Accident Overview
On January 26, 2018, at approximately 1125 UTC, an Airbus Helicopters SA330J, registration N339EV, sustained substantial damage while performing a fresh water engine rinse aboard the USNS Wally Schirra. The vessel was in international waters of the Pacific Ocean. The helicopter was operating under 14 CFR Part 133 as a visual flight rules external load flight, shuttling supplies from the ship to other naval vessels. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The two commercial pilots reported no injuries.
Engine Rinse Procedure
After landing on the ship's deck, the crew began a fresh water rinse of the engines to prevent corrosion from sea spray. The #1 engine was rinsed first, with power increased on #2 and #1 reduced to ground idle. The procedure was then reversed for the #2 engine. Once engine speeds stabilized, the lead mechanic squeezed the trigger on the rinse wand. A rumbling sound followed by a rough shudder through the airframe occurred. The pilot checked the instruments and found no abnormalities. Shortly after, another shudder was felt, accompanied by a change in engine and gearbox noise. The pilot observed the rotor speed (Nr) and the gas generator speed (Ng) of the #2 engine increasing. He secured the fuel boost pumps and engine control switches to off. When Nr slowed below 120 rpm, he applied the rotor brake. After the main rotor stopped, the lead mechanic noted that the tail rotor was still spinning.
The rinse procedure used was approved by Turbomeca in 2002, allowing both engines to run during rinsing. A later June 2013 Safran Helicopter Engines work card required only the engine being rinsed to run. The cockpit voice recorder confirmed the crew followed the 2002 approved procedure.
Post-Accident Examination
Inspection revealed a gash on the cover over the #7 tail rotor drive shaft. The shaft itself was fractured torsionally about mid-span.
Main Gearbox and Driveshaft Findings
On October 15, 2018, the main gearbox was examined at Airbus Helicopters. Metal fragments from freewheel unit roller bearing cages were found in cavities of both 8,000 RPM rear reduction pinions, coated with a black greasy substance. The left freewheel showed dislocation of small roller bearings, and the guide ramps of the freewheel cage had metal pitting. The right freewheel assembly had deformed tabs on the shur-lok washer and impact damage on the rotational stop of the roller cage. The 8,000 RPM shafts (part number 330A32-5059-03) had modification 07-52390, consisting of four half bushings and two O-rings to create an oil pool near splines. The input side splines exhibited small wear and damaged bushings, with the O-ring missing. The freewheel side O-ring was in place but brittle, fractured, and had lost flexibility. Laboratory examination showed age-related degradation due to long-term compression, without signs of excessive heating or chemical degradation.
Oil Analysis
Oil samples from the main gearbox, tail rotor gearbox, intermediate gearbox, and Engine #1 were analyzed. No significant anomalies were found. Slightly elevated iron in the main gearbox sample was typical of wear. Elevated phosphorus in Engine #1 indicated anti-wear additives. Two prior oil samples existed: one from February 2016 with iron at 3.3 ppm, and a November 2017 sample (misplaced until after the accident) that showed iron at 14.7 ppm when analyzed in February 2018. The maintenance manual required oil analysis every 150 hours for the main gearbox and every 50 hours for the intermediate gearbox; the two samples were taken 88.4 hours and two years apart. After the accident, the operator implemented an internal requirement for oil samples every 25 hours or three months.