Accident Overview
On August 3, 2018, about 1605 eastern daylight time, a Brantly B-2A helicopter, registration N5901X, sustained substantial damage during an accident near Midland, Michigan. The air transport pilot and passenger were not injured. The flight was conducted as a Title 14 Code of Federal Regulations Part 91 orientation flight.
Flight Sequence
The pilot reported that the purpose of the flight was to orient the passenger with the helicopter and demonstrate its performance. After departing, the helicopter was in level flight at about 2,000 ft above ground level, approximately ¼ mile south of the airport. Wind conditions were light from the west. While proceeding at about 60 knots on a heading of about 270°, the pilot felt a strong flutter followed by a bang from the rear of the helicopter. The helicopter immediately turned 180° to the right. The pilot reduced collective and maintained control but could not exert any yaw control with the tail rotor pedals. He increased airspeed and maneuvered for an emergency landing at the departure airport.
Originally intending a running landing on the runway, the pilot instead chose a dry, level grass field due to his higher altitude. During the landing, the helicopter slid forward about 50 yards on the grass while heading about 220°. As it slowed, the left skid caught on the grass and separated from the airframe. A dynamic rollover occurred, and the helicopter came to rest on its left side. The three main rotor blades struck the grass and separated at mid-span. The pilot executed an emergency shutdown, and both occupants egressed without further incident.
Damage and Examination
The helicopter sustained substantial damage to the main rotor system, fuselage, tailboom, and tail rotor system. A Federal Aviation Administration safety inspector examined the airframe at the accident site and noted that one tail rotor blade had fractured and separated at the spar. The tail rotor gearbox was also separated from the tailboom. The separated tail rotor blade and the right horizontal stabilizer were not recovered. Drive train continuity was established from the main rotor system to the point of tail rotor gearbox separation.
The pilot reported that his preflight inspection revealed no cracks, corrosion, or abnormalities with the tail rotor system. The tail rotor gearbox and attached blade were sent to the NTSB Materials Laboratory. Examination showed that the shoulder section of the tail rotor hub had fractured approximately 1.75 inches from the hub center. The shaft and gearbox housing that attach to the tail boom had also fractured. The remaining blade was torn and fractured in multiple locations. The hub boss fracture surface exhibited small features consistent with fatigue cracking, including ratchet marks and crack arrest marks, along with features consistent with subsequent overstress fracture. The report noted that accessing these areas requires specialized tooling and cannot be accomplished by a pilot's preflight visual inspection.
Maintenance Records
The owner provided historical maintenance records. These showed that maintenance was performed, including annual inspections and compliance with FAA Airworthiness Directive 68-04-04 R2 inspections on the tail rotor blade spar for cracks. Brantly service bulletins 1-20 (a hub replacement) and an early 1,200-hour inspection were performed on June 21–23, 2003, when the helicopter's total time was 773.3 hours.
The FAA Fort Worth ACO Branch reported one similar previous event in their database and no previous applicable ADs. They noted Brantly service bulletin 102, dated July 11, 1974, which requires inspections for cracks in the hub spindle shoulder radius areas every 100 hours and annually.
Support from the airframe manufacturer was unavailable during the investigation, as the current type certificate holder was non-responsive to inquiries from the FAA and the NTSB.