What happened
On June 7, 2016, during a scheduled hangar maintenance inspection at Warsaw-Babice (SOT), technical personnel performing a 12-month service check on an EC 135 P2+ (registration SP-HXB) discovered damage to the main rotor shaft components. Following instructions from service bulletin SB EC135-64-023, technicians inspected the main rotor shaft spacer tube for wear on the protrusions interacting with the main rotor shaft roller bearing. The inspection revealed a material chip measuring 6.6 x 5.7 x 1.02 mm on the internal race of the roller bearing (P/N 0750118442) and abnormal operational wear on one of the spacer tube protrusions (P/N 4649205015).
The manufacturer, Airbus Helicopters (AHD), was notified and recommended the replacement of all worn components along with a full inspection of the main gearbox. On June 17, 2016, the damaged parts were replaced. During the subsequent gearbox inspection, the missing metal fragment from the bearing race was not found, suggesting it had likely been flushed out during an oil change. Analysis of previous oil samples showed no anomalies. Following two gearbox flushes and a successful check of the oil filter and chip detector, the aircraft was returned to service.
The investigation
The user's investigation commission examined the maintenance history and the physical condition of the components. The investigation established that the main gearbox had been operated with the damaged bearing race for approximately 833:19 flight hours. The damage likely originated when the gearbox had 1511:06 flight hours. Despite regular scheduled inspections, oil changes, and oil analysis, the progressive wear of the bearing race and the spacer tube protrusion went undetected until the 2344:25 flight hour mark.
Findings
- The investigation concluded that the damage was likely caused by an assembly error occurring during the installation of the bearing while replacing the spacer tube, or alternatively, a material defect in the bearing race.
- Regular maintenance protocols, including oil sampling and inspections, were performed correctly but failed to detect the gradual wear caused by the initial damage.