phone. After about six sec- onds, the recording captured a puff of dark smoke from the engine bay that quickly dis- persed in the rotor wash. The ship suddenly yawed left and then right before descending in a right turn toward a wooded hillside and out of the witness’s sight.
The Aircraft The helicopter’s engine was the original unit installed when the aircraft was manu- factured in 1981. At the time of the accident, it had logged 11,519 hours in service. Between December 2008 and January 2009, at an engine total time (ETT) of 8,488 hours, the compressor sec- tion was removed for inspec- tion and overhaul; a new stage 1 and the stages 2 and 3 compressor wheels were installed, while the remaining stages’ origi- nal wheels remained installed. The helicopter was brought to
The Investigation NTSB analysis of the recorded cell-phone video concluded the helicopter’s ground speed was “about 39 kt.” before the smoke appeared, then decreased to “about 30” before increasing to “about 68 kt.” in descent. The high resolution and rapid cap- ture rate (equivalent to 60 frames per sec- ond) of the mobile phone’s camera also enabled analysts to calculate the main-rotor speed, which decayed from 390 rpm (99% of nominal speed) to just 74 rpm (19%) in the last second of the recording. The heli- copter’s rate of descent was estimated to have reached around 4,800 ft. per minute by the time the aircraft left the camera’s field of view. The wreckage was found on a steep,
The main wreckage of the St. Thomas accident. (NTSB Photo)
St. Thomas on Nov. 26, 2019, and although the US National Transportation Safety Board (NTSB) didn’t report the airframe and engine times as of that date, two 100-hour inspections were performed later, the first on Mar. 1, 2020, at 11,406 hours ETT, and the second on Jan. 25, 2021, at 11,504 hours ETT.
In a commercial service letter (CSL), the engine manufacturer recommends giving engines subjected to saltwater exposure a compressor rinse after the last flight of the day, spraying fresh water into the compres- sor inlet while turning the engine over with the starter motor. The helicopter’s logbook had record of
13 such compressor rinses during the 74 hours of operation between Jul. 20, 2020, and Jan. 13, 2021.
An earlier CSL, first issued in 1991 and most recently revised in 2007, calls for add- ing inspection of the case, blades, and vanes to the 300-hour inspection checklist
for aircraft “operating in a corrosive and/or erosive environment.” This CSL also sets a calendar time limit of 6 months for non- coated compressor wheels and 12 months for coated wheels such as those on the accident helicopter. The most recent 300-hour inspection documented in the logbook was signed off on Feb. 16, 2017, at an ETT of 10,960 hours—four years and 559 hours earlier than the accident date. The aircraft records also included a 300- hour inspection checklist from Jan. 11, 2018, that cited 11,197 hours of engine operation, but no corresponding entry was made in either the engine logbook or the airframe logbook. If that inspection in fact took place, the subsequent 300-hour engine inspection would still have been more than two years and 22 flight hours overdue.
An undated engine inspection checklist, possibly from the aircraft’s 2020 100-hour inspection, showed no initials or other markings in the 300-hour section. And in the records from the helicopter’s last 100- hour inspection, in 2021, the 300-hour sec- tion of the engine checklist was crossed out and marked “NA.”
JUNE 2023 ROTOR 55
heavily forested hillside. None of the four occupants survived. The post-impact fire consumed almost the entire fuselage; two sections of the tail boom were found sepa- rated from the main wreckage. Both
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