What happened
On 9 November 2014, an AgustaWestland 139 helicopter, registration SE-JRH, operated by the Swedish Maritime Administration, experienced a technical failure while returning to its base in Umeå. The aircraft was conducting a training mission and had activated its Full Ice Protection System (FIPS) due to approaching heavy cloud cover. Approximately ten minutes before landing, the crew received a caution alert indicating a fault in the FIPS related to the tail rotor. The crew requested priority landing, which was completed safely without any significant vibrations or further warnings.
Upon ground inspection and fault isolation, investigators found that a slip ring—a component used to transfer electrical power to the rotating parts of the tail rotor—had become detached from its mounting plate. The 5.9 kg component had shifted significantly, and only the electrical harness prevented it from making contact with the moving parts of the tail rotor, which could have led to a catastrophic failure.
The investigation
SHK examined the mounting plate and the broken hardware to determine why the component had moved. The investigation focused on the manufacturing quality of the slip ring, which is a Part Manufacturer Approval (PMA) part produced in the United States. Technical analysis of the hardware revealed that the screws used to secure the mounting plate to the tail rotor gearbox were of the incorrect model. Specifically, these screws were 4 mm longer than required, causing them to bottom out in the screw holes before the necessary clamping force could be established.
Findings
- The slip ring became loose because the mounting screws failed due to fatigue.
- The failure was caused by the use of incorrectly sized screws that were too long for the application.
- Because the long screws bottomed out, the bolted joint lacked sufficient tension, leading to insufficient friction and eventual fatigue breakage.
- The investigation also identified secondary manufacturing errors, including insufficient tightening torque and lock-wires that were installed in the wrong direction.
- Despite these significant assembly errors, the component had been issued an FAA Form 8130-3, certifying it as airworthy.
Safety action
Following the incident, the manufacturer of the slip rings implemented new procedures, including independent inspections and stricter controls to ensure the correct screw length is used during assembly. AgustaWestland also released a Technical Bulletin prescribing inspections and hardware changes for the type. SHK has recommended that the FAA increase its oversight of PMA holders to ensure all manufactured parts strictly comply with airworthiness requirements and internal procedures.