On March 15, 2013, a Sikorsky S-76A++ helicopter, registration N574EH, crashed near Grand Lake, Louisiana, resulting in 3 fatal injuries. The aircraft, operated by Era Helicopters LLC, was conducting a post-maintenance check flight of its avionics systems when the accident occurred.
What happened
The flight departed from Lake Charles Regional Airport (LCH) at 11:19. During the flight, the pilot and a mechanic performed various tests on the automatic flight control system and avionics. At approximately 11:43, the pilot transferred control of the aircraft to the mechanic at an altitude of 1,000 feet.
Shortly after the control transfer, the cockpit voice recorder (CVR) ceased recording. At 11:45, the pilot notified air traffic control that the aircraft was returning to LCH. Two minutes later, the pilot notified both the tower controller and the company dispatcher that an emergency existed and that an immediate off-airport landing was required. Witnesses on the ground reported hearing unusual grinding, screeching, or whistling noises coming from the helicopter. One witness observed the aircraft pitch down and spin before impacting the ground, which was followed by a post-crash fire.
The investigation
Investigators examined the wreckage and recovered the CVR. Analysis of the tail rotor assembly revealed that two blades, identified as the 'red' and 'yellow' blades, had fractured near the tail rotor hub. The investigation determined that the 'red' blade spar initially fractured, causing the blade to depart from the assembly. This separation created a significant imbalance, which subsequently caused the 'yellow' blade spar to fracture due to high centrifugal forces.
As the tail gearbox (TGB) output housing began to separate from the center housing, the gears likely moved out of mesh. This mechanical failure caused the unusual noises reported by witnesses and created drag on the tail rotor drive system. The investigation also looked into the possibility of a compression screw being left in the pivot bearing during maintenance, noting that such a screw could increase bearing stiffness and affect bending stresses on the spar.
Findings
- The initial fracture of the 'red' tail rotor blade spar led to the blade's separation from the helicopter.
- The loss of the 'red' blade caused a massive imbalance, leading to the subsequent failure of the 'yellow' blade spar.
- The resulting damage to the tail gearbox caused the gears to lose proper mesh, creating the noise and drag observed by witnesses.
- The pilot likely attempted an autorotative landing, but the compromised tail gearbox caused the main rotor speed to degrade, leading to a high vertical descent rate at impact.