What happened
On 23 September 2020, an Airbus Helicopters EC175B, registration G-EMEB, was undergoing a post-maintenance ground run at Aberdeen Airport. The aircraft was not intended for flight at the time, as it was undergoing scheduled maintenance to replace the main gearbox. During the run, approximately 18 minutes after the start of the No 1 engine, a chip detector warning illuminated in the left accessory gearbox (LAGB). Upon shutting down the engines, the crew noted a "crunching" sound from the left side of the gearbox and found it difficult to rotate the main rotors manually. The gearbox oil was found to be discoloured.
The investigation
An investigation by the AAIB focused on the maintenance activities performed prior to the ground run. The aircraft had recently undergone a main gearbox replacement, which required transferring certain components, including a 10 kVA alternator, from the old gearbox to the new one.
Investigators examined the installation process of the alternator to the LAGB. They found that the maintenance engineer had installed the alternator, V-band clamp, and interface spacer as a single assembly using four bolts. This deviated from the manufacturer's manual, which required the interface spacer to be installed first, followed by the alternator via the V-band clamp. Furthermore, the investigation looked into the application of grease within the alternator pinion cavity. While the manual required the cavity to be filled until it overflowed, the investigation found that the specific method of assembly used by the engineer significantly increased the potential for trapped volume.
Physical examination of the gearbox revealed that the F1 bearing had overheated, leading to deformed rollers and a melted bearing cage. Metallurgical analysis of debris found in the oil sump confirmed the material was from the failed bearing. Testing by the manufacturer demonstrated that the presence of excess grease and air within the sealed alternator shaft link cavity acted like a hydraulic piston. When the components were clamped together, this trapped volume was compressed, creating an excessive axial load that the bearing was not designed to withstand.
Findings
- The failure of the F1 bearing was caused by an axial load exceeding its rated capacity.
- This load was generated by the compression of trapped air and excess grease within the sealed alternator shaft link cavity.
- The installation method used by the engineer, attaching the components as an assembly rather than following the sequential steps in the maintenance manual, increased the axial load by up to four times the intended amount.
- The application of grease until it overflowed, as per the then-current instructions, created the latent condition for the pressure buildup.