CEN15LA002

The pilot’s failure to recognize and correct for flight conditions conducive to a loss of tail rotor effectiveness, which resulted in a rapid, uncommanded right yaw and subsequent hard landing.

The pilot was conducting an aerial pipeline observation flight when the helicopter suddenly yawed to the right and he received a low rotor rpm warning. The pilot applied left pedal to correct the yaw; however, the helicopter continued its right turn, impacted the ground, and rolled onto its side, resulting in substantial damage. Examination of the wreckage and testing of components did not reveal any preimapct anomalies that would have precluded normal helicopter operation. Nonvolatile memory obtained from the helicopter indicated that its groundspeed varied during the last 10 seconds of recorded data to a low of about 9 knots, increasing to about 25 knots, then decreasing to about 12 knots. During this time period, the helicopter's altitude decreased to about 50 feet above ground level (agl). The last recorded point showed the helicopter about 30 feet agl at a groundspeed of about 15 knots on a northwesterly heading. Weather observations from a nearby airport indicated variable wind from the south at 10 knots with gusts to 21 knots about the time of the accident. The helicopter's orientation to the prevailing wind and its slow airspeed made the helicopter susceptible to a loss of tail rotor effectiveness (LTE). Additionally, the helicopter's low altitude at the time of the LTE encounter was inadequate to allow for recovery before its impact with terrain.

The helicopter was not equipped with a cockpit voice recorder or flight data recorder nor was it required to be. However, the ALMA system included a Lenovo ThinkPad, Hitachi Hard Drives, an Argosy PCMCIA Card, and a Garmin GPSMAP 296. These items were shipped to the NTSB Vehicle Recorder Laboratory for examination and downloading. The GPS, hard drives, and PCMCIA card were examined by an NTSB Recorder Laboratory specialist. No useable data was able to be extracted from them that assisted in the investigation. However, the recorder specialist found that the laptop contained flight position data. This data was decoded and plotted in the specialist's factual report. The data showed that during the time between 1249 and 1255, the helicopter flew south before executing a left 270-degree turn to the west on a path along a roadway. The helicopter then turned right 90 degrees and its path paralleled another roadway toward the north. The helicopter then turned left 90 degrees and its path followed another roadway to the west. The helicopter's recorded groundspeed was varied between about 50 to 70 knots during this segment of the flight. The altitude flown began at about 200 feet MSL and trended down to about 70 feet before trending back up to about 120 feet. About 1255, the helicopter's ground track began to head to the north away from the road it had been flying along and its groundspeed decreased to about 15 knots. Simultaneously, the helicopter's altitude dropped from about 130 feet MSL to about 100 fee and then again to about 55 feet before it came back up to about 80 feet. The helicopter's groundspeed varied during its last 10 seconds of recorded data to a low of 9.4 knots, increasing to 25.2 knots one second later, and decreasing again to 12.3 knots. During this time period, the helicopter's altitude decreased to the 50-foot range. The last recorded point showed the helicopter was about 4.1 miles southeast of Woodsboro, Texas, on a track of 304 degrees true, an altitude of 53 feet MSL, and a groundspeed of 15.4 knots. Both the fuel control and governor were operational during testing on test benches. FAA Advisory Circular 90-95 - Unanticipated Right Yaw in Helicopters and the Helicopter Flying Handbook describe the phenomenon of loss of tail rotor effectiveness (LTE). The handbook, in part, stated: LTE or an unanticipated yaw is defined as an uncommanded, rapid yaw towards the advancing blade which does not subside of its own accord. It can result in the loss of the aircraft if left unchecked. It is very important for pilots to understand that LTE is caused by an aerodynamic interaction between the main rotor and tail rotor and not caused from a mechanical failure. Some helicopter types are more likely to encounter LTE due to the normal certification thrust produced by having a tail rotor that, although meeting certification standards, is not always able to produce the additional thrust demanded by the pilot. ... LTE is an aerodynamic condition and is the result of a control margin deficiency in the tail rotor. It can affect all single rotor helicopters that utilize a tail rotor of some design. 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. Translational lift is obtained by any amount of clean air through the main rotor system. Chapter 3 discusses translational lift with respect to the main rotor blade, explaining that the more clean air there is going through the rotor system, the more efficient it becomes. The same holds true for the tail rotor. As the tail rotor works in less turbulent air, it reaches a point of translational thrust. At this point, the tail rotor becomes aerodynamically efficient and the improved efficiency produces more antitorque thrust. The pilot can determine when the tail rotor has reached translational thrust. As more antitorque thrust is produced, the nose of the helicopter yaws to the left (opposite direction of the tail rotor thrust), forcing the pilot to correct with right pedal application (actually decreasing the left pedal). This, in turn, decreases the [angle of attack] AOA in the tail rotor blades. Pilots should be aware of the characteristics of the helicopter they fly and be particularly aware of the amount of tail rotor pedal typically required for different flight conditions. LTE is a condition that occurs when the flow of air through a tail rotor is altered in some way, either by altering the angle or speed at which the air passes through the rotating blades of the tail rotor system. An effective tail rotor relies on a stable and relatively undisturbed airflow in order to provide a steady and constant antitorque reaction as discussed in the previous paragraph. The pitch and angle of attack of the individual blades will determine the thrust output of the tail rotor. A change to any of these alters the amount of thrust generated. A pilot's yaw pedal input affects a thrust reaction from the tail rotor. Altering the amount of thrust delivered for the same yaw input creates an imbalance. Taking this imbalance to the extreme will result in the loss of effective control in the yawing plane, and LTE will occur. This alteration of tail rotor thrust can be affected by numerous external factors. 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 airflow relative to the helicopter; a. Worst case—relative wind within ±15 degrees of the 10 o'clock position, generating vortices that can blow directly into the tail rotor. This is dictated by the characteristics of the helicopters aerodynamics of tailboom position, tailrotor size and position relative to the main rotor and vertical stabilizer, size and shape. b. Weathercock stability—tailwinds from 120 degrees to 240 degrees, such as left crosswinds, causing high pilot workload. c. Tail rotor vortex ring state (210 degrees to 330 degrees). Winds within this region will result in the development of the vortex ring state of the tail rotor. 7. Combinations (a, b, c) of these factors in a particular situation can easily require more anti-torque than the helicopter can generate and in a particular environment LTE can be the result. Certain flight activities lend themselves to being more at high risk to LTE than others. For example, power line and pipeline patrol sectors, low speed aerial filming/photography as well as in the Police and Helicopter Emergency Medical Services (EMS) environments can find themselves in low and slow situations over geographical areas where the exact wind speed and direction are hard to determine. Unfortunately, the aerodynamic conditions that a helicopter is susceptible to are not explainable in black and white terms. LTE is no exception. There are a number of contributing factors but what is more important to understanding LTE are taking the contributing factors and couple them with situations that should be avoided. Whenever possible, pilots should learn to avoid the following combinations: 1. Low and slow flight outside of ground effect. 2. Winds from ±15 degrees of the 10 o'clock position and probably on around to 5 o'clock position 3. Tailwinds that m