WPR16LA056

The loss of helicopter control as a result of the loss of tail rotor effectiveness.

The student pilot was conducting approach and departure practice in the single main rotor helicopter. The practice area was a flood control basin located in the desert about 3 miles east of mountainous terrain. The basin was bounded on its east side by a 1-mile-long concrete levee oriented approximately north-south, which rose about 50 ft above the basin floor. The west-facing wall of the levee was sloped about 25 degrees to the horizontal and was topped by a level road. After an uneventful approach to the road from the north, the pilot slowed to a hover-taxi, and then, without touching down, he added power and control inputs to begin his departure straight ahead. The helicopter began to yaw to the right, and the pilot was unable to correct the rotation. The helicopter drifted west over the sloped part of the levee, continued to rotate while descending, and the pilot then attempted to land. The tail rotor struck the levee, and the helicopter landed hard on the sloped wall, facing uphill. Postaccident examination of the helicopter and several components did not reveal any deficiencies or failures that would have precluded normal operation. The only definitive wind information was from the pilot's departure airport about 9 miles away from the basin, which reported that the wind was about 6 knots from the east-southeast, and the pilot reported that the wind at the basin was similar. The late-morning timeframe, combined with the local topography and desert environment, led to unpredictable wind and/or gusts at the accident site. The pilot's description of the accident circumstances and events were consistent with a loss of tail rotor effectiveness that occurred as the pilot was attempting the transition from the hover-taxi to the departure segment.

FAA Post-Recovery Examination On January 25, 2016, subsequent to its recovery to Apex Aviation at VGT, the helicopter was examined further by the FAA inspector who initially examined it at the accident site. His findings included the following; Both tail rotor (TR) pedals were free to make full travel movements. All TR push rod attachments were secure, and the rods had full and free movement. There was a minor anomaly with the forward bell crank in the gear box compartment; it showed evidence of a one-time contact with the firewall. However, although the clearance was limited, the bellcrank did not contact the firewall throughout its travel range, and the push rod in the gear box compartment operated freely with full movement. The TR links and blades had full and complete pitch travel in both directions. The pitch change bearing operated normally, through its full travel range. All controls were free and easily movable. All the rod ends were intact and did not appear to be worn or eroded. Throughout the system, no foreign objects were found that would impede movement. The TR drive system was intact, without damage. The main rotor appeared to be undamaged. When the tail rotor was manually rotated in one direction, the main rotor also turned freely, per design. However, as expected and normal, rotation in the opposite direction was more difficult, due the clutch being engaged with tensioned drive belts. All TR drive shaft flex couplings were intact and undamaged. All "Telatemp" decals appeared normal, with no indications of abnormal operating temperatures on any components. The tail rotor gear box had sufficient oil. The only observed damage to the TR drive and control systems was to the TR blades, due to impact with the concrete levee. The inspector removed and examined the light bulb for the low rotor rpm warning light, and observed the filament to be intact. Component Removals On February 2, 2016, a technician from Apex Aviation and the FAA inspector removed three components (Governor Controller Box, Magneto, and Governor Motor) from the subject helicopter for subsequent examination. Component Examinations and Tests On February 9, 2016, at Robinson Helicopter Company (RHC), Torrance, California, examinations and some tests were performed on the three removed components, with oversight by investigators from Robinson and the NTSB. With one minor exception involving the magneto, all three components satisfactorily met all applicable condition and functionality criteria. A visual inspection of the Governor Controller Box revealed that the box had not been opened or overhauled since its production. No external damage, with the exception of a small area of bubbled paint, was present. The electrical connector was clean and undamaged. A functional test was performed using the production acceptance criteria. The controller satisfactorily met (passed) all of the test criteria. A visual examination of the interior found no indication of heat coming from the internal circuitry that would account for the bubbled paint, and no abnormalities were observed with the internal components. The bubbled paint was presumed to be the result of an unknown event during manufacture. The engine right (helicopter left) magneto was examined. A visual inspection of the magneto revealed that the data plate was yellow in color, indicating that it had been overhauled in the field (not by the manufacturer). No external damage was noted. The point cover was removed, and no oil or grease contamination was evident on or near the points. Although one holddown screw appeared to be slightly undertorqued, the tachometer point base was secure, and all the proper hardware was intact and in place. The points opened and closed on manual magneto rotation. The tachometer points, which provide a signal to the tachometer and the governor, had a gap between the contact points of less than .012 inches; (manufacturer's specification is .019 + .003 inches). The tachometer points were undamaged, and the contact pads were clean and smooth. A visual inspection of the governor motor revealed no external damage. A functional test was performed, with satisfactory results, and the motor was determined to be in compliance with production specifications. The functional test included directionality, and appropriate "friction disc slippage" checks, as indicated by motor current draw with a jammed actuator link. Loss of Tail Rotor Effectiveness (LTE) According to the Helicopter Flying Handbook (HFH, FAA-8083-H-21A), loss of tail rotor effectiveness (LTE) is an aerodynamic result of a control margin deficiency in the tail rotor that affects all single-rotor helicopters that utilize a tail rotor. The design of main and tail rotor blades and the tail boom assembly can affect the characteristics and susceptibility of LTE but will not nullify the phenomenon entirely. Rotor system efficiency is a direct function of the stability (absence of turbulence) of the air entering the rotor system. The less disturbed the air, the more efficient and effective the rotor system, and conversely, the more disturbed the air, the less efficient and effective the rotor system is. Highly disturbed air can result in a loss of tail rotor thrust and yaw control. The HFH stated that loss of tail rotor thrust can be affected by numerous external factors, and that the main factors contributing to LTE are: 1. Airflow and downdraft generated by the main rotor blades interfering with the airflow entering the tail rotor assembly. 2. Main blade vortices developed at the main blade tips entering the tail rotor. 3. Turbulence and other natural phenomena affecting the airflow surrounding the tail rotor. 4. A high power setting, hence large main rotor pitch angle, induces considerable main rotor blade downwash and hence more turbulence than when the helicopter is in a low power condition. 5. A slow forward airspeed, typically at speeds where translational lift and translational thrust are in the process of change and airflow around the tail rotor will vary in direction and speed. 6. The local ambient airflow relative to the helicopter. The worst case is a relative wind from about 285 to 315 degrees ( ±15 degrees from the 10 o'clock position) from the nose of the helicopter; this can generate vortices that can blow directly into the tail rotor. The conditions at the time of the pilot's loss of control were consistent with those cited in items 3 through 5, and the reported winds were in approximate agreement with the sector specified in item 6. The 1053 VGT automated weather observation included winds from 140 degrees at 6 knots, visibility 10 miles, clear skies, temperature 13 degrees C, dew point minus 6 degrees C, and an altimeter setting of 30.21 inches of mercury. Both the pilot's flight instructor and the FAA inspector, who had significant experience flying helicopters in that region, reported that the flood basin area was subject to unpredictable winds due to topography and local heating as each day progressed. The pilot reported that he held a student pilot certificate, and had a total flight experience of approximately 50 hours, all of which was in Robinson helicopters, and which included about 7 hours solo in the R22. He received his R22 solo endorsement in December 2015. His most recent FAA first-class medical certificate was issued in June 2015. The pilot was practicing along the roadway atop the levee, which extended for about 1.1 miles, and that is where the accident occurred. The site was located about 8.6 miles, on a true bearing of about 305 degrees, from VGT. A mountain range was situated about 3 miles to the west of the site. An FAA Inspector traveled to the accident location on the day of the accident, and conducted a follow-up examination 3 days later. The helicopter came to rest upright, facing uphill, approximately 2/3 of the way down the sloped levee wall. Scrape ma