Gremline Flight Safety Report: Causes of Human Error & Unanticipated Yaw in Helicopters

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What 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.

Aviation 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 jargon.
      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 component 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 is dangerous.

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 factors.

Physiological. Physical factors such as fatigue, hunger and general fitness can lower a pilot’s resistance to stress.
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 efficiency.

When 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.



Unanticipated 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.

The first 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 possible.

The LTE Phenomenom
Loss of 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.


Understanding LTE
To 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 rotating main rotor.
      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
Any 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 or incorrect.
      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.

Flight Characteristics
Flight and 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.



4. Loss of Translational Lift. Loss of translational lift can occur while turning at low airspeed with the relative wind from any direction. This condition is most significant when operating at or near maximum power and is associated with LTE for two reasons. First, if the pilot’s attention is diverted by an increasing right yaw rate, the pilot may not recognise that the relative headwind is being lost and thus translational lift is reduced. Second, if the pilot does not maintain airspeed while making a right downwind turn the power demand will increase, the aircraft will sink and the aircraft can suffer an accelerated right yaw rate. When operating at or near maximum power in this situation the increased power demand could result in a decrease of rotor rpm. Insufficient attention to wind velocity (speed and direction) can lead to an unexpected loss of translational lift.
      Allowing the aircraft to drift over the ground with the wind results in a loss of relative wind speed and a decrease in translational lift. This results in an increased power demand and increased anti-torque requirements.

Other Factors that can significantly influence the severity of the onset of LTE include gross weight and density altitude, low indicated airspeed and power droop.
An increase in either
gross weight or density altitude will decrease the power margin between maximum power available and the power required to hover. Low level, low airspeed operations should be conducted at minimum weight.
low airspeeds (below translational lift) the tail rotor is required to produce nearly 100% of the directional control. If the required amount of tail rotor thrust is not available for any reason the aircraft will yaw to the right.
A transient
power droop may occur during a rapid power application by the pilot. Any resultant decrease in main rotor rpm will cause a corresponding decrease in tail rotor thrust. The pilot must anticipate this and all power demands must be made as smoothly as possible to minimise power droop.

Reducing the onset of LTE
There are several actions that can be taken by a pilot to reduce the chances of encountering LTE. These include:

Ensure that the tail rotor is rigged in accordance with the maintenance manual.

Maintain maximum power-on rotor rpm. If the main rotor rpm is allowed to decrease, the anti-torque thrust available is decreased proportionally.

When manoeuvring between hover and 30 KIAS:

1. Avoid tailwinds. Loss of translational lift will result in an increased power demand and an additional anti-torque demand.

2. Avoid out of ground effect (OGE) hover and high power demand situations, such as low speed downwind turns.

3. Be especially aware of wind direction and speed when hovering in wind speed of about 8-12 knots, especially OGE. The pilot has no strong indications of a reduction in translational lift and the subsequent high power demand and increases anti-torque requirement can be unexpected.

4. If a considerable amount of left pedal is already required there may not be sufficient left pedal available to counteract an unexpected right yaw.

5. Be alert to changing aircraft flight and wind conditions when flying along ridge lines and around buildings.

6. Remain vigilant to power and wind conditions.


Recommended Recovery Techniques
If a sudden unanticipated right yaw occurs the pilot should apply FULL left pedal and simultaneously move the cyclic forward to increase airspeed. If height permits, reduce power. Then adjust the controls to normal flight as yaw control is recovered. Collective pitch reduction will aid in arresting the yaw rate but may cause an increase in the rate of descent. Any large, rapid increase in collective to prevent ground contact may further increase the yaw rate and decrease main rotor rpm. If the rotation cannot be stopped and ground contact is imminent then an autorotation may be best. The pilot should maintain FULL left pedal until the rotation stops, and then maintain a constant heading.

The various wind directions can cause significantly different rates of turn for any given pedal position. The pilot must remember that the tail rotor is not stalled. The corrective action is to immediately apply pedal to correct the yaw.
      The best way to avoid LTE is to avoid conditions conducive to LTE. Correct and immediate response to LTE is essential and critical to the outcome.
      The pilot can significantly reduce exposure to LTE by being acutely aware of wind and its effect on the helicopter when operating at low airspeeds.

This summary is based closely on AAIB Field Investigation EW/C2003/05/07 and on US Federal Aviation Administration Advisory Circular No 90-95, which sources are gratefully acknowledged. The figures are copied from FAA AC No 90-95.

Further References

“Supplemental Operating and Emergency Procedures”, Operations Safety Notice OSN 206-83-10 (31 Oct 83), Bell Helicopter Textron.

“Low Speed Flight Characteristics Which Can Result in Unanticipated Right Yaw”, Information Letter 206-84-41 and 206-84-27, Bell Helicopter Textron.

“OH-58 Loss of Tail Rotor Effectiveness – Why it Occurs”, Sneelen, DM., US Army Aviation Digest, September 1984.

“The Downwind Turn: Losing Directional Control”, Prouty RW, Rotor and Wing, May 1994.

“More on the OH-58 LTE Problem” Flightfax: Report of Army Aircraft Mishaps, Vol 13 No 32, May 1985.

“Loss of Tail Rotor Effectiveness - When It Is and When It Isn’t”, Flightfax: Report of Army Aircraft Mishaps, Vol 14 No 1, September 1985.



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FIGURE 1. Main Rotor Disc Vortex Interference

1. Main Rotor Disc Vortex Interference. Main rotor disc vortex interference with tail rotor effectiveness will occur with relative wind azimuths between 285° and 315° (taking the aircraft nose to be 360°). Figure 1 MAIN ROTOR DISC VORTEX INTERFERENCE illustrates the vortices streaming downwind from the edges of the main rotor disc with one of these vortices disrupting the airflow around the tail rotor. Winds of about 10 to 30 knots from the left front of the aircraft will cause a vortex from the main rotor to be blown into the tail rotor. This will cause the tail rotor to operate in an extremely turbulent airflow. The tail rotor does NOT stall.
      The sequence of events that occur during a
right turn at low airspeed with the relative wind from the left front is as follows. The turn begins with the relative wind from somewhere between, say, dead ahead and about 30° left of the aircraft’s heading. As the right turn is continued the angle off of the relative wind direction is increased relative to the aircraft’s heading. When the relative wind angle reaches 315° (45° left of the aircraft nose), the tail rotor will enter the main rotor vortex and experience a sudden reduction in anti-torque thrust. (Refer to Figure 1). This reduction in tail rotor anti-torque thrust will cause the nose of the aircraft to suddenly swing right at a rate faster than expected by the pilot and faster than anticipated from the pilot’s control input.
      This condition of disturbed airflow around the tail rotor, and reduced anti-torque thrust, can persist for another 30° of right turn, until the relative wind angle increases to more than 75° left of the aircraft nose. The pilot’s first impression may be of a tail rotor stall or of the mechanical failure of the tail rotor. The uncommanded right yaw will accelerate unless immediately checked by left pedal input and the aircraft will enter a rapid and uncontrollable right rotation about the mast.

2. Weathercock Stability. Weathercock stability is a factor producing uncommanded rates of yaw in either direction when operating at low airspeed with a relative tailwind blowing from an arc 60° either side of the tail of the aircraft. Tailwinds from this sector can cause an acceleration of the yaw rate because the wind, acting on the fuselage and vertical fin of the aircraft, will tend to weathercock the nose into the relative wind. The helicopter will make a slow uncommanded turn in either direction, depending upon the tailwind direction. If the aircraft is being yawed intentionally in these conditions, the yaw rate can accelerate without any increase in pedal input until corrective pedal is applied. If the pilot allows a right yaw to develop and the tail of the helicopter moves to an arc within 60° of the wind direction, then the yaw rate can accelerate rapidly. The pilot must concentrate on maintaining positive control of the yaw rate and devote full attention to flying the aircraft whenever the aircraft is operating at low airspeed in a downwind condition.



FIGURE 2. Weathercock Stability

3. Tail Rotor Vortex Ring State. A tail rotor vortex ring state can develop while operating at low airspeed with a wind from an arc extending 60° either side of 90° left of the aircraft nose. Figure 3 illustrates this condition. This tail rotor vortex ring causes a non-uniform, unsteady airflow into the tail rotor with rotor thrust variations and yaw deviations. The tail will tend to oscillate in yaw, demanding continuous pedal movements when hovering in left crosswind conditions. LTE can then occur if the pilot over-controls the aircraft. If a right yaw is allowed to develop then the aircraft can rotate until the tail swings into the area where the relative wind is from the rear sector (see Figure 2) and weathercock stability accelerates the right yaw rate until the pilot loses control.

FIGURE 3. Tail Rotor Vortex Ring State