On January 23, 2022, about 1030 Pacific standard time, a Bell UH-1E, N333XL, was substantially damaged when it was involved in an accident near Gustine, California. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight. The pilot reported that he was applying herbicide to a wheat field and that the accident occurred on the third load of the day and after the pilot had been operating the helicopter for about 45 minutes. At the apex of a turn, the pilot lost control of the helicopter, and it descended and struck terrain, which substantially damaged the main rotor system, tailboom, and fuselage. The pilot turned off the fuel boost and the master electrical switch and exited the helicopter through the roof window. The pilot reported that he had checked the oil level of the 90° gearbox before the flight but did not recall the quantity of oil at the time. A review of postaccident photos provided by the pilot revealed the helicopter came to rest on its left side in an open, level field. All major components were located in the debris area around the airframe. The tailboom had partially separated from the fuselage. One main rotor blade separated from the hub about 24 inches outboard of the blade grip. The other main rotor blade remained attached and exhibited upward bending about 24 inches from the blade grip. The tail rotor assembly and the 90° gearbox remained attached to the top of the vertical stabilizer. The 90° gearbox had an internal gear that had breached the housing. The green paint around the breach was thermally damaged, and no evidence of oil was observed around the exposed area of the gearbox. The 90° gearbox was shipped to the National Transportation Safety Board (NTSB) Materials Laboratory. Examination of the 90° gearbox revealed that the housing and the parts within the housing were dry and covered with a black deposit of fine metallic particles and that they contained no evidence of lubricating oil/fluid (see figures 1and 2). The gear and bearings exhibited evidence of heat damage and deformation. The pinion exhibited severe heat and wear damage, which is also shown in figure 2.

Figure 1. Overall view of the 90° gearbox assembly showing a portion of the exposed gear that breached the gearbox housing.

Figure 2. View of the internal components of the 90° gearbox and the damage to the pinion. The interior of the housing was covered in soot. When a flashlight was aimed at the oil level sight glass, no light passed through it (figure 3).

Figure 3. Oil level sight glass for the 90° gearbox. Review of the airplane’s maintenance records revealed that, on October 23, 2007, the 90° gearbox was installed on the accident helicopter at an airframe time of 8,412 hours. The gearbox had 260.9 hours since overhaul. The gearbox complied with Federal Aviation Administration Airworthiness Directive 90-10-05, dated June 5, 1990, which was issued to prevent the failure of the tail rotor gearbox duplex bearing. On February 15, 2022 (23 days after the accident), the 90° gearbox was removed for examination; at that time, the helicopter had an airframe time of 9,025.8 hours, and the gearbox had accumulated 3,124.5 historical hours and 298.9 hours since overhaul.