WPR18LA164

The pilot and observer were conducting a powerline patrol flight in the helicopter. About 40 minutes into the flight, while flying at 35 to 40 knots about 120 ft above ground level, the pilot heard the main rotor losing speed; the low rotor rpm light subsequently illuminated, and the associated aural warning sounded. The pilot entered an autorotation, during which he raised the collective to avoid impacting a set of power transmission lines. The pilot reported that the rotor speed remaining upon reaching the touchdown point was insufficient to cushion the landing, and the helicopter landed hard, resulting in substantial damage. After landing, the pilot noted that the engine was still partially producing power, and he shut it down using the throttle. Examination of the airframe and engine revealed no mechanical malfunctions or component failures that would have precluded normal operation. Based on the available information, the reason for the loss of engine power could not be determined.

On June 5, 2018, about 1245 eastern daylight time, a Bell 206B helicopter, N518Z, was substantially damaged when it was involved in an accident near Sanders, Kentucky. The pilot and passenger were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 aerial observation flight. The pilot stated that he departed on the powerline patrol flight with an observer onboard after filling the helicopter’s fuel tank to capacity. They were travelling on the right side of a set of power transmission lines, about 35 to 40 knots ground speed, and about 120 ft above ground level (agl), when he heard the engine speed reducing. He observed the low rotor rpm light illuminate then heard the low rotor rpm aural alert. He lowered the collective and verified that the throttle was at the fully open position and pushed the power turbine (N2) governor control switch to the “increase” position. Observing no increase in rotor speed, he continued with an autorotation to land to a nearby pasture. During the descent, the pilot observed a second set of power transmission lines in front of and perpendicular to the helicopter, and he increased the collective control to avoid the hazard. He stated that there was insufficient rotor speed remaining upon arrival to the landing area and the helicopter landed hard. After touchdown, the pilot noted that the engine was still producing partial power, and he used the throttle to shut it down. The helicopter came to rest on the incline of a hill. The skids collapsed, and the fuselage impacted the ground, damaging the underside, breaking the chin bubbles, and deforming the tail boom downward. The cross tubes deformed upward, which restricted the use of the doors to exit, as shown in the following figure. Figure. Right side of the helicopter at the accident site. (Photo courtesy of the Federal Aviation Administration) The engine met all the normal engine test parameters during a test run. The fuel control unit was removed, bench tested and found to function normally. The power turbine governor (PTG) was removed from the engine and bench tested. The PTG was disassembled, revealing a small amount of debris in the Py filter, a small amount of debris on the fuel side of the diaphragm and corrosion on the Pr regulator valve seat, as well as some chafing on a Teflon tube. About 75 gallons of fuel were removed from the airframe at the accident site. The airframe fuel filter was removed, and the filter assembly contained clear and bright fuel. The fuel was checked for the presence of water with negative results. The fuel filter was free of debris. The forward and aft fuel boost pumps were energized and produced pressurized fuel from the tank. The fuel shutoff valve opened and closed as designed. No obstructions were observed in the fuel line from the airframe fuel filter to the engine fuel system. The compressor (N1) and N2 tachometer generators were not examined. No mechanical malfunctions or component failuresof the airframe or engine were observed that would have precluded normal operation. The manager of the airport where the fuel was purchased reported that a postaccident quality assurance check performed on the Jet A pump revealed no evidence of fuel contamination.