Causes ‘Human Error’ in Aircrew?
An understanding of how we humans
make decisions, and of why we can get them wrong, can help pilots to prevent
incidents deteriorating into accidents.
psychology, like aviation itself and other areas of specialised study, is
riddled with jargon used as a kind of shorthand communication. This is fine
if you already have an understanding of the subject but can obscure the
meaning when the subject is first approached. I will try to avoid
We have a great capacity for gathering information through all of our
senses. There is a constant stream of inputs from sight, hearing, smell,
touch, balance mechanisms and memory, each coming into our brain on separate
channels and all arriving at more or less the same time. Unfortunately our
brains are limited to just one decision-making channel that has to be shared
between all the different sensory inputs. This means that while we are analysing
one input the inputs from all the other channels are held in our short-term
memory store to be retrieved later. There lies the potential for human error.
The storage and retrieval system is prone to error. The performance of
individuals’ short-term memories is also erratic.
The decision-making part of our brain has a limited capacity and can
become overloaded. When we reach this situation we tend to discard
information in a fairly random manner, sometimes causing vital inputs to be
either discarded or not recognised. Another reaction to overload or stress is
to concentrate totally on one piece of information input while ignoring other
inputs that may be more important. This is known as ‘perceptual
tunnelling’ when it affects visual input. We concentrate solely on one
object in view and simply do not see what is going on in the rest of the
visual field of view. Military pilots know this as ‘target
fixation.’ This is the recognised cause of military aircraft continuing
down the attack dive until the aircraft impacts the target. Total
concentration on one thing can be fatal.
Rationality vs Intuition
Decisions are arrived at by using two different components from our brains.
The rational component takes information from conscious thought and
choice, but the intuitive component takes stored information from our
long-term memory. The intuitive component may override the rational
under periods of high stress, which explains why a pilot handling an
emergency in an unfamiliar type may revert to action appropriate to a
different aircraft type on which he has had more experience.
We process information from our senses in five sequential steps.
These are, in order, sensing, perception, decision making, motor action and
feedback. Unfortunately, each of these processes is subject to error.
Decision making may be defined as the process of assessing all the
information available to our senses and then selecting an appropriate
response in often complex situations where several responses are possible.
The ability to make correct decisions on the information available is an
important part of situational awareness, airmanship and good flying
judgement. It is thought that the ability to make correct decisions improves
with experience and can also be improved with initial structured training and
regular continuation training. The quality of your basic training and the
frequency of your continuation training have great and lasting influence on
your abilities and standards as a pilot.
Several personal factors affect the efficiency of decision making by
individuals. These factors include the clarity of the individual’s
mental model of the current situation, the method used to solve the problem
(trial and error?), the assessment of likely outcome and the ability to
recognise personal limitations. Drugs, alcohol, medication and inattention do
nothing to enhance your mental model of the current situation. Overconfidence
Risk Assessment and Stress
The assessment of risk is a normal part of daily life. Each risk, however
large or small, has to be recognised, considered and a sensible conclusion
reached as to the action required. The risk must be recognised before it can
be assessed, but failure of recognition often leads to a hazardous outcome.
More complex judgements involving multiple factors, for example during the
approach and landing, develop later in training.
Flying training develops motor skills that should lead to the ability
to make correct decisions as each situation develops. Early training
emphasises basic motor skills and simple judgements such a controlling an
aircraft accurately and flying accurate speeds and correct flight paths. This
is another reason why student pilots should be able to exhibit regularly
their ability to fly accurate speeds, heights and headings before they are
introduced to circuit flying. Using things that can be seen, touched and
operated develop motor skills, but cognitive judgements and decisions are
abstract, using intelligence, experience and awareness. Cognitive judgement
takes longer to develop than motor skills.
Different pilots react to stress in different ways. High stress
levels will affect performance during flight. There are four sources of
stress affecting pilots in the air:
Physical. Factors in the immediate environment such as temperature,
vibration and noise can all induce stress especially in pilots unused to such
Physiological. Physical factors such
as fatigue, hunger and general fitness can lower a pilot’s resistance
Psychological. Emotion, workload, distraction and the need to
make critical decisions can increase stress.
Sociological. Emotional stress arising from outside the cockpit, such as
job pressure or marital problems, can raise stress levels and reduce
decisions have to be made quickly it is all too easy to make impulsive or
inappropriate decisions. Lack of time available to make decisions multiplies
the risk of
getting the decision wrong. It is possible to plan a course of action to be
taken in advance of the situation arising, thus arriving at decisions in an
unstressed state. The ideal place to plan your actions in an emergency is
while relaxing in your favourite armchair before beginning a flight.
Positive action Flight Safety programmes have reduced almost all
aircraft accident causal factors. The one factor that has proved most
difficult to reduce is ‘human error.’ The Confidential Human
Factors Incident Reporting Programme (CHIRP) aims to recognise causes of
human error incidents and to seek to remove these causal factors. The CHIRP
office receives many revealing reports from those involved in aviation that
recognise their own human error and want others to benefit from uncovering
the many causes of these incidents. Unfortunately, some human errors lead to
accidents where those involved do not survive.
In 1992 the United Kingdom Civil Aviation Authority (CAA) introduced
an examination in Human Performance and Limitations for applicants for all
private and professional pilot licences. The European JAR-FCL 1, implemented
in 1999, details the syllabus for the new examination in this subject. There
are probably many UK holders of PPLs who were not required to sit this
examination and who have never received any formal training in this subject.
It is strongly recommended that these pilots should study the subject.
“Human Performance and Limitations in Aviation” by RD Campbell
and M Bagshaw, available from our
Bookshop is well worth reading
– whether you sat the CAA examination or not.
Remember the old adage that there are only two kinds of pilot that
need refresher training, those not in regular flying practice and those in
regular flying practice.
A refresher of your understanding of ‘decision
making’ should help you avoid getting your decisions wrong.
Yaw in Helicopters
Two examples of accidents to UK
registered helicopters that were caused by a phenomenon that may not be
widely understood by some UK helicopter pilots.
accident involved a Hughes 369HS taking off from a hotel site. The wind
direction was 10-20 degrees left of the helicopter nose at about 10kt. As the
pilot lifted gently into a low hover that required about 90% torque the
helicopter began an uncommanded yaw to the right. Despite the application of
full left tail rotor control pedal the helicopter continued to yaw rapidly to
the right. The pilot believed the aircraft had suffered a tail rotor failure
and closed the throttle as the helicopter began a second rotation. The yawing
ceased as the landing skid contacted the ground. The aircraft suffered
extensive damage to the tail rotor and the tail pylon as the tail rotor
struck the ground. There was no evidence of an in-flight mechanical failure.
The second accident involved a Bell 206B Jet Ranger III engaged in
low-level photography. The flight profile involved an approach to the
‘target’ on an easterly track followed by a slow speed and low
level right turn around the ‘target’ while the cameraman filmed
from the right rear door of the helicopter. The surface wind was
approximately 140/10kt but there were thunderstorms and showers some distance
away. There is a possibility that a gust front moved through the area during
the flight. The local terrain was undulating. These factors combine to make
it difficult to be certain of the actual surface wind at the time and place
of the accident. The first run was judged slightly too fast and too close to
the structure. The second, slower, run went without incident until half way
around the turn the helicopter began to yaw to the right. The pilot applied
corrective left pedal that did not control the right yaw, leading the pilot
to suspect a tail rotor failure. He centred the pedals and then reapplied
full left pedal. The helicopter continued to rotate to the right, out of
control. Several revolutions were completed before the Jet Ranger struck
sloping ground at low forward speed and rolled onto its right side. The three
occupants vacated the aircraft with minor injuries. There was no evidence of
a technical malfunction before the accident.
The AAIB Field Investigation (EW/C2003/05/07) into the Jet Ranger
accident uncovered evidence that the helicopter may have been operating in
part of the flight envelope where loss of tail rotor effectiveness (LTE) was
The LTE Phenomenom
tail rotor effectiveness (LTE) is a critical, low speed aerodynamic
characteristic that can result in an uncommanded rapid yaw rate that does not
subside of its own accord and can result in loss of control.
LTE may occur in all single main rotor helicopters below 30 KIAS. It
is not necessarily a function of control margin deficiency. The
pre-certification flight testing determines that the helicopter has adequate
control authority for the approved sideways and rearward flight velocities
plus the ability to counteract gusts of reasonable magnitude. The
certification assumes that the pilot understands the critical wind azimuth
for the helicopter operated and maintains control of the helicopter by not
allowing excessive yaw rates to develop.
LTE has been identified as a contributing factor in several
helicopter accidents, both in the UK and the USA, involving loss of control.
These accidents have occurred in low-altitude, low-airspeed flight while
manoeuvring. Typical civil operations involved include powerline inspection,
low-level survey, agricultural spraying, police/traffic watch, emergency
medical rescue and filming flights.
understand the LTE phenomenon we must understand the function of the
anti-torque system. Helicopters manufactured in the USA have a main rotor
that rotates anti-clockwise viewed from above. Some European and Russian
helicopter main rotors rotate clockwise viewed from above. The
main rotor torque tends to rotate the fuselage in a direction opposite to the
rotation of the main rotor. The anti-torque system provides thrust to
counteract this rotation and to provide directional control, particularly at
low airspeeds. For simplicity, for the rest of this discussion we will only
consider USA manufactured single-rotor helicopters with their anti-clockwise
The pilot controls tail rotor thrust by using the anti-torque pedals.
If the tail rotor generates more than the thrust required to counteract the
torque from the main rotor the helicopter will yaw or turn left about the vertical
axis. If less tail rotor thrust is generated the helicopter will rotate to
the right. The pilot controls the heading while hovering by varying the tail
rotor thrust via the pedals.
In no-wind conditions, for any given main rotor torque setting, there
is an exact amount of tail rotor thrust required to prevent the helicopter
from yawing either left or right. This is known as the tail rotor trim
thrust. The pilot must maintain tail rotor thrust equal to trim thrust to
maintain a constant heading while hovering in still air.
Helicopters can be subjected to constantly changing wind speed and
direction. The tail rotor thrust required is modified by the effect of these
wind variations. If an uncommanded right yaw occurs this may be due to a
reduction in the effective tail rotor thrust because of the wind effect. The
wind effect can also add to the anti-torque thrust, producing an uncommanded
left yaw. Certain relative wind directions are more likely to produce tail
rotor thrust variations than others. These relative wind directions or
regions form an environment conducive to LTE.
Conditions under which LTE may occur
manoeuvre requiring the pilot to operate in a high-power, low-airspeed
environment with a left crosswind or tailwind produces a situation where
unanticipated right yaw may occur. There is a greater likelihood of LTE in
right turns. Immediate application of additional left pedal is an essential
response to an uncommanded right yaw. The pilot may not be able to stop the
rotation at low airspeeds. Recovery may be impossible if the reaction is slow
Computer simulation has shown that if the pilot delays in reversing
the applied pedal position when changing from a left crosswind situation
(where a lot of right pedal is required to counteract sideslip) to a downwind
situation, control would be lost and the aircraft would rotate rapidly
through more than 360° before stopping. The pilot must anticipate these
variations in pedal application, concentrate on flying the aircraft, and not
allow a yaw rate to develop. Particular caution must be exercised when
executing right turns in conditions conducive to LTE.
wind tunnel tests have identified four relative wind azimuth
regions and aircraft characteristics that can, singly or in combination,
create an environment conducive to LTE and loss of control. One result of
these tests is that operating a helicopter at low airspeed dramatically
increases the pilot’s workload. These characteristics occur only below
30 knots IAS and apply to all single rotor helicopters.