A
main reason for the
operators taking the wrong action is that they are not aware
of the system
state. If the the
operators are not aware of
state
changes, they might perform a wrong
procedure, or take the wrong action in
problem solving. For example, the
operators might rely on the
system
backup, unaware that it is disabled. The same procedure which was proper in the
previous state might be
risky in the actual state (as was the case of the TMI accident).
If
the operators are required to take some actions in order to learn the
system
state, they might spend to much time and mental effort, and neglect other
important tasks. Under stress, they might rely on their
memory of a previous
state, which might be wrong.
The
station design should include means, such as special
lights, to guarantee that
risky
states are visible and salient. The
system
testing should include a special test to verify that these means are effective,
namely that the operators always notice the
risky states.
If
the operators are required to memorize the
state of several variables, they
might make mistakes, and recall a wrong state.
To prevent memorization
errors, the the execution of
simple operational procedures should be automated, reducing the
operators
workload thereof.
If
the state indicators are located away from the operational panel, then the
operators might miss the
indications when they are busy with the panel operation.
The
state indicators should be located close to the
operational panel, so that any change in the indications is notice during the
panel operation.
If
the state indicators are dispersed then the
operators might miss some of them.
The state indicators should be arranged according to
operational scenarios, such that all indicators relevant to a
scenario are close
to each other.
If
a frequently used operational procedure changes, the
operators might conduct the
previous procedures (before the change), which might be wrong (as was the case
of the AF 296 accident in 1988).
The design should mitigate the
risks of employing the
obsolete, routine operation.
The designers should acknowledge the
operators of new
features and of changes in
operational procedures, and ensure that they are
aware of the risks of continuing the previous routines.
Designers typically expect that once the indications are designed according to
the rules, the operators will always notice them. Unfortunately, in certain
conditions, such as extreme stress or fatigue,
operators might miss visible and
salient indications.
The effectiveness of the indications in extreme
situations
should be tested in special
usability tests.
Updated on 29 May 2016.