CEN13FA192 · Sikorsky S-76A++

The pilot and two mechanics were conducting a post-maintenance check flight of the helicopter's avionics system. After testing the avionics, the pilot allowed the mechanic to fly the helicopter at 1,000 feet above ground level. The mechanic maneuvered the helicopter for about 1 minute and then stated that he was transferring control of the helicopter back to the pilot. Two seconds later, the cockpit voice recorder (CVR) cut off. About 1 minute after the CVR cut off, the pilot made a routine radio call to the tower controller that he was returning to land. Two minutes after this call, the pilot radioed the tower controller and his company's dispatcher, stating that he had a problem and would be landing off-airport immediately. Several ground witnesses noticed the helicopter as it flew toward the accident site, and it was making an unusual noise, described as grinding, screeching, or whistling. The helicopter impacted with a high vertical descent rate and a postcrash fire ensued.

Examination of the helicopter revealed that the main rotor and tail rotor systems had low rotational energy at the time of ground impact. Two of the tail rotor blades (yellow and red) were fractured adjacent to the tail rotor hub. The fracture signatures on the red/yellow tail rotor spar assembly were consistent with the red tail rotor blade spar initially fracturing and the red tail rotor blade departing from the tail rotor.

The resultant imbalance of the tail rotor fractured the tail gearbox (TGB) output housing studs and most likely tripped the CVR g-switch, which cut off the CVR. The yellow tail rotor blade spar fractured due to high centrifugal forces as a result of the imbalance, and the yellow blade departed from the tail rotor; the tail rotor was then rebalanced. The two remaining tail rotor blades continued to provide partial tail rotor anti-torque, and tail rotor drive remained continuous through the TGB. The entirety of the red and yellow blade separation event likely occurred very quickly, with only a momentary bump or vibration at the time of blade separation.

As the TGB output housing began to separate from the center housing, the gears likely began going out of mesh, allowing the output bevel gear to eventually contact the TGB center housing. This condition likely resulted in the loud, unusual noise reported by witnesses, as well as a drag force on the tail rotor drive system. As the pilot attempted to land, he likely shut the engines down in conjunction with an autorotative landing. Because the tail rotor drive system and main transmission remained mechanically linked, when the engines were shut down, it is likely that main rotor speed (Nr) degraded due to the compromised TGB. As a result, the helicopter developed a high vertical descent rate until ground impact.

The red blade spar fracture signatures were consistent with a fast-growth failure mode. Plausible fast-growth failure modes that were examined included a discrete impact event (e.g. object strike), anomalous operation of the pivot bearing, and a fracture of the pitch horn box (the structure which attaches the pitch horn to the blade and to which the pivot bearings and bumper attaches). With the lack of available evidence because the red blade was not recovered, the specific failure mode could not be determined during the investigation.

Verbatim NTSB analysis and probable cause from the NTSB dataset