What happened
On 26 February 2019, a Eurocopter EC1SBT1, registration VP-CPS, was preparing for a training and search mission at Owen Roberts International Airport in the Cayman Islands. The crew, consisting of a pilot, a safety pilot, and a tactical flight officer, had configured the aircraft with dual controls for instrument-flying practice. During the initial climb, the pilot lifted the aircraft to a height of approximately four feet above a transport dolly.
At this low altitude, the pilot experienced a sudden vibration in the cyclic control stick accompanied by a powerful rearward force that could not be overcome. In response, the pilot immediately lowered the collective, landed the aircraft heavily, and shut down the engines. The impact caused significant damage to the landing gear, the tail boom, the transmission deck, and the Fenestron shroud. There were 3 crew members on board, all of whom sustained minor injuries.
The investigation
Investigators examined the main rotor actuator (MRA) components, which move the swashplate in various axes. The inspection of the longitudinal axis actuator revealed that the tie bar had fractured approximately 30 mm from the fork end.
Laboratory analysis using Scanning Electron Microscopy (SEM-EDX) identified pitting corrosion on the tie bar surface, with cracks spreading through the material via intercrystalline corrosion. Chemical analysis of deposits near the fracture site revealed high levels of sodium and chlorine. This suggested that salt-laden moisture had entered the component, likely due to a gap between the compression ring segments in the actuator assembly.
Findings
- The primary cause of the accident was the fracture of the longitudinal axis actuator tie bar.
- The fracture originated from pitting corrosion caused by the accumulation of salt-laden moisture within the actuator.
- The unsealed gap between compression ring segments allowed moisture to penetrate the upper piston.
- The maritime environment of the Cayman Islands contributed to the high salt deposits found on the component.
- Over time, the corrosion reduced the effective cross-sectional area of the tie bar until it could no longer withstand normal operating loads, resulting in an overload failure.