Midland, MI, USA
N5901X
Brantly B-2
After departing from the airport, the helicopter was in a level flight profile about 2,000 ft above ground level, about 1/4 mile south of the airport. The pilot stated that, when the helicopter was about 60 kts and on a heading of about 270°, he felt a "strong flutter" emit from the helicopter and heard a subsequent "bang" from the rear of the helicopter. The helicopter immediately turned 180° to the right, and the pilot reduced collective and maintained control of the helicopter. The pilot increased the airspeed and maneuvered the helicopter for an emergency landing to the departure airport. During the landing sequence, the helicopter experienced a dynamic rollover and impacted terrain. A postaccident examination revealed that one of the tail rotor blades fractured and separated at the spar. Further, one of the bosses of the tail rotor hub fractured from fatigue. The fatigue crack initiated at multiple sites near the shoulder of the hub shank. These smaller cracks coalesced and propagated inward through over half the hub cross-section. During this propagation, as reverse bending stresses increased, a series of smaller fatigue cracks initiated on the opposite surface, coalescing and propagating inward. When the cracked hub could no longer withstand the stress, the remaining cross-section fractured from overstress. The tail rotor blade affixed to this portion of the hub liberated when the final fracture of the hub boss occurred.
On August 3, 2018, about 1605 eastern daylight time, a Brantly B-2A, N5901X, sustained substantial damage when it was involved in an accident near Midland, Michigan. The air transport pilot and the passenger sustained no injury. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 orientation flight. The pilot reported the purpose of the local area flight was to orient the passenger with the helicopter and demonstrate how it performed in flight. After departing from the airport, the pilot was in a level flight profile about 2,000 ft above ground level, located about ¼ mile south of the airport. The pilot reported that the wind condition was "light" and was originating from the west. Proceeding about 60 kts and heading about 270°, the pilot felt a "strong flutter" emit from the helicopter and heard a subsequent "bang" noise emit from the rear of the helicopter. The helicopter immediately turned 180° to the right, the pilot reduced collective, and he was able to maintain control of the helicopter. He reported he was unable to exert any yaw control with inputs to the tail rotor pedals. The pilot increased the airspeed and maneuvered the helicopter for an emergency landing to the departure airport. The pilot was originally intending to execute a running landing to the runway, but due to his higher altitude at the time, he decided to execute a running landing to a dry, level grass field at the airport instead. During the landing sequence, the helicopter slid forward about 50 yards on the grass while heading about 220°. The pilot reported as the helicopter slowed down, the left skid got "caught" on the grass. The left skid separated from the airframe, a dynamic rollover sequence occurred, and the helicopter came to rest on its left side. The three main rotor blades impacted the grass and separated about mid-span. The pilot executed an emergency shutdown and the two occupants egressed from the helicopter without further incident. The helicopter sustained substantial damage to the main rotor system, the fuselage, the tailboom, and the tail rotor system. A Federal Aviation Administration aviation (FAA) safety inspector (ASI) examined the airframe at the accident site and noted that one of the tail rotor blades (a two-blade system) had fractured and separated at the spar. The tail rotor gearbox was also found separated from the tailboom. The separated tail rotor blade, along with the right side horizontal stabilizer were not recovered. All other components of the helicopter were accounted for at the accident site. The FAA ASI was able to establish drive train continuity from the main rotor system to the point of separation of the tail rotor gear box. The pilot reported that during the preflight process, he conducted a detailed inspection of the helicopter and observed no cracks, no corrosion, and no other abnormalities with the tail rotor system. The tail rotor gear box, along with the attached tail rotor blade, were secured and transported to the NTSB Materials Laboratory in Washington, District of Columbia, for an examination. The shoulder section of the tail rotor hub had fractured approximately 1.75 inches from the center of the hub. In addition, the shaft and gear box housing that attach to the tail boom of the helicopter had fractured. The remaining tail rotor blade had torn and fractured in multiple locations. The hub boss fracture surface exhibited small features consistent with fatigue cracking (these include ratchet marks and crack arrest marks). The fracture surface also exhibited features consistent subsequent overstress fracture. Accessing the areas that had exhibited features consistent with fatigue require specialized tooling and is not something that can be accomplished solely by a preflight visual inspection by a pilot. The owner was able to share some of the helicopter's historical maintenance records. The records showed maintenance was being performed, including annual inspections and compliance with FAA Airworthiness Directive (AD) 68-04-04 R2 inspections on the tail rotor blade spar for cracks. The records also showed that 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 (ACO) responsible for the airframe reported that one similar previous event was found in their database and no previous applicable ADs. The FAA ACO also found Brantly service bulletin 102, dated July 11, 1974, that inspects 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.
The in-flight separation of the tail rotor blade due to fatigue, which resulted in the subsequent emergency landing and dynamic rollover.
Source: NTSB Aviation Accident Database
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