N79LPBell 407 · 1996

On December 7, 2019, at 0918 central standard time, a Bell 407 helicopter, N79LP, was destroyed when it was involved 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 operated as a Title 14 Code of Federal Regulations Part 135 business flight.

In a statement provided by the operator, the helicopter had departed oil platform SP77A about 0834 and was en route to platform WD73, located about 17 nautical miles northwest. The pilot was to conduct pollution control inspections while en route to the destination platform. At 0853, the pilot landed at platform WD109 for additional fuel, but discovered that the fuel nozzle was broken and was unable to refuel. At 0910, the helicopter departed platform WD109 with 1 hour and 20 minutes of fuel reported, 2 persons onboard, and estimated time en route of 20 minutes. The operator tracked the helicopter via Sky Connect Tracker Systems.

Recorded ADS-B data revealed that the helicopter’s flight track began at 0912:50 about 1.5 miles west-northwest of platform WD109 at 700 ft mean sea level (msl) and 115 knots groundspeed. The helicopter continued northwest for about 10.5 miles and gradually descended to 375 ft msl. At 0918:10, the helicopter’s heading was 292° at a groundspeed of 114 knots and altitude of 375 ft msl. The final recorded point, at 0918:18, showed that the helicopter made a left course deviation to 270°, descended to 150 ft msl, and slowed to 72 knots. Figure 1 shows the platforms, recorded ADS-B flight path, and initial main wreckage location.

Figure 1 – Main wreckage location, flight track and platforms.

At 0915, the pilot contacted WD73 personnel and reported that he was 10 minutes from the platform. At 1011, when the helicopter had not landed at WD73, platform personnel reported it overdue. An alert notice (ALNOT) was issued at 1121.

The United States Coast Guard recovered several small pieces of the helicopter, including a cargo door, a compressed gas cylinder, and seat cushions. The debris was found near another platform about 25 miles west of where the main wreckage was eventually located.

On December 14, 2019, during a side-scan sonar mission near the last ADS-B point, the sonar hooked onto the left skid tube of the helicopter. The sonar boat pulled the skid tube up to the boat and transferred it to the wreckage recovery company. Pieces of helicopter debris were visible on the sonar images, and divers marked the location for a future recovery. The main wreckage location was about 350 ft southwest of the last ADS-B point and was on the sea floor, about 190 ft underwater. Due to adverse weather in the Gulf of Mexico, the recovery was postponed until December 20, 2019.

On December 16, 2019, a shrimp trawler unintentionally caught the helicopter wreckage in its nets and dragged the wreckage about 3 miles. When the wreckage was noticed to be caught in the net, the trawler stopped, and the wreckage eventually broke loose and sank back to the sea floor. The main wreckage was recovered from the Gulf of Mexico on December 20, 2019, and transported to a recovery facility in Louisiana.

The pilot initially departed Grand Isle, Louisiana, about 0655 on the morning of the accident. He accumulated over 2 hours of flight time and was on the eighth leg of the day when the accident occurred.

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 the accident helicopter 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 an engine oil change were last performed on May 12, 2019, at an ATT of 7,786.3 hours (ETT of 6,870.2 hours) as well as on October 21, 2019, at an ATT of 8,382.1 hours (ETT of 7,466.0 hours). According to the operator’s director of maintenance, during a typical engine oil change, the combined engine filter assembly (CEFA) oil filter, which filters engine scavenge oil, is inspected for contamination and reinstalled if none is found. A review of the engine and aircraft maintenance records from April 2018 until December 2019 found no entries regarding engine chip indications. During that timeframe, there were two instances of the CEFA oil filter bypass indicator extending, the first on April 24, 2018, at an ATT of 7,180.7 hours (ETT of 6,264.6 hours) and the second on September 25, 2019, at an ATT of 8,286.6 hours (ETT of 7,370.5 hours), neither of which found contamination of the CEFA oil filter. According to the director of maintenance, the CEFA oil filter bypass indicator extending is not a common occurrence but can happen in colder ambient temperatures.

The maintenance records showed that the accident engine was last overhauled in September 2016 at an ETT of 5,894.1 hours. The last overhaul included completion of the 2,000-hour nonintrusive gearbox inspection, a visual inspection primarily aimed at the power turbine pinion and torquemeter gear mesh. No anomalous findings were reported.

The weather in the area was reported by the operator as 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 the clouds were 800 to 1,000 ft, and 7 to 10 miles visibility with a gray sky.

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 the accident helicopter 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 an engine oil change were last performed on May 12, 2019, at an ATT of 7,786.3 hours (ETT of 6,870.2 hours) as well as on October 21, 2019, at an ATT of 8,382.1 hours (ETT of 7,466.0 hours). According to the operator’s director of maintenance, during a typical engine oil change, the combined engine filter assembly (CEFA) oil filter, which filters engine scavenge oil, is inspected for contamination and reinstalled if none is found. A review of the engine and aircraft maintenance records from April 2018 until December 2019 found no entries regarding engine chip indications. During that timeframe, there were two instances of the CEFA oil filter bypass indicator extending, the first on April 24, 2018, at an ATT of 7,180.7 hours (ETT of 6,264.6 hours) and the second on September 25, 2019, at an ATT of 8,286.6 hours (ETT of 7,370.5 hours), neither of which found contamination of the CEFA oil filter. According to the director of maintenance, the CEFA oil filter bypass indicator extending is not a common occurrence but can happen in colder ambient temperatures.

The maintenance records showed that the accident engine was last overhauled in September 2016 at an ETT of 5,894.1 hours. The last overhaul included completion of the 2,000-hour nonintrusive gearbox inspection, a visual inspection primarily aimed at the power turbine pinion and torquemeter gear mesh. No anomalous findings were reported.

The main fuselage was fractured, folded, and twisted in multiple locations, but remained connected via wiring and netting from the shrimp trawler. The forward portion of the tail boom remained connected to the intermediate fuselage but was fractured at the tail boom tube structure. The remainder of the tail boom, including the vertical and horizontal stabilizers, was not recovered. The main rotor gearbox remained attached to the transmission deck. The gearbox housing exhibited extensive corrosion due to saltwater immersion. The main rotor gearbox chip detectors were removed and exhibited extensive corrosion, but no metallic chips were present. The emergency flotation system was installed under the provisions of Supplemental Type Certificate SR01450LA. All floats were found outside their covers but did not show evidence of inflation and exhibited a vacuum-deflated appearance.

The engine was separated from the airframe but remained partially attached via the wiring harness. The majority of the accessory gearbox housing was consumed by corrosion due to saltwater immersion, and the compressor section was partially separated from the combustion and turbine sections. The airframe-mounted fuel filter remained attached to the airframe and was removed from its mount and disassembled. The fuel filter bowl contained about 0.25 ounces of liquid with a color and odor similar to that of Jet A fuel and the fuel filter was clear of contaminants. The throttle lever was present and indicated about 15 degrees, which corresponded to below idle and beyond the throttle lock-out. The CEFA was impact-separated from the wreckage. The CEFA’s oil filter bypass indicator was not extended, which is the position for when the filter is not in bypass mode. Residual oil and fuel, along with salt water, were present in the CEFA, including the filter bowls. The fuel filter element was removed, and no debris was observed. The scavenge oil filter element was removed and metallic debris was found on the filter element and its filter bowl.

Several gears and engine accessories from the accessory gearbox were recovered separated from the engine assembly. The power turbine pinion gear, part of the power turbine-to-pinion (N2) gear train, was found loose in the accessory gearbox and exhibited severe wear and smearing damage on its gear teeth and bearing shoulders. Figure 2 shows a comparison of the accident power turbine pinion gear (left) to an exemplar power turbine pinion gear (right).

Figure 2 – Accident power turbine pinion gear (left) and an exemplar power turbine pinion gear (right).

The No. 4 roller bearing, normally installed on the aft end of the power turbine pinion gear, was severely worn and its roller elements were found loose in the gearbox cavity, along with the pinion gear. Nine of the twelve roller elements from the No. 4 bearing were recovered. The No. 3 roller bearing outer ring was present and exhibited severe wear. The inner race, roller elements, and bearing cage for the No. 3 bearing were not recovered. The engine was relocated to a specialized engine shop and disassembled. The oil nozzle for the Nos. 4 and 5 bearings remained attached to the gearbox and were corroded but did not exhibit other damage. The No. 5 bearing did not exhibit anomalous wear. Corrosion and mud were observed throughout the accessory gearbox, and additional roller elements from the Nos. 3 or 4 bearings were not found in the remnant mud or within the gearbox. The oil delivery tube for the Nos. 3 and 4 bearings, also known as the “piccolo tube,” exhibited severe corrosion.

The following engine components were identified for additional testing and analysis: the No. 4 bearing; the piccolo tube oil nozzle; the oil nozzle for the Nos. 4 and 5 bearings; the housing for the Nos. 3 and 4 bearing; the power takeoff gear; the power turbine pinion gear; the scavenge oil filter and its filter bowl; the pressurized oil filter; the N2 shaft; the No. 2.5 bearing and its support; and an oil sample from the CEFA oil filter.

Engine Electronic Control Unit (ECU) Data

The non-volatile memory was successfully recovered from the ECU’s interval chips, which included engine fault history, engine history, and engine incident recording data. The recovered data from the maintenance terminal showed multiple faults logged in the fault history of the primary governor, all of which related to power turbine or rotor functions. Three incidents were logged in the incident recorder: 1) an engine power turbine speed (Np) high exceedance; 2) helicopter rotor speed (Nr) droop; and 3) a Np overspeed.

The ECU data indicated that the engine had performed normally until a sudden overspeed of the power turbine and a steep drop in torque occurred. Nr decreased rapidly and went below 80% within 2.5 seconds of the power turbine overspeed. Immediately after the overspeed occurred, collective position gradually decreased over the next 3 seconds, which slowed the rate of the Nr decay, but not enough to recover Nr. Collective then increased until the end of the ECU data, resulting in an increased rate of Nr decay until its lowest value of 17% at the end of recorded data.

The pilot’s body was not recovered during the course of the investigation.

Metallographic Examination of Gears and Bearings

Rolls-Royce completed a metallographic examination of the retained engine components. The power turbine pinion gear exhibited evidence of fatigue fractures on six consecutive gear teeth. The fatigue fractures originated on the drive side tooth root fillets and exhibited multiple origins. The fatigue fractures progressed toward the coast side (the non-drive and non-contact side) of the gear tooth. No dimensional anomalies were observed at the fracture origins or the gear tooth root surfaces adjacent to the origins. All pinion gear teeth exhibited circumferential rubbing and smearing. The Nos. 3 and 4 bearing journals on the power turbine pinion gear also exhibited circumferential rubbing damage and thermal distress, with the No. 3 bearing journal showing significant deformation and material loss compared to the No. 4 bearing journal. Analysis of the material composition and core microstructure of the power turbine pinion gear, away from areas of thermal distress, showed that they met drawing requirements. The No. 3 bearing outer ring exhibited material removal on its roller path and circumferential rubbing on the forward shoulder corner. The outer ring’s raceway and forward shoulder corner displayed thermal distress. Analysis of the material composition and microstructure of the outer ring, away from areas of thermal distress, showed that they met drawing requirements. The No. 4 bearing outer ring exhibited material removal on its roller path and circumferential rubbing on the aft shoulder corner. Transferred material and roller impressions were observed on the raceway of the inner ring. The inner ring’s forward shoulder also exhibited deformation in the forward direction. The inner diameter of the inner ring exhibited circumferential rubbing and transferred material. The outer ring’s raceway and forward shoulder corner, the roller diameters, and the inner ring exhibited thermal distress. The recovered rollers all exhibited a circumferential rub step on one end. Analysis of the material composition of the outer ring, inner ring, and rollers met drawing requirements. Microstructure analysis away from the area of thermal distress could only be accomplished on the outer ring and a representative roller. The microstructure for both components met drawing requirements. The No. 2.5 bearing support exhibited circumferential rubbing consistent with contact with the inner diameter of the power turbine pinion gear. The No. 2.5 bearing support was corroded, and its aft face was deformed forward at the 12 o’clock position. The area of deformation on the aft face also showed evidence of circumferential rubbing.

Computed Tomography (CT) Scans of Piccolo Tube and Oil Nozzle

The piccolo tube showed material deposits on the internal passageway walls and all of its oil jets appeared to be blocked. Additionally, material was observed within the oil screen of the remnant gearbox housing material attached to the piccolo tube. The piccolo tube was sectioned in multiple locations to analyze the tube, the remnant gearbox housing material, and deposits within the tube. The oil delivery tube and gearbox housing material were consistent with drawing requirements. The deposits within the oil jets were consistent with byproducts of aluminum (piccolo tube) corrosion and dirt. Additionally, fine metallic particles observed in the deposits were consistent with an iron-based alloy. A representative iron-based alloy particle measured about 0.0008 in. long. Material seen within the oil screen was consistent with byproducts of magnesium (gearbox housing) corrosion. The CT scan of the oil nozzle for the Nos. 4 and 5 bearings showed material deposits on the internal passageway walls and all of the oil jets appeared to be blocked. The oil nozzle was sectioned in multiple locations for further analysis. The oil nozzle material was consistent with drawing requirements. The blockage material was consistent with byproducts of aluminum (oil nozzle) corrosion and dirt. Additionally, fine metallic particles observed within the deposits were consistent with an iron-based alloy, as well as byproducts of magnesium corrosion. A representative iron-based alloy particle was measured to be about 0.0028 in. long.

Caution and Warning Panel

The caution and warning panel was examined by the NTSB Materials Laboratory using a stereomicroscope. Each annunciator light consisted of three bulbs. For several bulbs, the glass globe had a clouded appearance, and the filaments could not be visually examined; these bulbs were removed from the panel and the individual bulb was x-rayed to examine the filaments. Seven lights exhibited stretched filaments. Table 1 contains excerpts from Bell 407 rotorcraft flight manual applicable to those annunciator lights in which filaments were found stretched.