**8.2 Human Performance Issues:**

Perception

Memory (types)

Attention

Skill

Intelligence and personality

Aptitude and aptitude testing

Motivation

Decision making process

Response/reaction

Workload and workload measurement

Pilot error (types)

Controlled flight into terrain (CFIT)

**Personality**

A logical starting point in a discussion of human performance is
personality, the innate, learned, modified, and evolved collection of
ways in which an individual interacts and reacts to persons and events.
In a more analytical perspective, personality has been described as the
characteristic integration of the individual's modes of behavior,
interests, aptitudes, capacities, attitudes, and abilities.

During the 100 years of human flight many studies have attempted to
define both desirable and undesirable personality traits as related to
piloting success. As would be expected, studies from different periods
tend to place value on different traits depending on the type of
aircraft systems and mission requirements required at any given time.
Suffice it to say, there is no perfect constellation of traits. Factors
such as motivation, acceptance and adherence to proven safety and
performance procedures, commitment to continued learning, and ability to
invoke or control the right emotion at the proper time all make large
contributions to an individual aviator's success. It is critically
important that aviation personnel have, or develop, positive behaviors
appropriate to the type of aviation activities in which they
participate.

Much effort has been expended in the effort to develop "tests" to
assess, describe, or otherwise quantitate personality. The first notion
to dispel is the tendency to refer to these instruments as tests, since
there are no "right" or "wrong" answers. Rather these should be thought
of as assessment tools or, batteries, which attempt to describe how an
individual reacts, thinks he / she would react, or would prefer to
react. The term used most frequently to describe such efforts is,
"personality inventory." Such instruments are widely used throughout
industry and government in human capital management efforts for career
development, team building and, indirectly, for efforts such as Crew
Resource Management (see below). One of the main benefits derived from
such profiling is to help individuals better understand their strengths
and preferred modes of interaction with others so that individuals and
groups will be able to apply resources wisely to increase both
individual and group effectiveness. Some of the better known personality
inventory instruments are the Personal Profile System^TM^, the
Myers-Briggs Type Indicator®, and the Kirton Adaptation-Innovation
Inventory©.

**Skill**

Skill is a human attribute, or set of attributes, for which we all have
a perspective, but rarely stop to think about in detail. All skills are
learned and, therefore, require specific innate ability as a start
point, coupled with motivation to learn. In general though, a skill can
be thought of as a distinctive behavior, which allows one to effectively
apply knowledge in the execution of performance. Performance may entail
flying an airplane, making a complex clinical diagnosis, directing an
orchestra, solving complex mathematical problems etc. In the education
or training process, this entails skill development and may include
elements of both cognitive and motor performance. A skill requires
consistent use, practice and often, retraining in order to maintain
proficiency. This is the sense in which the term skill is most often
used in aviation activities, and is often referred to as skilled
behavior.

Skill acquisition is thought to occur in three stages:

Cognitive phase -- provides an understanding of what the skill entails,
what it is intended to accomplish, detailed descriptions of the task,
and how it may be accomplished in an acceptable manner.

Associative phase -- the learner practices the various elements of the
task until the desired level of performance is achieved; normally, at
this stage of learning many tasks can be performed without strict
cognitive control.

Autonomous phase -- the learner is able to accomplish the task as
"second nature" and does not have to think through the "when, how, and
why" in order to apply the skill in the context of performance. At this
stage, the learner frequently acquires the capability to integrate
multiple discrete skills in series to accomplish complex actions.

In the aviation environment, it is often necessary to seamlessly
integrate many discrete skills. The following example illustrates the
linkage of several independent skills, resulting in supreme airmanship
and an outcome that was the best that could be achieved under the
circumstances. The example involves a flight which took place during
practice for the World Aerobatic Championships in 1971:

"The aircraft took off on its third flight of the day and went through a
set aerobatic sequence twice, during which time the aircraft behaved
normally. During the third sequence the pilot was recovering level
flight from a vertical dive; on pulling plus 5 'G' at a height of about
1000 ft. there was a loud bang and a jolt was felt in the aircraft,
which then commenced to roll to the left. This roll could not be
checked, despite use of full right aileron and right rudder, and it
\[the left wing\] was deflecting upwards \[threatening to separate from
the aircraft\]. The pilot's intention at this time was to land the
aircraft as soon as possible, but by the time he had descended to 300
ft., it was clear that all control was about to be lost. He therefore
reversed the aileron, applied negative 'G' and rolled the aircraft to
the inverted position. The pilot, who was not wearing a parachute, found
that he could fly the aircraft normally in inverted flight. A circuit of
the airfield was completed, climbing to 1000 ft. while the handling of
the aircraft was assessed. Having decided to land \[with\] wheels
retracted, an inverted approach was made and at the last moment the
pilot rolled the aircraft to the right into a normal \[upright\]
attitude. Negative 'G' was held during the roll-out to hold the wing in
position and as this was released, just prior to impact, the wing
started to fold \[separate from the aircraft\]. The aircraft came to
rest after a ground slide of 70 yards, during which it had swung 90
degrees to the left. The pilot was slightly bruised, but was able to
leave the aircraft without assistance." ^1^

In the preceding example, although one may question the pilot's judgment
for not wearing a parachute during aerobatic flight, there are few who
would try to second guess the set of skills brought to bear to diagnose
the situation, consider alternatives, implement and execute the
solution. Several characteristics inherent to skilled behavior were
demonstrated by the pilot:

Behavior is goal directed -- choices made reflect selectivity in
perception of appropriate cues followed by responses all calculated to
achieve a given planned, desired outcome.

Sequential behavior -- a skilled response usually involves a chain of
related acts, rather than isolated acts; the chain must occur in the
right order and be properly timed.

Feedback -- assessment of real-time information to constantly update
response actions.

Adaptation -- flexibility in determining which individual actions are
needed to fit the response needs of a particular situation, especially
when the situation is not an expected normal occurrence.

Timing -- although extreme speed of response may be appropriate in some
situations, typically, in skilled behavior the individual will appear
*not* to have time pressures because the skill has become an innate
behavioral capability which can be effectively employed at the correct
sequence point in the achievement of the desired goal.

**Intelligence**

The term, intelligence, is generally used to describe the capacity to
comprehend facts, their relationships, and to reason about them. David
Wechsler, the developer of the Wechsler Adult Intelligence Scale (WAIS),
defined intelligence as the aggregate capacity of an individual to act
purposely, to think rationally, and to deal effectively with the
environment. As commonly applied, the notion of intelligence tends to
account for a individual's capacity to effectively make use of all
abilities, not just to have them.

Although there remains a strong tendency to view intelligence as a
purely intellectual or cognitive function, considerable evidence
suggests that intelligence has many facets. For instance, it is
generally accepted that intelligence is related to both heredity and
environmental factors. Studies done on families, particularly among
identical twins and adopted children, have shown that heredity is an
important factor in determining intelligence; but they have also
suggested that environment is a critical factor in determining the
extent and manner of its expression. For example, children raised in
orphanages or other environments that are comparatively unstimulating
tend to show less intellectual development. In recent years, controversy
regarding intelligence has centered primarily on how much of each
factor, heredity and environment, is responsible for an individual's
level of intelligence. These arguments may be as much related to testing
validity as to the capacity purportedly being measured.

Although a strict definition of intelligence has proven elusive, a
number of psychologists have argued that it can be quantified, primarily
through testing. One criticism of intelligence testing is that it is
difficult to insure that test items are equally meaningful or difficult
for members of different socio-cultural groups. Testing is often
considered validated in part, however, by the finding that the quantity
measured by the tests can be closely correlated in American society with
career and academic achievement. There has been a decline in interest in
pure intelligence tests since the 1920s, with a corresponding increase
in the number of mental tests that measure special aptitudes and
personality factors which have be correlated with likelihood of success
for specific endeavors (see Personality discussion, above).

Nonetheless, success in many aviation activities has been related to
levels of intelligence as measured by several different intelligence
testing instruments. In general, the findings have indicated that, for
most aviation activities, including piloting of aircraft, so called
"normal" levels of intelligence are adequate. Individuals with above
average levels of intelligence tend to learn quicker and are of course
better suited to complex tasks such as test piloting and research
applications, but as is true in virtually any occupational setting,
underlying ability and motivation to apply intelligence are often more
important than is having a super-normal intelligence capability.

**Perception**

Perception associates meaning to sensory stimuli. Two challenges, which
must be overcome, are signal detection and signal selection. The ability
of humans to distinguish a signal from background "noise" is termed
*sensitivity*, while the human's reaction to the signal is termed
*response bias*. Response bias may fall into one of four categories: HIT
-- a signal is present and it is correctly detected; MISS -- a signal is
present, but not detected; FALSE ALARM -- operator thinks a signal is
present when there is none; CORRECT REJECTION -- operator correctly
determines no signal is present. This may be visualized in a standard
two-by-two table:

![](media/image1.wmf)

For application of human factors principles involving perception in
information processing it is critical to separate the effects of
sensitivity from response bias in signal detection performance. This is
especially important in order to optimize human-to-system interface. For
example, the goal in the design of new displays is to produce maximal
gains in sensitivity. Sensitivity can only be improved with increased
correct detection of the signal (HITs) while minimizing the FALSE
ALARMs. An undetected signal cannot be acted upon; failure to detect a
signal may compromise safety (through a lack of action) while false
detection of a signal may result in an inappropriate action.

**Memory**

Memory is the capacity to retain, recall, and recognize data. It
underlies conditioning, learning, and many other higher mental
functions. In general, the capacity to retain and recall discrete,
structured, and coded data is much greater than the capacity to retain,
recall, and effectively use ambiguous or amorphous data.

In the flying environment, information perceived by an operator usually
results in an action. However, in some situations immediate action is
inappropriate and the operator must *remember* the information. This
usually involves the use of *working memory* skills. The information to
be retained is almost always visual or verbal code. Regardless of the
type of information, working memory capacity is quite limited. Research
has found most individuals can successfully hold only 5-9 unrelated
items in working memory when *full concentration* is available to
process the items. When attention must be divided to carry out other
tasks, working memory capacity will be significantly attenuated. Most
items in working memory will be forgotten after 15 to 20 seconds, unless
special effort is made to update and reinforce retention. Four actions
may be taken to increase retention in working memory:

1\. Reduce the rate of information inflow.

2\. Reduce the similarity between items; insure individual item
distinctiveness.

3\. Eliminate unnecessary redundancy.

4\. Distribute information between verbal and visual codes; like-coded
items tend to interfere more than do dissimilar coded items.

Other information not immediately used but necessary for future action
is retained in *long term memory.* Long term memory consists of two
sub-types: *Semantic memory and episodic memory.* Semantic memory
retains such things as word meanings, how to perform previously acquired
skills (such as tying a shoe, playing a sport, driving an automobile, or
flying a plane). Episodic memory contains recollections of specific
events and becomes important for activities such as recalling accident
scenarios and separating actual events from expected events. Many
studies have shown that what an individual remembers about a specific
event is influenced by what the person thought *should* happen and that
when a difference occurs, the expectation often overrules reality.

**Aptitude and aptitude testing**

Aptitude is a human quality which we associate with a natural ability, a
general suitability, and a capacity for learning a certain task or
skill. In military and commercial aviation there has long been a quest
to determine what or which aptitudes are most important for success in
flying and determining valid methods for identifying whether or to what
degree individuals possess those aptitudes. Thus, the basis for
application of aptitude testing in the selection of personnel who seek
to become engaged in aerospace operations. As noted above in the
discussion on personality, aviation operations have changed
significantly through the years . Within the aerospace industry today
there is tremendous variation in job requirements and in the aptitudes
which contribute to an individual's likelihood of success.

In general, aptitude tests have become increasingly popular among
employers as a means of assessing and predicting candidates' subsequent
job performance. There are various types of aptitude tests used,
including verbal, numerical and logical reasoning. The tests are
designed to test a candidate's potential to learn, the ability to gather
and understand information quickly, the ability to cope under pressure,
and ability to apply knowledge in a logical way. Firms expect their
employees to be flexible at work, able to adapt to change quickly, and
able to sift through vast amounts of data, picking out relevant facts,
making sound decisions, and implementing actions accordingly. In
essence, there are two important purposes for aptitude testing: To serve
as an aid to individuals in career selection and preparation, and as a
tool for employers to help select and place the right individuals in the
right jobs.

One of the first comprehensive attempts to document a rational process
for aptitude traits and selection techniques in aviation was Dr. Ross
McFarland's textbook, *Human Factors in Air Transportation,* written 50
years ago. Dr. McFarland was one of the first to recognize the critical
need to perform a job task analysis before one could identify aptitudes
to be screened for in the air crew selection process. In the textbook,
Dr. McFarland identified several traits and aptitudes which were
important to success in aviation operations -- most of those identified
have withstood the test of time and remain important in today's
environment. Some of those included were, mental ability, mechanical
comprehension, judgment, motor skill, emotional control, character, and
leadership. Subsequent research has stressed the importance of
intelligence, conscientiousness, emotional stability, and self control.

**Motivation**

Motivation is a description of what induces an individual to act, to
decide upon and execute goal directed behavior. In the short term this
may be as simple as withdrawing from a hot surface to prevent a burn. In
the long term this may entail multiyear effort leading to the award of
advanced academic degrees. As applied to work, motivation is frequently
linked to an emotional dimension, or value judgment associated with the
outcome of a task. This often becomes manifest by an individual's
"motivation" to do something better the next time it is attempted.

Success in aerospace operations often depends on skill sets which must
be consistently learned, applied, maintained, and on occasion changed.
Often it is necessary to examine the environment in which individuals
and teams function to assess what factors may enhance or detract from
creating an environment in which participants will strive for their best
individual and collective performance in all situations. For highly
qualified technical and professional individuals, adequate compensation
and benefits are an important part of the environment, but certainly far
from being all that is necessary to ensure top-notch performance. For
most individuals, once adequate compensation has been achieved, job
satisfaction is the most important factor in determining the sustained
motivation to perform at peak levels. When job satisfaction is not
achieved, problems in the form of frustration and conflict result and
individual and group morale suffers.

Frustration results when an individual is unable to successfully attain
goals. Sometimes this is due to a lack of innate ability or a training
system, but it can also result from deficiency which has failed to
instill a capability necessary for goal accomplishment. At other times,
frustration may result from administrative practices which preclude one
from reaching a goal. Frequently, when frustration goes unrecognized or
is not dealt with effectively, aggressive behavior will result. Such
behavior is almost always counter productive and may at times become so
severe as to create operational sabotage or induce other unsafe
behaviors.

For individuals who pursue careers in aviation, motivation at entry is
seldom a significant issue, other than the occasional person who enters
because of a perception of parental expectation. Rather, the problem
becomes the maintenance of motivation throughout all stages of career
development, especially when external stressors related to non-career
responsibilities require a diversion of personal investment.

**Decision making process**

Pilots and other aircrew are frequently challenged with the need for
making decisions, some of which must be made very quickly and with
incomplete information. It is critical that aircrew be able to make
correct decisions quickly, often without optimal information (see also,
Response / Reaction discussion below).

In general, making a decision involves three major features:

Gathering and evaluation of information, often from multiple sources
(cues), in order to assess the current situation, determining that an
action is needed, and picking an appropriate response.

The quality of the information gathered may not be perfect (or may be
interpreted incorrectly) so that the decision making process becomes
*probabilistic*. In other words, a correct decision may still lead to an
undesirable outcome due to "bad luck", and a wrong decision may turn out
OK due to "good luck".

Most decisions will involve some elements of value judgment, which may
include economic criteria, comfort criteria, time criteria, safety
criteria, etc. In many cases, deciding how much risk one is willing to
take, is coupled with a value judgment on how manageable the risk is,
will determine the action taken.

**Response / Reaction**

Actions in the airborne environment may be time critical. One of the
most important learned skills of air crewmembers is to be able to
quickly distinguish which actions are time critical from those which are
not so time critical to preclude errors in decision making which can be
induced by time pressure. For those events in which there is a true time
criticality, there has been considerable research in recent decades to
determine how people best respond to information, how that information
can most effectively be conveyed to decision makers, and the processes
involved in those decisions and following actions. In the aviation human
factors research community this collection of effort has been referred
to as the *choice reaction-time task* paradigm. This paradigm has
elucidated many elements which influence both the speed and the accuracy
of response to unexpected environmental conditions, such as the
identification of another aircraft with which a midair collision is
highly likely if no action is taken. Such a situation is illustrated in
Figure 1.

![](media/image2.wmf)

**Figure 1. Response / reaction \-- process, steps, and action.**

Several factors are thought to be involved in the response -- reaction
sequence. These involve issues related to signal reception and
identification, information transmission, recipient preparation,
speed-accuracy trade offs, and dual-task performance. Operator inherent
delays are a key area of concern for humans in complex systems. These
delays are very difficult to modify to any significant degree. During
routine flight operations, there is a fundamental delay associated with
sensory-neural pathway transit times and basic central processing.
Assuming all neuroanatomical and physiological parameters are intact,
this baseline time is on the order of 150 to 250 milliseconds. A simple
requirement to make a choice between two possible responses will double
the delay. Receptor delay -- processing time is up to 15 times longer
for visual as compared to auditory transduction, however, the visual
system is able to process much more complex data.

Operators who are alerted, prepared, or even waiting for an event will
normally exhibit a much faster response since they are anticipating a
known event and may even rehearse the correct response prior to
executing it. When an unexpected event occurs, the operators must first
identify the event, determine that a response is needed, evaluate
multiple courses of potential action, select one, and execute the
response. In aviation, these responses tend to be influenced positively
by recent proficiency flying experience. When one is time pressured to
make a choice response, the probability of choosing an errant course of
action is much greater -- a phenomenon known as the *speed-accuracy
trade-off.*

The speed-accuracy trade-off is vitally important in aviation
environments in the following ways:

Phases of operation in which fast reaction is most important, such as
take off and approach to landing, are the phases in which an error will
be most likely to result in significant consequences.

Slower responses, as a result of system design, will be more likely to
result in errors that may produce catastrophic results in time critical
situations. Therefore, even fractions of a second can become
significant, although there may be no discernable difference in overall
system performance during normal operations.

Speed may be selected in preference to accuracy depending on the level
of arousal. Strong arousal stimuli tend to favor a speedy response at
the expense of accuracy. For example, a loud auditory stimulus tends to
be more arousing than a visual stimulus, and loud auditory alarms tend
to invoke erroneous responses more often then a visual stimulus. Of
course, visual stimuli may be easily overlooked during periods of high
intensity workload when attention is already channeled ("perceptual
MISS").

Preparation (see above) may enhance a quick and accurate response.
However, if the anticipated situation does not occur, and the operator
executes the response for the anticipated situation, obviously an error
will occur.

Age increases the response time. Age increase from 20 to 60 is
correlated with a noticeably slower, but more accurate response, an
attribute that undoubtedly serves the older pilot well in many
situations (and lends support to the old adage, "there are bold pilots
and there are old pilots, but there are not many old bold pilots").

Frequently in the course of piloting activities, there are situations in
which pilots must engage in more than one task at a time. This may be
relatively simple, as climbing to an assigned altitude while changing
course and talking to Air Traffic Control. These are tasks in which
professional pilots are highly skilled and can conduct almost
"automatically." This requires pilots to invoke multiple resources in
parallel actions, as opposed to sequential actions. However, humans tend
to be very limited in their ability to perform tasks in parallel; only
fairly simple tasks, which have been practiced in training situations,
tend to be performed successfully. In emergency situations, which
require multiple actions in brief periods, pilots must be able to
prioritize accurately and follow through with responses. Predictable
emergencies are the types of training scenarios, which are practiced
repeatedly so that crews are able to respond automatically in a
predictable fashion. It is when unpredicted situations occur that novel
responses are called for, and response time tends to be prolonged,
especially when multiple parallel actions are required, but have not
been specifically trained.

**Workload and workload measurement**

Humans are most reliable when performing under moderate levels of
workloading. When the workload level changes unexpectedly, or when
operating at the extremes of workload capacity, the probability of human
error occurring rises proportionately. With high or increasing
workloads, at some level of activity every individual will become unable
to receive, process, and react appropriately to information flow.
Conversely, when workload levels fall too low, attention may lapse, or
worse, individuals or groups may engage in distracting, nonproductive
conversation, other activities, or unplanned sleep.

Measurement of workload remains a vexing challenge. Physical workload is
reasonably easy for exercise physiologists to objectively measure and
describe. In modern aircraft, there is no longer any significant
physical workload involved; workload now is mental.

The development of an objective standard to measure and quantify mental
workload has never been achieved and does not appear to be on the
horizon. In the absence of an objective metric much attention has been
devoted toward the development of useable subjective ratings. Several
subjective ratings scales have been used, none of which is universally
applicable. Examples of such scales include:

Cooper-Harper Scales. Originally designed to assess aircraft handling
qualities, pilots using this instrument answered specific questions in a
binary decision tree. The results of statistical analysis then led to an
assigned rating between 1 and 10. This scale, or modifications of it,
has been successfully integrated into numerous aircraft development
programs.

Subjective Workload Assessment Technique (SWAT). This technique draws on
the findings of earlier psychometric research. SWAT ratings are derived
from subjective ratings of three major factors -- time load, mental
effort, and psychological stress. Each factor is rated on a 3-point
scale; twenty-seven 3 x 3 x 3 data points are created.

These intermediate data points are then subjected to conjoint
measurement techniques to derive a unitary measurement scale, which
presents results with interval properties.

Weighted Bipolar Ratings. This technique combines the information from
several subscales into a single weighted measure of workload for
specific flying tasks, or combination of tasks. Pilots answer series of
paired questions each from a different subscale to determine which of
the pair contributes more to their perceived workload.

Eventually all possible combinations from all subscales are compared and
through statistical evaluation a weighted result is obtained. Further
refinement of this technique has allowed results from individual tasks
to be combined to predict probable workload during the simultaneous
occurrence of more than one flying task (parallel task performance).

Another critical, but exceptionally difficult workload measurement
challenge occurs when pilots or crews are distracted by secondary tasks.
In this situation, the primary task is flying, in which varying degrees
of difficulty can be programmed, e.g., ranging from straight and level
flight to situations entailing weather or emergency operational
challenges. A secondary task is then required and the performance of the
secondary task is then critically evaluated. Decrements in the
performance of the secondary task are thought to provide indirect
information related to workload and pilot's workload capacity. As with
the primary task, the level of difficulty imbedded into the secondary
task can be varied.

In addition to efforts to assess workload through subjective means, much
effort has been devoted to the use of certain "biocybernetic measures."
These are basically physiologic parameters such as ECG, measurement of
stress related chemicals (cortisol, epinephrine, etc.) in blood or
urine, EEG, EMG, evoked potentials, and pupillary responses. Although
each of these potential parameters can produce easily quantifiable
objective data, there are drawbacks associated with their application to
workload assessment.

Two primary difficulties occur: (1) There is an enormous volume of data
generated, imbedded within normal background "biologic noise\" which
must be carefully filtered, evaluated, and correlated with the
psychological observations, and (2) The theoretical framework for
evaluation and integration of physiologic measures in workload
evaluations is imperfect and subject to debate. Nonetheless, the use of
selected physiological measures, interpreted in the context of overall
results, appears promising.

Workload control is most critical when unexpected events result in
emergency conditions. Frequently pilots must to perform multiple actions
in a brief time span. When faced with such conditions, the phenomenon of
*perceptual narrowing* may occur. In this condition, available physical
and mental resources become channeled toward the solution of narrow,
immediate problems, while excluding additional peripheral inputs. By
studying such conditions in simulator settings, a final determination of
acceptable pilot workload depends on how the individual pilots and the
crew perform during critical situations. Usually, the key issue is
whether the pilot or crew is able to achieve a successful outcome;
keeping in mind there may be a wide range of acceptable outcomes.

However, for the time being there is no single standard or method that
adequately describes and objectively measures workload in aerospace
applications. Some researchers have suggested that future attention may
also need to focus on measuring a lack of workload, which could occur in
the setting of advanced automation, in which the aircrew becomes subject
to a lack of stimulation with a resulting loss of vigilance. A related
concern has been voiced, that attention may be diverted to nonproductive
activities, similar to the phenomenon of perceptual narrowing. The
example, as frequently expressed, states that the probability for
human-induced accidents may *increase* with the use of automated systems
as a result of pilots attempting to program themselves out of
difficulty, rather than simply switching off automated systems and
reverting to manual control.

**Attention**

Attention refers to an individual's ability to concentrate on, and
perform, specific tasks. Often times in the aviation environment,
attention refers to an individual's ability to perform more than one
task at the same time. Extensive psychological research has shown
human's ability to accurately perform more than one task simultaneously
is severely limited. Many attempts have been made to train individuals
to perform more than one task at a time or to develop enabling
technology to allow such performance. To date though, the best human
factors specialists have been able to do is to automate many functions
and arrange the format and content of others so that they may be
performed in rapid sequence, if necessary. The goal is to allow
individuals to function accurately at full, or near-full, capacity.
Other research has indicated that cockpit errors in managing concurrent
tasks are more than a matter of work overload. In many occurrences, the
central challenge for the crew is division of attention (or effective
delegation of tasks to individual crew members), which requires rapid
focusing of attention from one task to another, such that individual
actions occur in sequence, even though multiple more complex actions are
being processed in parallel. This difficulty seems to be especially
prevalent if one of the tasks is temporarily suspended and there are no
strong environmental reminders that the task is incomplete, or the need
for a new, subsequent task arises.

In general, the consensus of the human factors / psychological research
indicates that well trained operators can process several streams of
incoming information at once, but output is limited to a single response
at any given instant. With high levels of training and current
proficiency, many output responses may be accomplished accurately in
rapid serial sequence. Since there are both inter- and intra- individual
variations, support systems must be designed with a high degree of
integrative flexibility so as not to over or under load a given operator
at any given time. The following discussion illustrates limitations of
human abilities for information reception, processing, and action:

"Many experiments have shown that our ability to attend to several
sources of information simultaneously is severely restricted. As a first
approximation, we can regard the human as a *limited-capacity channel*
who can only transmit a small amount of information per second. Whenever
this small amount (on the order of 10 bits/second) is exceeded, people
make errors. So, according to this model of attention, a pilot who must
process information that exceeds his channel capacity will make
mistakes. Perhaps an analogy will make this important point clearer. An
airport can land only so many planes per hour. This is its landing
capacity. If there is a need to land planes in excess of this capacity,
holding patterns must be assigned to augment the storage capability of
the system. If this were consistently required, the airport would try to
expand its landing capacity, perhaps by adding an extra runway.

The limited-capacity model states that the human is similar to an
airport with only one runway. At one time psychologists thought this
runway was very short and could handle only a single aircraft at one
time; this was known as the single-channel model of attention. Now we
know that the mental runway is long enough to hold more than one
aircraft under certain circumstances, analogous to landing one propjet
quickly after another. However, it is possible for one cognitive
function to take up all the runway space if it requires lots of mental
calculation, analogous to landing a heavy, wide-bodied jet with a
turbulent wake that prevents lighter aircraft from landing too soon
behind it." ^2^

Pilot (human) error

We are all well aware of the voluminous data attributing the majority of
aviation accidents to "human (pilot) error." The world's flight safety
specialists have recognized that total elimination of human error is not
a realistic goal, and have redirected the major effort in aviation human
factors safety training to error identification, containment, and
management (see CRM discussion below). For these efforts to be
successful however, we must gain an understanding of the root causes and
types of errors committed by the human operators of aviation systems.
Only then can effective measures be developed and deployed to manage
human errors. An important characteristic of human error accidents is
that they seldom, if ever, have a single cause.

Several methods are available to study human error; each has strengths
and limitations:

Direct observation. This requires a highly trained individual who is
able to remain in a position to observe flight crew actions for
prolonged periods. The observer must be skilled in the interpretation of
what the crew is doing at any given time and whether the observed
actions are correct for the given situation. However, it is important
that the presence of the observer not influence the crew to behave
differently than their normal patterns. This has been mostly overcome in
recent years through the use of sophisticated simulator devices in which
the observer need not be physically present in the same compartment with
the crew to make necessary observations (see 4, below). Two relative
weaknesses associated with this technique include potential errors made
by the observer and difficulty (especially in actual flying) in gaining
control of all variables.

Accident data and post accident analysis. Although this can be a rich
source of data to determine the probable causes of accidents, drawbacks
are associated with incomplete data in most cases. Although errors are
relatively easy to identify, it is often impossible to determine why the
errors were made.

Self report. The best know self-reporting system is the Aviation Safety
Reporting System (ASRS) operated by the NASA for the FAA. ASRS allows
flight crews to self-report errors that the crew recognized but did not
result in an accident or incident. The reporting individuals are
afforded confidentiality and immunity from disciplinary action, unless
there has been an act with criminal intent. Probably the greatest
strength of this system is the recognition of the magnitude of the error
problem in aviation. Whereas error identification formerly had been
mainly from accident investigation, after several years this system
produced voluminous data from which a much clearer picture of the
enormity of error emerged. Experts came to recognize a number of
patterns which had gone undetected previously, but which clearly would
have led to accidents had the chain of events not been resolved by the
crews.

These findings led to proactive actions such as revisions to crew
training and the issuance of "alert bulletins" through which end users
have amplified the value of the information gained from the reporting
system.

Simulator and/or laboratory conditions. As the sophistication and
complexity of simulation has evolved, the use of these devices for the
study of human error and its root causes have increased dramatically.
One of the problems associated with the use of simulation is its
tendency to oversimplify the environment so that observations gained in
the simulator setting cannot always be generalized to real world
application. In order to gain significant insight into the root
processes, which ultimately result in unwanted outcomes in aviation
operations, the operational conditions must be reproduced very
carefully. Present generations of simulators have been able to bridge
that gap in many respects so that crews are now able to conduct full
mission simulation for research purposed and simultaneously conduct line
oriented flight training (LOFT, see section on crew resource management
below).

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Investigation of human error proceeds from a thesis that accidents
happen for reasons, and prevention of accidents requires a detailed
knowledge of the underlying chains of reasons. The lack of detailed
knowledge makes the development of effective solutions difficult and
inefficient. Development of effective solutions will require that we not
only identify and understand the underlying factors, but solutions
involve being able to *predict* how humans will respond to certain
situations. A simplified conceptual framework to address these needs is
the "information -- decision -- action model" of piloting, which
includes the following stages (each is discussed in greater detail
elsewhere in this module):

Information acquisition and processing errors. In this stage the crew
receives or must obtain the right information at the right time. The
information must be filtered, analyzed, and shared appropriately.
Problems may occur with a lack of information, which the crew may or may
not recognize, physiological limitations that compromise information
acquisition (sight, hearing), poorly designed instrument interfaces,
momentary information overload, ineffective crew interactions.

Errors in deciding. A decision to act, or perhaps not to act, must be
made. This part of the process may require coordination with more than
one individual, and interaction with automated systems, both within and
external to the aircraft.

Action errors. At this stage decisions are implemented. Mistakes in the
form of poor decisions, decisions made too slowly, or an inability to
carry out a decision may all occur.

All three stages of this process occur within the context of goal
directed behavior. The goals may be simple or complex, set by the crews
themselves or established by predetermined protocols. In all cases it is
assumed that the crews possess adequate personal resources
(intelligence, skill, motivation, etc.) and that the intent of the crews
is *not to commit* errors. This conceptual model can be better
understood within the framework of the previously discussed SHEL model.
One can easily identify the multiplicity of interactions between the
liveware (flight crews, air traffic controllers, and cabin crews),
software and hardware elements as the three stages of the information --
decision -- action model of aviation operations are considered. Root
cause analysis of human error will frequently identify a system design
in which human limitations, or the abilities of the human to effectively
interface with the system were not adequately assessed.