2.6.1 It must always be remembered that every
system is designed to be operated by or to serve human beings. Consequently,
the man-machine interface is of paramount importance. The human being
is very complex and has both physical and mental capabilities and
limitations. Human beings are of varying shape and size and respond
differently to external stimuli, especially under stressful conditions.
It is probably impossible to design a system that will be suited to
every person who will ever use it, but it can at least be designed
for the expected user population.
2.6.2 Physical differences in human beings such
as height, reach, weight, and strength are relatively easy to take
into account during design. Numerous charts and tables have been published
in which limiting body dimensions for a range of a given population
are presented. These dimensions typically are tabulated as 5th and
95th percentile values. For any body dimension, a 5th percentile value
is equal to or larger than the smallest five percent of the population.
That particular dimension is exceeded by 95 percent of the population.
Similarly, a 95th percentile value is equal to or larger than 95 percent
of the population; it is exceeded by only five percent of the population.
Therefore, if a system is designed to accommodate 5th and 95th percentile
dimensions, 90 percent of the user population will theoretically be
able to operate the system.
2.6.3 Inspectors should satisfy themselves that
the lifeboat has been designed to allow access within the boat and
to controls and equipment for the largest expected users (probably
the 95th percentile male). At the same time the system should be designed
to allow the smallest expected users (possibly the 5th percentile
female) to reach all controls, lift any required weights, and have
adequate visibility from their stations. Other design considerations
may include the ability to operate valves, switches, or other controls
while wearing gloves, immersion suits, or suitable lifejackets and
the ability to read a label or observe an indicator light from the
normal operating or seating position.
2.6.4 Accounting for cognitive and perceptual
differences between people when designing a system is much more difficult
and is the subject of continuous research. However, some basic principles
can be followed easily. These include clear and unambiguous instructions
inscribed on placards that are posted at eye-level, color coding of
lights and displays so that the indications are intuitively obvious,
fail-safe design features, and interlocks in release systems to prevent
a dangerous situation from developing. Another design feature essential
to safe operation is that the system must provide automatic feedback
to the operator indicating whether an operation was successful. For
instance, a click can indicate if a hatch is properly closed (audio
feedback), a green light can be illuminated to indicate that all tasks
required prior to release have been accomplished (visual feedback),
and a steering control can become progressively harder to turn as
the rate of turn is increased (tactile feedback).
2.6.5 This discussion has not been intended to
be a comprehensive guide to human engineering. Rather, its intent
is to inform the lifeboat designer, manufacturer, and inspector about
the types of issues that are important and that should be noted during
prototype certification tests so that a free-fall lifeboat system
can be as safe as possible.