the current rate
Fatalities in midair collisions per
flight hour or per departure
Approximately the same
3.3 THE ACCEPTABILITY OF RISK
In assessing the acceptability of risk, a number of industries (e.g., nuclear) now use an
approach known as the As-Low-As-Reasonably-Practicable (ALARP) approach. Studies
are currently underway within EUROCONTROL to investigate the feasibility of adapting
the ALARP approach for use in assessing midair collision risk.
As illustrated in Figure 3-1, risk, in this approach, is classified as being in one of three
categories: intolerable, tolerable if ALARP, and negligible.
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a concept paper for separation safety modeling(13)
