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“worst case” scenario. Major failures were not included in the remaining 800 incapacitations so that “the
simulation was of a subtle incapacitation, still taking place at a critical phase of flight, but as an event in
ICAO Preliminary Unedited Version — October 2008 I-3-2
itself and not complicated by other major failures.” This latter scenario is the more realistic, since the risk
of an incapacitation occurring simultaneously with a major technical failure is extremely remote.
In the simulator it was found that only 1 in 400 “uncomplicated” incapacitations resulted in a simulator
“crash”, because the second pilot successfully took control on the 399 other occasions. If certain
assumptions about a typical multi-crew flight are made, this knowledge can be used to calculate an
acceptable risk of incapacitation for an individual pilot. These assumptions (see Figure 1) are:
1. Each flight lasts one hour.
2. Only 10% of the flight time is critical, viz. take-off and initial climb, approach and landing (in a
one hour flight this comprises the first and last three minutes).
3. Pilot incapacitations occur randomly during a flight.
4. 1 in 100 real life incapacitations occurring in the critical periods would result in a fatal accident, a
more pessimistic view than that suggested by the simulator studies mentioned above (1 in 400),
where simulated incapacitations could be anticipated by the flight crew.
Based on these four assumptions, the so-called 1% rule has been developed.
Figure 1.— Critical and non-critical phases of flight
in a flight of one hour1
The 1% rule
During the last decades of the 20th century, a number of Contracting States were approaching a fatal
accident2 rate of one in 107 flying hours. Some Contracting States therefore set as their target all cause
maximum fatal accident rate a figure of one in 107 flying hours, with human “failure” constituting one
tenth of the risk and human failure caused by medical incapacitation comprising one tenth of the human
failure risk, or one hundredth of the total risk, i.e. medical incapacitation should not result in a fatal
1 From Rainford, D.J., Gradwell, D.P. eds. Ernsting’s Aviation Medicine, Hodder Arnold, 2006
2 A fatal accident is an accident in which one or more persons are fatally injured as a result of being in the aircraft,
or being struck by an aircraft or its parts.
ICAO Preliminary Unedited Version — October 2008 I-3-3
accident more often than one in 109 hours. Based on the assumptions stated above, a pilot flying a twopilot
aircraft can have an incapacitation risk of no more than one in 106 hours and the operation will
achieve the target medical cause fatal accident rate of no more than one in 109 hours, since the presence of
a second pilot reduces the risk by a factor of 1000. This is because:
• In a multi-pilot aircraft only 10% of flight time is critical (risk reduced by a factor of 10) as
incapacitations are assumed to occur randomly. Therefore only one in ten in-flight incapacitations
will occur during a critical stage of flight and thus pose a flight safety risk
• Only one in 100 incapacitations occurring at a critical stage of flight is likely to result in a fatal
accident (risk further reduced, by a factor of 100)
• Therefore the total risk reduction with the addition of a second pilot is 1/10 × 1/100 = 1/1000, i.e.
the risk is one 1000th of the risk of single pilot operations
• For a pilot with an incapacitation risk of one in 106 hours, a second pilot therefore reduces the risk
of a fatal accident from pilot incapacitation from one in 106 hours to one in 109 hours.
In other words, only one fatal accident in one thousand in-flight pilot incapacitations would be expected
to result in a fatal accident, because the other pilot would take over safely in the other 999 times. For an
individual pilot flying a multi-crew aircraft the acceptable risk of incapacitation may therefore be
increased by a factor of 1000 from one in 109 to one in 106 hours.
An incapacitation rate of one in 106 hours approximates to a rate of 1% (or one in 102) per annum (since
there are 8760 - close to 10 000 (or 104) - hours in one year). More explicitly:
• 1 in 106 hours = 0.01 in 104 hours (dividing both figures by 100)
• 0.01 in 104 hours = 1% in 104 hours
• 1% in 104 hours approximates to 1% in one year (because there are 8760 hours per year)
The acceptable maximum incapacitation rate of 1% p.a. outlined above has become known as the “1%
rule”. This rule specifies a predicted annual medical incapacitation rate which, if exceeded, would
exclude a pilot from flying in a multi-crew aircraft. This is widely regarded as an acceptable risk level and
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Manual of Civil Aviation Medicine 1(49)
