f m+ 3 

U.S. Department Advisory 

of Transportation 

Federal Aviation 

Administration Circular 

Subject: UNANTICIPATED RIGHT YAW Date: 12/26/95 

IN HELICOPTERS Initiated by: AFS-804 

1. PURPOSE. This advisory circular (AC) will 

examine unanticipated right yaw phenomenon, the 

circumstances under which it may be encountered, 

how it can be prevented, and how the pilot should 

react if it is encountered. 

2. RELATED READING MATERIAL. Bell 

Helicopter Textron, Supplemental Operating and 

Emergency Procedures, Operations Safety Notice, 

OSN 206-83-10 (October 3 1, 1983), Bell Helicopter 

Textron; Bell Helicopter Textron, Low Speed Flight 

Characteristics Which Can Result in Unanticipated 

Right Yaw, Information Letter 206-84-41 and 2060 

84-27 (July 6, 1984), Bell Helicopter Textron; 

Sneelen, D.M., OH-58 Loss of Tail Rotor Effectiveness - Why It Occurs, U.S. Army Aviation Digest 

(September 1984), U.S. Army Aviation Center; 

Prouty, R.W., The Downwind Turn: Losing Directional Control, Rotor and Wing (May 1994), Phillips 

Business Information, Inc.; More on the OH-58 LTE 

Problem, Flightjbx: Report of Amzy Aircraft Mishaps, Vol. 13, No. 32 (May 22, 1985), U.S. Army 

Safety Center; Loss of Tail Rotor Effectiveness...When It Is and When It Isn’t, Flightfax: Report 

ofArmy Aircraft Mishaps, Vol. 14, No. 1 (September 

25, 1985), U.S. Army Safety Center; U.S. Army, 

OH-58 Helicopter Operators Manual, TM 55.15200 

228-10, U.S. Army; U.S. Naval Air Training Command, Flight Training Instructions, TH-57 (1989), 

U.S. Naval Air Training Command. 

in several civil helicopter accidents wherein the pilot 

lost control. In most cases, inappropriate or late 

corrective action may have resulted in the development of uncontrollable yaw. These mishaps have 

occurred in the low-altitude, low-airspeed flight 

regime while maneuvering, on final approach to a 

landing, or during nap-of-the-earth tactical terrain 

flying. Typical civil operations include powerline 

patrol, electromagnetic survey, agricultural spraying, 

livestock herding, police/radio traffic watch, emergency medical service/rescue, and movie or television support flights. 

4. THE PHENOMENA OF LTE. 

CI, LTE is a critical; low-speed aerodynamic 

flight characteristic which can result in an 

uncommanded rapid yaw rate which does not subside 

of its own accord and, if not corrected, can result 

in the loss of aircraft control. 

3. BACKGROUND. Unanticipated right yaw, or 

loss of tail rotor effectiveness (LTE), has been determined to be a contributing factor in a number of 

accidents in various models of U.S. military helicopters. The National Transportation Safety Board 

(NTSB) has identified LTE as a contributing factor 

6. LTE is not related to a maintenance malfunction and may occur in varying degrees in aZZ single 

main rotor helicopters at airspeeds less than 30 knots. 

LTE is not necessarily the result of a control margin 

deficiency. The anti-torque control margin established during Federal Aviation Administration 

(FAA) testing is accurate and has been determined 

to adequately provide for the approved sideward/ 

rearward flight velocities plus counteraction of gusts 

of reasonable magnitudes. This testing is predicated 

on the assumption that the pilot is knowledgeable 

of the critical wind azimuth for the helicopter operated and maintains control of the helicopter by not 

allowing excessive yaw rates to develop. 

c. LTE has been identifid as a contributing factor in several helicopter accidents involving loss of 

AC No: 90-95 

Change: 
AC 90-95 12l26495 

control. Plight operations at low altitude and low airspeed in which the pilot is distracted from the 

dynamic conditions affecting control of the helicopter are particularly susceptible to this phenomena. 

The following are three examples of this type of 

accident: 

(1) A helicopter collided with the ground following a loss of control during a landing approach. 

The pilot reported that he was on approach to a ridge 

line landing zone when, at 70 feet above ground level 

(AGL) and at an airspeed of 20 knots, a gust of 

wind induced loss of directional control. The helicopter began to rotate rapidly to the right about the 

mast. The pilot was unable to regain directional control before ground contact. 

(2) A helicopter impacted the top of Pike’s 

Peak at 14,100 feet mean sea level (MSL). The pilot 

said he had made a low pass over the summit into 

a 40knot headwind before losing tail rotor effectiveness. He then lost directional control and struck the 

ground. 

(3) A helicopter entered an uncommanded 

right turn and collided with the ground. The pilot 

was maneuvering at approximately 300 feet AGL 

when the aircraft entered an uncommanded right 

turn. Unable to regain control, he closed the throttle 

and attempted an emergency landing into a city park. 

5. UNDERSTANDING LTE PHENOMENA. To 

understand LTE, the pilot must first understand the 

function of the anti-torque system. 

a. On U.S. manufactured single rotor helicopters, the main rotor rotates counterclockwise as 

viewed from above. The torque produced by the 

main rotor causes the fuselage of the aircraft to rotate 

in the opposite direction (nose right). The anti-torque 

system provides thrust which counteracts this torque 

and provides directional control while hovering. 

b. On some European and Russian manufactured helicopters, the main rotor rotates clockwise 

as viewed from above. In this case, the torque produced by the main rotor causes the fuselage of the 

aircraft to rotate in the opposite direction (nose left). 

The tail rotor thrust counteracts this torque and provides directional control while hovering. 

NOTE: This AC will focus on U.S. manufactured helicopters. 

c. Tail rotor thrust is the result of the application of anti-torque pedal by the pilot. If the tail rotor 

generates more thrust than is required to counter the 

main rotor torque, the helicopter will yaw or turn 

to the left about the vertical axis. If less tail rotor 

thrust is generated, the helicopter will yaw or turn 

to the right. By varying the thrust generated by the 

tail rotor, the pilot controls the heading when hovering. 

d. In a no-wind condition, for a 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 tail rotor trim 

thrust. In order to maintain a constant heading while 

hovering, the pilot should maintain tail rotor thrust 

equal to trim thrust. 

e. The environment in which helicopters jly, 

however, is not controlled. Helicopters are subjected 

to constantly changing wind direction and velocity. 

The required tail rotor thrust in actual flight is modified by the effects of the wind. If an uncommanded 

right yaw occurs in flight, it may be because the 

wind reduced the tail rotor effective thrust. 

J: The wind can also add to the anti-torque system thrust. In this case, the helicopter will react with 

an uncommanded left yaw. The wind can and will 

cause anti-torque system thrust variations to occur. 

Certain relative wind directions are more likely to 

cause tail rotor thrust variations than others. These 

relative wind directions or regions form an LTE 

conducive environment. 

6. CONDITIONS UNDER WHICH LTE MAY 

OCCUR. 

a Any maneuver which requires the pilot to 

operate in a high-power, low-airspeed environment 

with a left crosswind or tailwind creates an environment where unanticipated right yaw may occur. 

6. There is greater susceptibility for LTE in 

right turns. This is especially true during flight at 

low airspeed since the pilot may not be able to stop 

rotation. The helicopter will attempt to yaw to the 

right. Correct and timely pilot response to an 

uncommanded right yaw is critical. The yaw is usually correctable if additional left pedal is applied 

immediately. If the response is incorrect or slow, 

the yaw rate may rapidly increase to a point where 

recovery is not possible. 

Page 2 
U/26/95 AC 90-95 

c. Computer simulation has shown that if the 

pilot delays in reversing the pedal control position 

when proceeding from a left crosswind situation 

(where a lot of right pedal is required due to the 

sideslip) to downwind, control would be lost, and 

the aircraft would rotate more than 360’ before stopping . 

d. The pilot must anticipate these variations, 

concentrate on flying the aircraft, and not allow a 

yaw rate to build. Caution should be exercised when 

executing right turns under conditions conducive to 

LTE. 

7. FLIGHT CHARACTERISTICS. 

a. Extensive jlight and wind tunnel tests have 

been conducted by aircraft manufacturers. These 

tests have identified four relative wind azimuth 

regions and resultant aircraft characteristics that can, 

either singularly or in combination, create an LTE 

conducive environment capable of adversely affecting aircraft controllability. One direct result of these 

tests is that flight operations in the low speed flight 

regime dramatically increase the pilot’s workload. 

b. Although specific wind azimuths are identified for each region, the pilot should be aware that 

the azimuths shift depending on the ambient conditions. The regions do overlap. The most pronounced 

thrust variations occur in these overlapping areas. 

c. These characteristics are present only at airspeeds less than 30 knots and apply to all single 

rotor helicopters. Flight test data has verified that 

the tail rotor does not stall during this period. 

d. The aircraft characteristics and relative wind 

azimuth regions are: 

(1) Main rotor disc vortex interference (285” 

to 315”). (See figure 1.) 

(a) Winds at velocities of about 10 to 

30 knots from the left front will cause the main rotor 

vortex to be blown into the tail rotor by the relative 

wind. The effect of this main rotor disc vortex is 

to cause the tail rotor to operate in an extremely 

turbulent environment. 

(b) During a right turn, the tail rotor will 

experience a reduction of thrust as it comes into the 

area of the main rotor disc vortex. The reduction 

in tail rotor thrust comes from the air flow changes 

experienced at the tail rotor as the main rotor disc 

vortex moves across the tail rotor disc. The effect 

of this main rotor disc vortex is to increase the angle 

of attack of the tail rotor blades (increase thrust). 

(c) The increase in the angle of attack 

requires the pilot to add right pedal (reduce thrust) 

to maintain the same rate of turn. 

(d) As the main rotor vortex passes the tail 

rotor, the tail rotor angle of attack is reduced. The 

reduction in the angle of attack causes a reduction 

in thrust and a right yaw acceleration begins. This 

acceleration can be surprising, since the pilot was 

previously adding right pedal to maintain the right 

turn rate. 

(e) This thrust reduction will occur suddenly and, if uncorrected, will develop into an 

uncontrollable rapid rotation about the mast. When 

operating within this region, the pilot must be aware 

that the reduction in tail rotor thrust can happen quite 

suddenly and the pilot must be prepared to react 

quickly and counter that reduction with additional 

left pedal input. 

(2) Weathercock stability (120” to 240”). (See 

figure 2.) 

(a) Tailwinds from 120’ to 240”, like left 

crosswinds, will cause a high pilot workload. The 

most significant characteristic of tailwinds is that 

they are a yaw rate accelerator. Winds within this 

region will attempt to weather-vane the nose of the 

aircraft into the relative wind. This characteristic 

comes from the fuselage and vertical fin. 

(b) The helicopter will make a slow 

uncommanded turn either to the right or left depending upon the exact wind direction unless a resisting 

pedal input is made. If a yaw rate has been established in either direction, it will be accelerated in 

the same direction when the relative winds enter the 

120’ to 240’ area unless corrective pedal action is 

made. 

(c) If the pilot allows a right yaw rate to 

develop and the tail of the helicopter moves into 

this region, the yaw rate can accelerate rapidly. It 

is imperative that the pilot maintain positive control 

of the yaw rate and devote full attention to flying 

the aircraft when operating in a downwind condition. 

(d) The helicopter can be operated safely 

in the above relative wind regions if proper attention 

is given to maintaining control. If the pilot is inattentive for some reason and a right yaw rate is initiated 

in one of the above relative wind regions, the yaw 

rate may increase. 

Page3 I 
AC 90-95 12/26/95 

360’ 

0” 

FIGURE 1. MAIN ROTOR DISC VORTEX INTERFERENCE 

Page 4 
0 a 

t I 

12/26/95 AC 90-95 

(3) Tail rotor vortex ring state (210” to 330”). 

(See figure 3.) 

(a) Winds within this region will result in 

the development of the vortex ring state of the tail 

rotor. As the inflow passes through the tail rotor, 

it creates a tail rotor thrust to the left. A left crosswind will oppose this tail rotor thrust. This causes 

the vortex ring state to form, which causes a nonuniform, unsteady flow into the tail rotor. The vortex 

ring state causes tail rotor thrust variations which 

result in yaw deviations. The net effect of the 

unsteady flow is an oscillation of tail rotor thrust. 

This is why rapid and continuous pedal movements 

are necessary when hovering in left crosswind. 

(b) In actuality, the pilot is attempting to 

compensate for the rapid changes in tail rotor thrust. 

Maintaining a precise heading in this region is difficult. LTE can occur when the pilot overcontrols 

the aircraft 

(c) The resulting high pedal workload in the 

tail rotor vortex ring state is well known and helicopters are operated routinely in this region. This 

characteristic presents no significant problem unless 

corrective action is delayed. 

(d) ‘When the thrust being generated is less 

than the thrust required, the helicopter will yaw to 

the right. When hovering in left crosswinds, the pilot 

must concentrate on smooth pedal coordination and 

not allow an uncontrolled right yaw to develop. 

(e) If a right yaw rate is allowed to build, 

the helicopter can rotate into the wind azimuth region 

where weathercock stability will then accelerate the 

right turn rate. Pilot workload during vortex ring 

state will be high. A right yaw rate should not be 

allowed to increase. 

(4) Loss of translational lift (all azimuths). 

(a) The loss of translational lift results in 

increased power demand and additional anti-torque 

requirements. 

(b) This characteristic 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 as a result of an increasing right yaw rate, the pilot may not recognize that 

relative headwind is being lost and hence, 

translational lift is reduced. Second, if the pilot does 

not maintain airspeed while making a right downwind turn, the aircraft can experience an accelerated 

right yaw rate as the power demand increases and 

the aircraft develops a sink rate. Insufficient pilot 

attention to wind direction and velocity can lead to 

an unexpected loss of translational lift. When operating at or near maximum power, this increased power 

demand could result in a decrease in rotor rpm. 

(c) The pilot must continually consider aircraft heading, ground track, and apparent ground 

speed, all of which contribute to wind drift and airspeed sensations. Allowing the helicopter to drift 

over the ground with the wind results in a loss of 

relative wind speed and a corresponding decrease 

in the translational lift. Any reduction in the 

translational lift will result in an increase in power 

demand and anti-torque requirements. 

8. OTHER FACTORS. The following factors can 

significantly influence the severity of the onset of 

LTE. 

a. Gross Weight and Density Altitude. An 

increase in either of these factors will decrease the 

power margin between the maximum power available and the power required to hover. The pilot 

should conduct low-level, low-airspeed maneuvers 

with minimum weight. 

6. Low Indicated Airspeed. At airspeeds below 

translational lift, the tail rotor is required to produce 

nearly 100 percent 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. 

c. Power Droop. A rapid power application may 

cause a transient power droop to occur. Any decrease 

in main rotor rpm will cause a corresponding 

decrease in tail rotor thrust. The pilot must anticipate 

this and apply additional left pedal to counter the 

main rotor torque. All power demands should be 

made as smoothly as possible to minimize the effect 

of the power droop. 

9. REDUCING THE ONSET OF LTE. In order 

to reduce the onset of LTE, the pilot should: 

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

b. Maintain maximum power-on rotor rpm. If 

the main rotor rpm is allowed to decrease, the antitorque thrust available is decreased proportionally. 

Page 7 
AC 90-95 n/26/95 

c. When maneuvering between hover and 

30 knots: 

(1) Avoid tailwinds. If loss of translational lift 

occurs, it will result in an increased high power 

demand and an additional anti-torque requirement. 

(2) Avoid out of ground effect (OGE) hover 

and high power demand situations, such as lowspeed downwind turns. 

(3) Be especially aware of wind direction and 

velocity when hovering in winds of about 8-12 knots 

(especially OGE). There are no strong indicators to 

the pilot of a reduction of translational lift. A loss 

of translational lift results in an unexpected high 

power demand and an increased anti-torque requirement. 

(4) Be aware that if a considerable amount of 

left pedal is being maintained, a sufficient amount 

of left pedal may not be available to counteract an 

unanticipated right yaw. 

(5) Be alert to changing aircraft flight and 

wind conditions which may be experienced when flying along ridge lines and around buildings. 

(6) Stay vigilant to power and wind conditions. 

10. RECOMMENDED RECOVERY TECHNIQUES. 

a. If a sudden unanticipated right yaw occurs, 

the pilot should perform the following: 

(1) Apply full left pedal. Simultaneously, 

move cyclic forward to increase speed. If altitude 

permits, reduce power. 

(2) As recovery is effected, adjust controls for 

normal forward flight. 

6. 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 or obstacle contact may further 

increase the yaw rate and decrease rotor rpm. 

c. The amount of collective reduction should be 

based on the height above obstructions or surface, 

gross weight of the aircraft, and the existing 

atmospheric conditions. 

d. If the rotation cannot be stopped and ground 

contact is imminent, an autorotation may be the best 

course of action. The pilot should maintain full left 

pedal until rotation stops, then adjust to maintain 

heading. 

11. SUMMARY. 

a. The various wind directions can cause 

significantly differing rates of turn for a given pedal 

position. The most important principle for the pilot 

to remember is that the tail rotor is not stalled. The 

corrective action is to apply pedal opposite to the 

direction of the turn. 

b. Avoiding LTE may best be accomplished by 

pilots being knowledgable and avoiding conditions 

which are conducive to LTE. Appropriate and timely 

response is essential and critical. 

c. By maintaining an acute awareness of wind 

and its effect upon the aircraft, the pilot can significantly reduce LTE exposure. ~ 

William J. White v 

Deputy Director, Flight Standards Service 

Page 8