History of Flight
On December 7, 2019, at 0918 central standard time, a Bell 407 helicopter, registration N79LP, was destroyed in an accident in the Gulf of Mexico, about 25 nautical miles southeast of Grand Isle, Louisiana. The pilot and passenger were fatally injured. The helicopter was operating as a Title 14 Code of Federal Regulations Part 135 business flight.
The helicopter had departed oil platform SP77A at 0834 and was en route to platform WD73, about 17 nautical miles northwest. The pilot was to conduct pollution control inspections along the route. At 0853, the pilot landed at platform WD109 for additional fuel but found the fuel nozzle broken and could not refuel. At 0910, the helicopter departed WD109 with 1 hour and 20 minutes of fuel reported, two persons on board, and an estimated 20-minute flight time. The operator tracked the helicopter via Sky Connect Tracker Systems.
ADS-B data showed the flight track began at 0912:50 about 1.5 miles west-northwest of WD109 at 700 ft msl and 115 knots groundspeed. The helicopter continued northwest, descending to 375 ft msl. At 0918:10, the heading was 292° at 114 knots and 375 ft. The final recorded point at 0918:18 showed a left turn to 270°, descent to 150 ft msl, and 72 knots.
At 0915, the pilot contacted WD73 personnel, reporting 10 minutes out. At 1011, when the helicopter had not landed, WD73 reported it overdue. An ALNOT was issued at 1121.
The United States Coast Guard recovered small debris near another platform about 25 miles west of the main wreckage location. On December 14, 2019, a side-scan sonar mission located the left skid tube. The main wreckage was found about 350 ft southwest of the last ADS-B point on the sea floor at 190 ft depth. Recovery was postponed to December 20 due to bad weather. On December 16, a shrimp trawler unintentionally caught the wreckage, dragging it about 3 miles before it broke loose. The main wreckage was recovered on December 20 and transported to a facility in Louisiana.
Personnel Information
The pilot had departed Grand Isle, Louisiana, about 0655 on the morning of the accident. He had accumulated over 2 hours of flight time and was on the eighth leg of the day.
Aircraft Information
At the time of the accident, the helicopter had an aircraft total time (ATT) of 8,500.8 hours and an engine total time (ETT) of 7,584.7 hours. The engine was installed on November 16, 2016, at an ATT of 6,811.6 hours and ETT of 6,264.6 hours. A 600-hour/12-month inspection and oil change were last performed on May 12, 2019, at ATT 7,786.3 (ETT 6,870.2) and on October 21, 2019, at ATT 8,382.1 (ETT 7,466.0). According to the operator’s director of maintenance, during an oil change, the combined engine filter assembly (CEFA) oil filter is inspected for contamination and reinstalled if clean. Maintenance records from April 2018 to December 2019 showed no engine chip indications. Two instances of the CEFA oil filter bypass indicator extending occurred (April 24, 2018, and September 25, 2019), but no contamination was found. The director of maintenance stated bypass extension is not common but can occur in colder temperatures. The engine was last overhauled in September 2016 at ETT 5,894.1 hours, including a 2,000-hour nonintrusive gearbox inspection with no anomalous findings.
Meteorological Information
Weather reported by the operator indicated wind from 050° at 10 knots, broken clouds at 1,700 ft, no precipitation, and 5 miles visibility in haze. A platform WD73 worker reported clouds at 800 to 1,000 ft, 7 to 10 miles visibility, and a gray sky.
Wreckage and Impact Information
The main fuselage was fractured, folded, and twisted but remained connected by wiring and netting from the trawler. The forward tail boom remained connected to the intermediate fuselage but was fractured at the tail boom tube. The remainder of the tail boom, including stabilizers, was not recovered. The main rotor gearbox remained attached to the transmission deck and exhibited extensive corrosion from saltwater. Chip detectors showed no metallic chips. The emergency flotation system was installed under STC SR01450LA; all floats were outside their covers, uninflated, and vacuum-deflated.
The engine separated from the airframe but remained partially attached. The accessory gearbox was heavily corroded, and the compressor section partially separated. The airframe-mounted fuel filter contained about 0.25 ounces of liquid with color and odor of Jet A, and was clear of contaminants. The throttle lever indicated about 15 degrees, corresponding to below idle. The CEFA was impact-separated; its bypass indicator was not extended. Residual oil, fuel, and salt water were present. The fuel filter element had no debris. The scavenge oil filter element contained metallic debris.
Several gears and engine accessories were recovered separated from the engine. The power turbine pinion gear, part of the N2 gear train, was found loose in the accessory gearbox with severe wear and smearing damage on gear teeth and bearing shoulders. The No. 4 roller bearing was severely worn with roller elements loose. Nine of twelve roller elements from No. 4 were recovered. The No. 3 roller bearing outer ring had severe wear; inner race, rollers, and cage were not recovered. The oil nozzles for Nos. 4/5 bearings were corroded but not otherwise damaged. The No. 5 bearing did not show anomalous wear. Corrosion and mud were throughout the gearbox. The oil delivery tube (piccolo tube) for Nos. 3/4 bearings exhibited severe corrosion.
Additional Information
The engine ECU non-volatile memory was recovered. It showed multiple faults related to power turbine or rotor functions. Three incidents were logged: a power turbine speed (Np) high exceedance, rotor speed (Nr) droop, and a Np overspeed. ECU data indicated normal engine performance until a sudden power turbine overspeed and steep torque drop. Nr decreased rapidly below 80% within 2.5 seconds of overspeed. Collective gradually decreased over 3 seconds, then increased, leading to further Nr decay to 17% at the end of recorded data.
Medical and Pathological Information
The pilot’s body was not recovered during the investigation.
Tests and Research
Rolls-Royce metallographic examination of the power turbine pinion gear revealed fatigue fractures on six consecutive gear teeth, originating on drive side tooth root fillets with multiple origins, progressing toward the coast side. No dimensional anomalies were found at fracture origins. All pinion teeth showed rubbing and smearing. No. 3 and 4 bearing journals had rubbing and thermal distress, with No. 3 showing significant deformation. Material composition met drawing requirements. The No. 3 bearing outer ring had material removal on its roller path and thermal distress, meeting composition requirements. The No. 4 bearing outer ring had similar damage; inner ring showed deformation and rubbing. Recovered rollers had a circumferential rub step. Material composition met requirements. The No. 2.5 bearing support had rubbing consistent with contact with the power turbine pinion gear inner diameter.
Computed tomography of the piccolo tube showed material deposits on internal walls and all oil jets blocked. Material in oil screen was consistent with magnesium corrosion. The oil nozzle for Nos. 4/5 bearings also had blocked jets with deposits from aluminum corrosion and dirt, plus fine iron-based particles.
The caution and warning panel was examined; seven lights had stretched filaments. Bell 407 rotorcraft flight manual excerpts apply to those annunciators.