Loss of tail-rotor effectiveness


" Loss of 'tail rotor effectiveness "' shall not be taken  literally. It has been used in the early 1980s by the US Army to designate unanticipated yaw of a helicopter and does not mean that at any time the efficiency of the anti-torque rotor is diminished. Such a behavior has never been demonstrated.
This phenomenon remains poorly understood, and literature on the subject provides little light on how it occurs. It is true that flight mechanics struggles to explain a helicopter rotating rapidly to the right with the pedals fully extended to the left. Complex aerodynamic phenomena that cause rapid variations in tail rotor thrust have only been identified, and it has been recommended that the areas in which they occur be avoided.
Accidents characterized by a loss of yaw control on a fully operative aircraft are numerous. An analysis by Airbus Helicopters has shown that they mostly occur close to the ground, where the maneuver usually recommended for recovery is not applicable. Airbus Helicopters explains the phenomenon based on the pedal curve. This explanation assumes that the pilot only counters the yaw start with a limited amplitude control and does not reach the control stop. Objective data collected during such accidents are beginning to become more numerous and confirm this limited amplitude.
Airbus Helicopters' explanation covers not only the majority of events in the opposite direction to the rotation of the main rotor, but also the rarer cases in the direction of this rotation and even accidents occurring on NOTAR aircraft for which it is difficult to incriminate the interactions between the main rotor and the tail rotor or the vortex regime of the tail rotor.

'''Historical'''

'''US Army and Bell'''

At the end of the 1970s, the US Army records a series of similar accidents on its OH‑58 helicopters, a military version of the Bell 206. At low speed, during a right turn, the yaw rate increases without any action from the pilot and the tail rotor appears incapable of stopping it.
A complex theory is proposed, involving an alternation of stalls and recoveries of the tail rotor assumed to be at its maximum pitch. The phenomenon is then called " tail rotor stall ", without this theory being experimentally validated.
A new procedure is defined by the US Army which consists, when the yaw movement to the right is detected, to add foot to the right, in the direction of the turn and cyclic stick to the front-right. A working group, bringing together different components of the US Army with the participation of Bell Helicopter Textron, is also set up to understand the problem and remedy it. Its findings are released in 1984.
How unanticipated yaw occurs is not explained, showing that it is not well understood. A curve measured in a wind tunnel giving, as a function of the wind direction relative to the helicopter, the value of the yaw moment created by the tail rotor—at constant main rotor and tail rotor collective pitches—is used to identify the wind directions giving rise to the most significant changes, estimated to be most conducive to the occurrence of UY. Three areas are thus highlighted at low speeds, which pilots are recommended to avoid as much as possible:
1. wind in the sector between 280° and 330°, in an area corresponding to the passage of the main rotor horseshoe vortex to the tail rotor,
2. wind in the sector between 210° and 330°, zone where the tail rotor can enter into vortex regime ,
3. Tailwind, between 120° and 240°, where the instability of the helicopter places a significant workload on the pilot.
Further lessons are learned from these tests and a flight campaign.
  1. the OH-58 is not the only helicopter affected and any single-rotor helicopter can experience unanticipated yaw.
  2. the hypothesis of a tail rotor stall is eliminated, explaining why the new procedure did not improve anything
  3. an increase in tail rotor pitch never has adverse consequences.
  4. the basic tail rotor of the OH-58 has always been able to stop yaw rates up to 115° per second by applying maximum pitch together with forward cyclic. It is not said whether this forward cyclic is mandatory or only accelerates the exit from the problem.
  5. the recovery procedure is modified with the application of full left pedal and, simultaneously, forward cyclic to increase speed. If the height allows it, lowering the collective can help.The forward cyclic in addition to the pedals, ''the natural control around the yaw axis, suggests that the tail rotor alone is not enough''.
Three improvements to the OH-58 are also proposed: a power transient optimization kit, a 3-axis automatic pilot and finally the increased diameter tail rotor.
A further research program is finally recommended, of which no trace is found subsequently. LTE accidents reported in Flightfax, the US Army's mishap prevention magazine, are becoming fewer, although some still occur—and not always on OH-58s—as acknowledged on the occasion of Flightfax 's 20th anniversary.
At about the same time, Bell publishes an Operations Safety Notice and then an Information Letter containing roughly the same information on the phenomenon. Bell uses " unanticipated right yaw " to describe it, but reports in the second letter that the US Army calls it " Loss of Tail Rotor Effectiveness ", a term Bell considers misleading.

'''AC 90-95'''

Such accidents also affect civil helicopters and in 1995 the FAA publishes AC 90-95. This is still considered today as the "bible" of unanticipated yaw. Other documents follow, all over the world, which clearly take up the information.
The AC 90-95 is consistent with the results given by the US Army with one notable exception. While the US Army Information Digest gave the reassuring message that even the extreme yaw rates encountered during the tests could always be stopped, the AC 90-95 suggests on several occasions that this might not be the case.
  • “…since the pilot may not be able to stop rotation”
  • “…the yaw rate may rapidly increase to a point where recovery is not possible”
  • “If the rotation cannot be stopped…”
Finally, there are some awkward formulations, particularly in the use of the term LTE. While it is initially taken as a synonym for " unanticipated yaw ", we can also read on the same page " LTE is a critical, low -speed aerodynamic flight characteristic which can result in an uncommanded rapid yaw rate which does not subside… ". After stating that LTE was the yaw motion, it becomes the cause. This makes it difficult to properly understand this term.
20 years later, the problem is still not resolved and the NTSB, the body responsible for investigating aviation accidents in the USA, publishes a Safety Alert in March 2017 reporting that between 2004 and 2014 LTE was the cause of 55 helicopter accidents in the United States and recalling the content of AC 90-95.

'''Airbus Helicopters'''

In 2019, Airbus Helicopters presents a paper with a reminder of the above history, an analysis of accidents recorded during the period 2000-2016 and, for the first time, an explanation of the phenomenon, based on the pedal curve giving the position of the pedals as a function of the heading relative to the wind in hover, at constant mass, altitude, temperature and wind speed.
Over the period under review, more than 300 accident reports were found involving loss of yaw control on civilian helicopters, without any prior failure. These are encountered in many countries, although they are much more numerous in the United States, which must reflect the greater use of this vehicle there. The vast majority of these events occur in the opposite direction to the rotation of the main rotor, although there are a few occurrences in the rotor rotation direction. The multiplicity of helicopter types involved validates the AC90-95 statement that all single-rotor helicopters are affected. There are even a few cases on aircraft equipped with NOTAR, for which the use of the term LTE can raise a smile if we forget the meaning originally given to it.
Finally, we see that accidents most often occur close to the ground, which does not allow for the recommended recovery maneuver. The translation start suggested by the forward cyclic may not be easy to achieve on a machine in rapid yaw rotation, in the middle of obstacles.

'''The pedal curve'''

This approach of Airbus Helicopter is presented in the article already cited and in a Safety Information Notice for pilots.
The pedal curve represents, for a given condition of mass, altitude, temperature and wind speed, the evolution with the wind heading of the pedals position in hovering flight. The following figure shows how it evolves as a function of wind speed on a Bell 206-B1.
On this aircraft, the rotor thrust at trim in hover is oriented towards the right of the aircraft and the maximum collective pitch of the tail rotor corresponds to the left stop of the pedal.
Without wind, this pedals position is unique and the curve is therefore flat. The higher the wind speed, the greater the amplitude of the curve. When the wind comes from the left, it increases the blade angle of attack and consequently the tail rotor thrust. The pilot must therefore reduce the tail rotor's collective pitch to maintain thrust, as with a main rotor in descending flight. The opposite is true when the wind comes from the right. This is what gives the foot curve its sinus wave shape.
The effect of a collective pitch change on this curve is easy to understand. An increase in collective pitch comes with an increase in torque and therefore requires more anti-torque, i.e. more collective from the tail rotor. Increasing the main rotor collective therefore shifts the pedal curve downward.
The pedal curve then allows simulating, at least qualitatively, what happens when moving the collective stick or the pedals. The starting point is positioned at trim, on the curve. An action on the pedals moves the point vertically without changing the curve. An action on the collective moves the curve without modifying the point. Once the command is entered, the point moves to the right as long as it is above the curve and to the left when it is below.

'''Unanticipated yaw'''

With a pedals position above the highest point of the pedal curve, a right yaw builds that will never stop, since there is no equilibrium corresponding to such a pedals position. With a position below the lowest point, an uninterrupted left yaw will start. With an intermediate position, the yaw motion eventually stops when crossing the stable part of the pedal curve.
A right yaw that does not stop by itself can therefore be explained very easily. All that is required is to place the pedals in a position above the highest point of the foot curve. If you start from a headwind hover and gradually add right pedal, each change initially increases the heading and stabilizes it, until the maximum of the pedal curve is exceeded. The aircraft enters then an endless rotation until the pilot brings the pedals back sufficiently to the left. This change in the aircraft's behavior is unpredictable, if the pilot is not warned and does not know precisely the wind direction.
Another way to trigger this permanent yaw movement is to increase the collective pitch without pedals compensation when starting close to the maximum of the pedal curve, with wind coming from the left. The curve shifts downward, its maximum comes below the pedals position and a right yaw rotation begins that will not stop until the pilot puts his foot back sufficiently to the left.
Here again, this behavior is difficult to predict. At any other point in the headwind sector, such an increase in collective will only result in a limited change of heading to the right, the yaw rate being cancelled as soon as the new pedal curve is reached.

'''Why so many accidents?'''

While it is understood that the pilot may be surprised by a change in the helicopter's response to small pedals inputs, it is more difficult to interpret the lack of recovery of the maneuver. Indeed, simply bringing the pedals back within the range of the pedal curve is enough for the aircraft to stop as soon as the stable part of the pedal curve is reached.
When the aircraft yaws to the right from the maximum of the pedal curve, the equilibrium pedals position corresponding to the actual wind heading begins to move downward. This downward movement of the trim control reflects a decrease in the thrust of the tail rotor at constant pitch due to the change of heading relative to the wind. If the pilot reacts with a delay to the unexpected start of yaw, the pedals step that is necessary to stop the maneuver is of a completely different amplitude than that which triggered it. If he has never been warned, if the phenomenon has not been explained to him, the pilot will simply note that his action has no effect and conclude that the tail rotor has lost its effectiveness. Since it has been suggested to him that the tail rotor may not be sufficient, he will then try to land while limiting the damage.
Recordings made during accidents all point in this direction, even when the pilot is convinced that he has put his foot to the stop. The US Army reports 5 accidents in 2023-2024 on AH-64E Apache helicopters due to unanticipated yaw and flight recorder data shows that the crews never reached the left pedals stop. A Russian article similarly claims that all analyses of accident data on Mi-8 helicopters equipped with recorders also conclude that the maximum tail rotor pitch was never reached.
Accidents do not occur despite the pedals being fully depressed, as was thought when Bell and the US Army concluded their tests or when the FAA wrote AC 90-95, but because the pedals correction is insufficient.
Unfortunately, apart from EASA which has put online a video for pilots presenting the problem by the pedal curve, the authorities have not yet reacted to the new information brought by the recent accidents.
US Army analyses, based on the erroneous belief that full pitch of the anti-torque rotor did not stop the phenomenon, are still taught to pilots. However, they have shown that they do not prevent accidents due to "LTE". Since 2017, we can still count more than 50 of them, which occurred in the United States, in the NTSB database. However, it is enough to note that such accidents also affect aircraft equipped with NOTAR to doubt the importance of the interactions between the main rotor and the tail rotor or the vortex regime of the tail rotor on the phenomenon
'''References'''