and limitations are. In general, controls serve one of four actions:
activation, discrete setting, quantitative setting, and continuous control.
Activation controls are those that toggle a system on or off, like a light
switch. Discrete setting switches are variable position switches with three or
more options, such as a fuel selector switch with three settings. Quantitative
setting switches are usually knobs that control a system along a predefined
quantitative dimension, such as a radio tuner or volume control. Continuous
controls are controls that require constant equipment control, such as a
steering wheel. A control is a system, and therefore follows the same
guidelines for system design described above. In general, there are a few
guidelines to control design that are unique to that system. Controls should be
easily identified by color coding, labeling, size and shape coding and location
(Bailey, 258-64). When designing controls, some general principles apply.
Normal requirements for control operation should not exceed the maximum
limitations of the least capable operator. More important controls should be
given placement priority. The neutral position of the controls should
correspond with the operator’s most comfortable position, and full control
deflection should not require an extreme body position (locked legs, or arms).
The controls should be designed within the most biomechanically efficient design.
The number of controls should be kept to a minimum to reduce workload, or when
that is not possible, combining activation controls into discrete controls is
preferable. When designing a system, it should be noted that foot control is
stronger, but less accurate than hand control. Continuous control operation
should be distributed around the body, instead of focused on one particular part,
and should be kept as short as possible (Damon, 291-2). Detailed studies have
been conducted about control design, and some concerns were such things as the
ability of an operator to discern one control with another, size and spacing of
controls, and stereotypes. It was stated that even with vision available,
easily discernible controls were mistaken for another (Fitts, 898; Adams, 276).
A study by Jenkins revealed a set of control knobs that were not prone to such
error, or were less likely to yield errors (Adams, 276-7). Some of these have
been incorporated in aircraft designs as recent as the Boeing 777. Another
study, conducted by Bradley in 1969 revealed that size and spacing of knobs was
directly related to inadvertent operation. He believed that if a knob were too
large, small, far apart, or close together, the operator was prone to a greater
error yield. In the study, Bradley concluded that the optimum spacing between
half-inch knobs would be one inch between their edges. This would yield the
lowest inadvertent knob operation (Fitts, 901-2; Adams, 278). Population
stereotypes address the issue of how a control should be operated (should a
light switch be moved up, to the left, to the right, or down to turn it on?).
There are four advantages that follow a model of ideal control relationship.
They are decreased reaction time, fewer errors, better speed of knob adjustment,
and faster learning. (Van Cott & Kinkdale, 349). These operational advantages
become a great source of error to the operator unfamiliar with the aircraft and
experiencing stress. During a time of high workload, one characteristic of the
Liveware component is to revert to what was first learned (Adams, 279-80). In
the case of the Bonanza and Baron pilots, this was the case in mistaking the
gear and flap switches.
VI. Displays
In late 1986, the NTSB released the following recommendation to the FAA
based on three accidents that had occurred within the preceding two years:
“A-86-105. Issue an Air Carrier Operations Bulletin-Part 135, directing
Principal Operations Inspectors to ensure that commuter air carrier training
programs specifically emphasize the differences existing in cockpit
instrumentation and equipment in the fleet of their commuter operators and that
these training programs cover the human engineering aspects of these differences
and the human performance problems associated with these differences” (NTSB
database).
The instrumentation in a cockpit environment provides the only source of
feedback to the pilot in instrument flying conditions. Therefore, it is a very
valuable design characteristic, and special attention must be paid to optimum
engineering. There are two basic kinds of instruments that accomplish this
task: symbolic and pictorial instruments. All instruments are coded
representations of what can be found in the real world, but some are more
abstract than others. Symbolic instrumentation is usually more abstract than
pictorial (Adams, 195-6). When designing a cockpit, the first consideration
involves the choice between these two types of instruments. This decision is
based directly on the operational requirements of the system, and the purpose of
the system. Once this has been determined, the next step is to decide what sort
of data is going to be displayed by the system, and choose a specific instrument
accordingly.
Symbolic instrumentation usually displays a combination of four types of
information: quantitative, qualitative, comparison, and reading checking (Adams,
197). Quantitative instruments display the numerical value of a variable, and
is best displayed using counters, or dials with a low degree of curvature. The
preferable orientation of a straight dial would be horizontal, similar to the
heading indicator found in glass cockpits. However, conflicting research has
shown that no loss accuracy could be noted with high curvature dials (Murrell,
162). Another experiment showed that moving index displays with a fixed pointer
are more accurate than a moving pointer on a fixed index (Adams, 200-1).
Qualitative readings is the judgment of approximate values, trends, directions,
or rate of variable change. This information is displayed when a high level of
accuracy is not required for successful task completion (Adams, 197). A study
conducted by Grether and Connell in 1948 suggested that vertical straight dials
are superior to circular dials because an increase in needle deflection will
always indicate a positive change. However, conflicting arguments came from
studies conducted a few years later that stated no ambiguity will manifest
provided no control inputs are made if a circular dial is used. It has also
been suggested that moving pointers along a fixed background are superior to
fixed pointers, but the few errors in reading a directional gyro seem to
disagree with this supposition (Murrell, 163). Comparisons of two readings are
best shown on circular dials with no markings, but if they are necessary, the
markings should not be closer than 10 degrees to each other (Murrell, 163).
Check reading involves verifying if a change has occurred from the desired value
(Adams, 197). The most efficient instrumentation for this kind of task are any
with a moving pointer. However, the studies concerning this type of
informational display has only been conducted with a single instrument. It is
not known if this is the most efficient instrument type when the operator is
involved in a quick scan (Murrell, 163-4).
The pictorial instrument is most efficiently used in situation displays,
such as the attitude indicator or air traffic control radar. In one experiment,
pilots were allowed to use various kinds of situation instruments to tackle a
navigational problem. Their performance was recorded, and the procedure was
repeated using different pilots with only symbolic instruments. Interestingly,
the pilots given the pictorial instrumentation performed no navigation errors,
whereas those given the symbolic displays made errors almost ten percent of the
time (Adams, 208-209). Regardless of these results, it has long been known that
the most efficient navigational methods are accomplished by combining the
advantages of these two types of instruments.
VII. Summary
The preceding chapters illustrate design-side techniques that can be
incorporated by engineers to reduce the occurrence of mishaps due to Liveware-
Hardware interface problems. The system design model presented is ideal and
theoretical. To practice it would cost corporations much more money than they
would save if they were to use less cost-efficient methods. However, today’s
society seems to be moving towards a global concensus to take safety more
seriously, and perhaps in the future, total human factors optimization will
become manifest. The discussion of biomechanics in chapter three was purposely
broad, because it is such a wide and diverse field. The concepts touched upon
indicate the areas of concern that a designer must address before creating a
cockpit that is ergonomically friendly in the physical sense. Controls and
displays hold a little more relevance, because they are the fundamental control
and feedback devices involved in controlling the aircraft. These were discussed
in greater detail because many of those concepts never reach the conscious mind
of the operator. Although awareness of these factors is not critical to safe
aircraft operation, they do play a vital role in the subconscious mind of the
pilot during critical operational phases under high stress. Because of the
unpredictable nature of man, it would be foolish to assume a zero tolerance
environment to potential errors like these, but further investigation into the
design process, biomechanics, control and display devices may yield greater
insight as far as causal factors is concerned. Armed with this knowledge,
engineers can set out to build aircraft not only to transport people and
material, but also to save lives.