Accident Sequence
On November 2, 2015, at 1923 central standard time, a Bell 407 helicopter, registration N496AE, operated by AirEvac EMS, Inc., under 14 CFR Part 135 as an on-demand air medical flight, was preparing to depart from San Antonio International Airport (KSAT), Texas, for Northeast Methodist Hospital (XS83). Night visual meteorological conditions prevailed, and no flight plan had been filed. As the pilot lifted the helicopter to a 3-foot hover, a loud bang was heard, and the helicopter began an uncommanded right yaw. Despite the pilot applying full left pedal, the yaw continued. The pilot then lowered the collective, and the helicopter touched down, rotating approximately 270 degrees to the right before coming to a stop. The pilot, flight nurse, and paramedic were not injured.
Post-accident Examination
Post-accident examination revealed substantial damage to the tail rotor spline, coupling, and hanger bearings. The tail rotor could be manually rotated, but a grinding noise was heard from the damaged No. 3 hanger bearing. The main rotor blades did not rotate in synchronization with the tail rotor blades. The No. 3 tail rotor shaft adapter and shaft splines were completely ground down, and the adapter and tail rotor shaft connecting point had failed. Evidence of excessive torsion beyond intended allowable tolerances was found. Excessive grease was present on the hanger bearing. The aluminum adapter washer hole was elongated, consistent with a loose through-bolt. A 1/8-inch gap existed between the adapter and the base of the hanger bearing, consistent with the adapter not being fully in contact with the bearing and potential stress within the adapter and shaft splines. The smaller stainless steel washer was mushroom-shaped, and the drive shaft through-bolt had worn threads.
On November 18 and 19, 2015, the tail rotor assembly was examined at Able Engineering in Mesa, Arizona, under the auspices of an FAA aviation safety inspector from the Scottsdale, Arizona, FSDO. Bell Helicopters and AirEvac EMS participated. The shaft, freewheeling unit, and tail gear box were examined, with no issues noted except for the failed coupling and shaft. The retaining stud had migrated approximately 0.025 inch outward and was very loose compared to others, consistent with loss of torque that would have allowed movement between the coupling and shaft, leading to eventual failure.
Fleet Inspection
Following the examination, AirEvac EMS ordered an inspection of their entire Bell 407 fleet. Two helicopters were found to have loose tail rotor drive shafts. The drive shaft, carrier bearing, coupling, and spline from one of those helicopters were examined the following day. The retaining nut was loose; torque measured 20 inch-pounds, with minimum specified torque of 30 to 50 inch-pounds and tare of 5 to 0 inch-pounds. Stud extension was within limits. There was slight motion between the coupling face and the face of the bearing, causing wear on the coupling face and slight wear on some splines. In the Bell 407 Maintenance Manual and AirEvac EMS' Approved Aircraft Inspection Program (AAIP), there is no requirement for the assembly to be inspected for security.