Unanticipated Right Yaw in Helicopters

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f3m+U.S. Department

Advisoryof Transportation

Federal 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 , hecircumstances under 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 3 1, 1983), Bell HelicopterTextron; Bell Helicopter Textron, Low Speed FlightCharacteristics 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 DigestSeptember 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 several civil 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-ea rth 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 does not subsideof its own accord and, if not corrected , can resultin the loss of a ircraft control.

AC No: 90-95Change:


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c. Computer simulation has shown that if thepilot delays in reversing the pedal control positionwhen proceeding from a left crosswind situationwhere a lot of right pedal is required due to the

sideslip) to downwind, control would be lost, andthe aircraft would rotate more than 360 before stop-ping

d. The pilot must anticipate these variations,concentrate on flying the aircraft, and not allow ayaw ra te to build. Caution should be exercised when

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 resultant aircraft characteristics hat can,either singularly or in combination, create an LTE

conducive environment capable of 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 depending on 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 thatthetail ro tor does not stall during this period

of this main rotor disc vortex is to increase he angleof a ttack 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 causes a reductionin thrust and a right yaw acceleration begins. 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. Whenoperating within this region, the pilot must be awarethat the reduction n tail rotor thrust can happen quitesuddenly and the pilot must be prepared to reactquickly and counter that reduction with additional

left pedal nput.2) Weathercock stability 120 to 240). See

figure 2.)a) Tailwinds from 120 to 240, like left

crosswinds, 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-vane he nose of theaircraft into the relative wind. This characteristiccomes rom the fuselage and vertical fin.

b) The helicopter will make a slowuncommanded urn either to the right o r left depend-ing upon the exact wind direction unlessa resisting


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

See igure 3.)a) Winds within this region will result in

the development of the vortex ring state of the tailrotor. As the inflow passes hrough the tail rotor,it creates a tail rotor thrust to the left. A left cross-wind will oppose his tail rotor thrust. This causesthe vortex ring state to form, which causes a 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 ro tor thrust.This is why rapid and continuous pedal movementsare necessary hen hovering n left crosswind.

b) In actuality, the pilot is attempting tocompensate or the rapid changes n tail rotor thrust.Maintaining a precise heading n this region is dif-ficult. LTE can occur when the pilot overcontrols

the aircraftc) The resulting high pedal workload in the

tail rotor vortex ring state is well known and heli-copters are operated outinely in this region. Thischaracteristicpresents no significant 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 concentrate on smooth pedal coordination andnot allow an uncontrolled ight yaw to develop.

e) If a right yaw rate is allowed to build,h h li h i d i h i

wind turn, the aircraft can experience an accelerated

right yaw ra te as the power demand increases andthe aircraft develops a sink rate. 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 owerdemand could result n a decrease n rotor rpm.

c) The pilot must continually consider air-craft heading, ground track, and apparent ground

speed, all of which contribute to wind drift and air-speed sensations. Allowing the helicopter to driftover the g round with the wind results in a loss ofrelative wind speed and a correspond ing decreasein 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 onse t 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 rotor thrust is not availablefor any reason the aircraft will yaw to the right


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c. When maneuvering between hover and

30 knots:1) Avoid tailwinds. If loss of translational ift

occurs, 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 andvelocity when hovering n winds of about 8-12 knotsespecially OGE). There are no strong indicators to

the pilot of a reduction of translational ift. A lossof translational lift results in an unexpected highpower demand and an increased anti-torque equire-ment.

4) Be aware hat if a considerable amount 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 and

wind conditionswhich may be experienced when fly-ing along ridge lines and around buildings.

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 arresting

the 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 and decrease otor rpm.

c. The amount of collective reduction should bebased on the height above obstructions or surface,gross weight of the aircraft, and the existingatmospheric onditions.

d. If the rotation cannot be stopped and groundcontact s imminent, an autorotation may be the bestcourse of 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 for 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 accomplished bypilots being knowledgable and avoiding conditionswhich are conducive o LTE. Appropriate and timelyresponse s essential and 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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Unanticipated Right Yaw in Helicopters