ac90-95 (uncommanded right yaw)

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f3m+U.S. Department Advisoryof TransportationFederal AviationAdministration Circular

Subject: UNANTICIPATED RIGHT YAW Date: 12/26/95IN HELICOPTERS Initiated by: AFS-804

1. PURPOSE. This advisory circular (AC) willexamine unanticipated right yaw phenomenon , hecircumstancesunder which it may be encountered,how it can be prevented,and how the pilot shouldreact f it is encountered.2. RELATED READING MATERIAL. BellHelicopter Textron, Supplemental Operating andEmergency Procedures, Operations Safety Notice,OSN 206-83-10 October 31, 1983),Bell HelicopterTextron; Bell Helicopter Textron, Low SpeedFlightCharacteristics Which Can Result in UnanticipatedRight Yaw, Information Letter 206-84-41 and 206084-27 (July 6, 1984), Bell Helicopter Textron;Sneelen,D.M., OH-58 Loss of Tail Rotor Effective-ness - Why It Occurs, U.S. Army Aviation D igest(September 1984), U.S. Army Aviation Center;Prouty, R.W., The Downwind Turn: Losing Direc-tional Control, Rotor and Wing (May 1994),PhillipsBusiness nformation, Inc.; More on the OH-58 LTE

in severalcivil helicopter accidentswherein the pilotlost control. In most cases, inappropriate or latecorrective action may have resulted n the develop -ment of uncontrollable yaw. These mishaps haveoccurred in the low-altitude, low-airspeed flightregime while maneuvering, on final approach o alanding, or during nap-of-the-earth actical terrainflying. Typical civil operations include powerlinepatrol, electromagnetic urvey, agricultural spraying,livestock herding, police/radio traffic watch, emer-gency medical service/rescue,and movie or tele-vision support lights.4. THE PHENOMENA OF LTE.

CI, LTE is a critical; low-speed aerodynamicflight characteristic which can result in anuncommanded apid yaw rate which doesnot subsideof its own accord and, if not corrected , can resultin the loss of a ircraft control.

AC No: 90-95Change:


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AC 90-95 12l26495control. Plight operationsat low altitude and ow air-speed in which the pilot is distracted from thedynamic conditions affecting control of the heli-copterareparticularly susceptible o this phenomena.The following are three examples of this type ofaccident:

(1) A helicopter collided with the ground fol-lowing a loss of control during a landing approach.The pilot reported hat he was on approach o a ridgeline landing zonewhen, at 70 feet aboveground evel(AGL) and at an airspeed of 20 knots, a gust ofwind induced loss of directional control. The heli-copter began o rotate rapidly to the right about themast. The pilot was unable to regain directional con-trol beforegroundcontact.

(2) A helicopter impacted the top of PikesPeak at 14,100 eet mean sea evel (MSL). The pilotsaid he had made a low pass over the summit intoa 40knot headwindbefore osing tail rotor effective-ness.He then lost directional control and struck theground.(3) A helicopter entered an uncommandedright turn and collided with the ground. The pilotwas maneuvering at approximately 300 feet AGLwhen the aircraft entered an uncommanded rightturn. Unable to regain control, he closed the throttle

and attemptedan emergency anding nto a city park.5. UNDERSTANDING LTE PHENOMENA. TounderstandLTE, the pilot must first understand he

c. Tail rotor thrust is the result of the applica-tion of anti-torquepedal by the pilot. If the tail rotorgeneratesmore thrust than is required o counter themain rotor torque, the helicopter will yaw or turnto the left about the vertical axis. If less tail rotorthrust is generated, he helicopter will yaw or turnto the right. By varying the thrust generatedby thetail rotor, the pilot controls the headingwhen hover-ing.

d. In a no-wind condition, for a given main rotortorque setting, there is an exact amount of tail rotorthrust required o prevent he helicopter rom yaw ingeither left or right. This is known as tail rotor trimthrust. In order to maintain a constantheadingwhilehovering, the pilot should maintain tail rotor thrustequal o trim thrust.e. The environment in which helicopters jly,however, s not controlled. Helicopters are subjectedto constantly changing wind direction and velocity.The required ail rotor thrust in actual flight is modi-fied by the effects of the wind. If an uncommandedright yaw occurs in flight, it may be because hewind reduced he tail rotor effective thrust.J: The wind can also add to the anti-torquesys-tem thrust. In this case, he helicopter will reactwithan uncommanded eft yaw. The w ind can and will

cause anti-torque system thrust variations to occur.Certain relative wind d irections are more likely tocause tail rotor thrust variations than others. Theserelative wind directions or regions form an LTE


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c. Computer simulation has shown that if thepilot delays in reversing the pedal control positionwhen proceeding from a left crosswind situation(where a lot of right pedal is required due to thesideslip) to downwind, control would be lost, andthe aircraft would rotate more than 360 before stop-pingd. The pilot must anticipate these variations,concentrateon flying the aircraft, and not allow ayaw ra te to build. Caution shouldbe exercisedwhen

executing right turns under conditions conducive toLTE.7. FLIGHT CHARACTERISTICS.

a. Extensive jlight and wind tunnel tests havebeen conducted by aircraft manufacturers. Thesetests have identified four relative wind azimuthregions and resultantaircraft characteristics hat can,either singularly or in combination, create an LTEconducive environment capableof adversely affect-ing aircraft controllability. One direct result of thesetests s that flight operations n the low speed lightregime dramatically increase the pilots workload.

b. Although specific wind azimuths are identi-fied for each region, the pilot should be aware thatthe azimuths shift dependingon the ambient condi-tions. The regions do overlap. The most pronouncedthrust variations occur in these overlapping areas.c. These characteristics are present only at air-speeds ess than 30 knots and apply to all singlerotor helicopters. Flight test data has verified that

of this main rotor disc vortex is to increase he angleof attack of the tail rotor blades (increase hrust).(c) The increase in the angle of attackrequires the pilot to add right pedal (reduce thrust)to maintain the same ate of turn.(d) As the main rotor vortex passes he tailrotor, the tail rotor angle of attack is reduced.Thereduction in the angle of attack causesa reductionin thrust and a right yaw accelerationbegins. Thisacceleration can be surprising, since the pilot was

previously adding right pedal to maintain the rightturn rate.(e) This thrust reduction will occur sud-denly and, if uncorrected, will develop into anuncontrollable rapid rotation about the mast. Whenoperatingwithin this region, the pilot must be awarethat the reduction n tail rotor thrust can happenquitesuddenly and the pilot must be prepared to reactquickly and counter that reduction with additionalleft pedal nput.(2) Weathercockstability (120 to 240). (Seefigure 2.)

(a) Tailwinds from 120 to 240, like leftcrosswinds, will cause a high pilot workload. Themost significant characteristic of tailwinds is thatthey are a yaw rate accelerator.Winds within thisregion will attempt to wea ther-vanehe nose of theaircraft into the relative wind. This characteristiccomes rom the fuselageand vertical fin.(b) The helicopter will make a slowuncommanded urn either to the right o r left depend-


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(3) Tail rotor vortex ring state 210 to 330).(See igure 3.)(a) Winds within this region will result inthe development of the vortex ring state of the tailrotor. As the inflow passes hrough the tail roto r,it createsa tail rotor thrust to the left. A left cross-wind will oppose his tail rotor thrust. This causesthe vortex ring state to form, which causesa non-uniform, unsteady low into the tail rotor. The vortexring state causes ail rotor thrust variations whichresult in yaw deviations. The net effect of theunsteady flow is an oscillation of tail roto r thrust.This is why rapid and continuouspedal movementsarenecessary hen hovering n left crosswind.

(b) In actuality, the pilot is attempting tocompensate or the rapid changes n tail rotor thrust.Maintaining a precise head ing n this region is dif-ficult. LTE can occur when the pilot overcontrolsthe aircraft(c) The resulting high pedalworkload in thetail rotor vortex ring state is well known and heli-copters are operated outinely in this region. Thischaracteristicpresentsno sign ificant problem unlesscorrectiveaction s delayed.

(d) When the thrust being generated s lessthan the thrust required , the helicopter will yaw tothe right. When hovering n left crosswinds, he pilotmust concentrateon smooth pedal coordination andnot allow an uncontrolled ight yaw to develop.(e) If a right yaw rate is allowed to build,

wind turn, the aircraft can experiencean acceleratedright yaw rate as the power demand increasesandthe aircraft deve lops a sink ra te. Insufficient pilotattention to wind direction and velocity can lead toan unexpected oss of translational ift. When operat-ing at or near maximum power, this increased owerdemandcould result n a decreasen rotor rpm.

(c) The pilot must continually consider air-craft heading, ground track, and apparent groundspeed,all of which contribute to wind drift and air-speed sensations.Allowing the helicopter to driftover the ground with the wind results in a loss ofrelative wind speed and a correspondingdecreasein the translational lift. Any reduction in thetranslational ift will result in an increase n powerdemandand anti-torque equirements.8. OTHER FACTORS. The following factors cansignificantly influence the severity of the onset ofLTE.

a. Gross Weight and Density Altitude. Anincrease n either of these factors will decrease hepower margin between the maximum power avail-able and the power required to hover. The pilotshould conduct low-level, low-airspeed maneuverswith minimum weight.6. Low Indicated Airspeed. At airspeeds elowtranslational ift, the tail rotor is required o producenearly 100 percent of the directional control. If therequired amount of tail ro tor thrust is not available


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c. When maneuvering between hover and30 knots:(1) Avoid tailwinds. If loss of translational iftoccurs, it will result in an increased high powerdemand and an additional anti-torque requirement.

(2) Avoid out of ground effect (OGE) hoverand high power demand situations, such as low-speed ownwind turns.(3) Be especially aware of wind direction and

velocity when hovering n winds of about 8-12 knots(especially OGE). There are no strong indicators tothe pilot of a reduction of translational ift. A lossof translational lift results in an unexpected highpower demandand an increasedanti-torque equire-ment.(4) Be aware hat if a considerableamount ofleft pedal is being maintained, a sufficient amount

of left pedal may not be available to counteract anunanticipated ight yaw.(5) Be alert to changing aircraft flight andwind conditionswhich may be experiencedwhen fly-ing along ridge lines and aroundbuildings.(6) Stay vigilant to power and wind condi-tions.

10. RECOMMENDED RECOVERY TECH-NIQUES.

a. If a sudden unanticipated right yaw occurs,the pilot shouldperform the following:

6. Collectivepitch reduction will aid in arrestingthe yaw rate but may cause an increase n the rateof descent.Any large, rapid increase n collectiveto prevent ground or obstacle contact may furtherincrease he yaw rate anddecreaseotor rpm.

c. The amount of collective reduction shouldbebased on the height above obstructions or surface,gross weight of the aircraft, and the existingatmospheric onditions.d. If the rotation cannot be stoppedand groundcontact s imminent, an autorota tionmay be the bestcourseof action. The pilot should maintain full leftpedal until rotation stops, then adjust to maintainheading.

11. SUMMARY.a. The various wind directions can causesignificantly differing ratesof turn for a given pedal

position. The most important principle fo r the pilotto remember s that the tail rotor is not stalled. Thecorrective action is to apply pedal opposite to thedirection of the turn.b. Avoiding LTE may best be accomplishedbypilots being knowledgable and avoiding conditionswhich are conducive o LTE. Appropriate and timelyresponses essentialand critical.c. By maintaining an acute awareness of windand its effect upon the aircraft, the pilot can signifi-cantly reduceLTE exposure. ~


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ac90-95 (uncommanded right yaw)

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