History of Flight
On February 15, 2021, about 1514 Atlantic standard time, a Bell 206B-III helicopter, registration N13AT, was destroyed in an accident near St. Thomas, U.S. Virgin Islands. The pilot and three passengers were fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 sightseeing flight. According to the Director of Operations, the flight was planned as a 17-minute sightseeing tour around the island.
A witness standing in his front yard saw the helicopter fly over his house and out over the ocean. He stated that the helicopter began a 180° turn back toward him, at which point he started recording a video. About 6 seconds after the start of the video, a puff of dark-colored smoke emanated from the vicinity of the engine compartment and dissipated in the rotor wash. The helicopter then abruptly yawed nose-left, then nose-right, and descended in a right turn over wooded terrain downhill from the witness.
Wreckage and Impact Information
The accident site was located in heavily wooded, steep terrain. The wreckage came to rest upright, oriented on a heading of about 200° magnetic. The landing skids, main rotor system, main rotor drive system, engine, hydraulic system, and forward portion of the tail rotor drive system were thermally damaged by a postimpact fire. The majority of the cockpit, cabin, and flight controls were consumed by fire. A portion of the tailboom with horizontal stabilizers attached was embedded upright in a tree adjacent to the main wreckage. The aft portion of the tailboom, with vertical fin and tail rotor, was found about 15 feet from the tailboom section.
Engine Examination
The engine case showed no evidence of uncontained failure. The 1st and 2nd stage compressor blades displayed no evidence of foreign object debris ingestion. Two blades from the third stage compressor wheel had fractured near their respective root ends and were not present. The remainder of the 3rd stage blades were present but damaged, primarily on trailing edges. The 4th, 5th, and 6th stage compressor wheel blades were all fractured near their root ends and absent. The impeller inducer exhibited evidence of hard body debris ingestion.
Material analysis of the compressor section revealed that both fractured stage 3 compressor blades showed signatures consistent with fatigue. The fatigue origins were impact damaged, precluding further analysis. On one blade, fatigue progressed about 0.31 inches toward the leading edge, with the remainder overload. On the other, fatigue progressed about 0.023 inches. The stages 4 and 5 compressor wheel fracture surfaces, where not smeared, exhibited signatures consistent with overload; generalized corrosion was present on inner and outer diameters. The stage 6 compressor wheel showed generalized corrosion, and all but one blade exhibited fatigue with multiple origins near the suction-side crown root; one blade had a crack showing fatigue progression.
Maintenance History
The accident engine, serial number CAE-833459, was installed new on the airframe at manufacture on February 6, 1981. The helicopter had previously operated in Alaska and Texas before arriving in Saint Thomas on November 26, 2019, for operation by Caribbean Buzz Helicopters. Between December 2008 and January 2009, at an engine total time (ETT) of 8,487.6 hours, the compressor section was overhauled, including installation of a new stage 1 and stage 2-3 compressor wheel and impeller; original stage 4 and 5 wheels remained. The most recent engine maintenance occurred on February 12, 2021, at an airframe total time (ATT) of 11,516.9 hours, when the power turbine governor was replaced. At the start of the accident day, the helicopter had an ATT and ETT of 11,519.3 hours; compressor section had 3,031.7 hours since overhaul.
Two annual/100-hour inspections were completed at Caribbean Buzz Helicopters: the first on March 1, 2020, at 11,406.2 hours, and the second on January 25, 2021, at 11,504.2 hours. The 100-hour engine inspection included visual inspection of compressor inlet guide vanes and visible blades. The 300-hour inspection checklist required inspection of case, blades, and vanes in erosive/corrosive environments, recommending 10X magnification to inspect for corrosion pits. Rolls-Royce Commercial Service Letter (CSL) 1172 discusses compressor case inspection intervals. The engine logbook indicated a 300-hour inspection completed on February 16, 2017, and another endorsed on January 11, 2018, but with no logbook entry. An undated checklist from March 1, 2020, had no marks in the 300-hour section. During the last annual/100-hour inspection, the 300-hour section was crossed out and marked “N/A.” According to the operator’s director of maintenance, compressor case halves were not opened; only borescope was used. CSL 1135 discusses engine contamination removal using a water rinse. The aircraft logbook showed multiple engine rinse entries, with the most recent on January 13, 2021.