WPR22LA340

The pilot and two passengers were returning to the base airport following an aerial firefighting mission. During the approach to the airport, the helicopter began an uncontrolled right rotation that could not be arrested despite full use of the left pedal. The pilot attempted to land in the back yard of a private residence; however, the helicopter landed hard and rolled onto its left side. The helicopter sustained substantial damage to the main rotor system, fuselage, and tailboom. An on-scene examination revealed the tail rotor drive shaft had separated at a flex coupling and components critical to the assembly were missing. Postaccident metallurgical examination of tail rotor drive shaft assembly, the pulley, and the recovered hardware revealed that tail rotor drive shaft flange was likely directly attached to the air conditioning pulley flange without a drive ring. The bolt length and recovered hardware would have been adequate to secure the drive shaft flange and the pulley flange together to a drive ring; however, the drive ring, two bolts, six washers and one nut were not found amongst the wreckage after a thorough search. The examination also revealed that the two bolts that were recovered failed from fatigue, likely from the cyclic stress due to the absence of a drive ring. According to the helicopter maintenance records, there had been no recent work on the tail rotor drive shaft or air conditioning unit that would have required disassembly and reassembly of the failed flex coupling.

On September 10, 2022, about 1550, a Bell 206L-1 helicopter, N242BH, was substantially damaged when it was involved in an accident near Banning, California. The pilot and two passengers received minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 firefighting flight. According to the pilot, he was contracted by the California Department of Forestry and Fire Protection (Cal Fire) and was transporting two of their employees to the Banning Municipal Airport (BNG), Banning, California. The pilot had established the helicopter on a final approach path to land between the runway and a taxiway. While on approach, the helicopter made an uncommanded yaw to the right about the vertical axis. The right yaw slowly increased during the approach until he applied full left pedal. The pilot announced to the passengers that he thought he had lost his tail rotor. While about 100 to 150 ft above ground level (agl) he elected not to autorotate while over people and aircraft, and instead kept his power low and kept up his speed. After the helicopter had turned about 180°, the pilot saw a landing area in a back yard and maneuvered the helicopter toward it. As he neared the yard, about 50 ft agl, the helicopter began a continuous right rotation about its vertical axis despite the application of full left pedal. When the helicopter was close to the ground, he applied collective to cushion the landing. The helicopter landed hard, bounced, and rolled onto its left side. Multiple witnesses reported seeing the helicopter “spinning,” followed by a descent to the ground. The pilot turned off the fuel switch and exited the helicopter with assistance from first responders. One passenger exited unassisted and helped the other passenger exit. A review of photographs provided by the pilot showed substantial damage to the main rotor assembly, fuselage, and tailboom. A postaccident examination, which included photographs provided by the pilot, identified all major components near the main wreckage except for some fragments of the main rotor blades that were on a nearby street. The tail rotor drive shaft had separated from the air conditioner pulley flange at the flex coupling (figure 1). Two bolt shank fragments and the attached nuts were found trapped in the two holes of the pulley flange. A bolt head piece, a nut, and six washers were found in the enclosed area. Neither the drive ring, nor any of the hardware other than one nut for connecting the drive shaft flange to the drive ring, were recovered from the fuselage despite a thorough search. Figure 1. View of the separated drive shaft from the pulley at the flex coupling. According to the diagram, 4 nuts, 4 bolts, and a total of 14 washers are required for the drive shaft to pulley flex coupling assembly, as shown in figures 2 and 3. Figure 2. Illustration of a flex coupling for a drive shaft to pulley assembly. Illustration courtesy of Bell. Figure 3. Photograph of an exemplar flexcoupling for a tail rotor drive shaft. Photo courtesy of Bell. The drive shaft section, the pulley, and the recovered hardware were shipped to the National Transportation Safety Board’s Materials Laboratory for further examination. Examination of the drive shaft revealed a complete and intact flex coupling on one end and a separated flex coupling on the other end. The fracture surfaces of the bolt pieces trapped in the pulley flanges had relatively flat features with curving crack arrest lines across most of the fracture surfaces, which are features consistent with a fatigue fracture. A matching fracture surface was observed on the recovered bolt head piece and one of the bolt shanks trapped in the pulley flange. The trapped nut end of one bolt was removed from the pulley flange and mated to the recovered head piece. The total bolt shank length was measured at 0.494 inch between the clamping faces of the head and the nut. The thickness of the pulley flange was 0.164 inch, and the thickness of the flange on the drive shaft assembly was 0.161 inch. For reference, the total thickness of the drive ring adjacent to 1 of the bolts on the intact coupling was 0.119 inch. The 6 recovered washers consisted of 4 washers that were 0.058 inch thick, and 2 washers that were 0.029 inch thick. The specified washers for the pulley flange to drive ring have 2 thicknesses; the thinner washer has a specified nominal thickness of 0.032 inch, and the 3 thicker washers have a specified nominal thickness of 0.063 inch. A maintenance records review revealed no recent work to the tail rotor drive shaft or air conditioning unit. No anomalies were noted in the examination of the maintenance records.