Occurrence
On 21 December 2022, at 2108 local time, a Regional Express Saab 340B aircraft departed Perth Airport, Western Australia, for a passenger transport flight to Albany, Western Australia. The flight crew climbed the aircraft to a cruising level of 15,000 feet. Shortly after leveling off, the captain felt two bumps pass through the airframe prior to the aircraft noticeably yawing. The flight crew identified a right engine failure, conducted the associated checklists, and secured the right engine. The aircraft was returned to Perth Airport and landed without further incident.
Investigation Findings
The Australian Transport Safety Bureau (ATSB) conducted post-occurrence troubleshooting, component testing, examination, and analysis on the right engine. The investigation found that the right engine hydro-mechanical unit (HMU) was incorrectly seated, resulting in a misalignment with the engine’s accessory gearbox (AGB). This misalignment caused significant wear and eventual decoupling of the HMU’s drive shaft. The decoupling led to a loss of the fuel pump function of the HMU, resulting in an in-flight engine failure.
Wear and deformation were identified on the splines of the HMU drive shaft and the internal splines of the AGB shouldered shaft. The polished O-ring land on the HMU drive shaft and corresponding circumferential wear marks within the AGB shouldered shaft indicated that the drive shaft had decoupled and was spinning within the shouldered shaft during operation. Damage on the HMU drive shaft splines, along with corresponding wear and damage to the AGB shouldered shaft receptacle, indicated that rotation had occurred between the two shafts. Additionally, the measured minimum internal diameter between the splines of the AGB shouldered shaft receptacle was larger than the maximum HMU spline diameter, making it impossible for the AGB to drive the HMU.
Functional testing of the HMU revealed no faults with the HMU or the high-pressure fuel pump. The likely reason for serviceable operation during testing was that the HMU drive shaft was being driven by the shouldered shaft of the test stand rather than the worn shouldered shaft within the engine’s AGB.
Wear marks from the AGB flange were observed on the inner surface of the V-band coupling. These wear marks were located on an area of the coupling that, when the flanges of the HMU and AGB are correctly aligned and the coupling is aligned correctly prior to and during torquing, an apex gap is maintained between the flanges and the inner surface of the V-band coupling. The presence of interference wear marks on the inner surface of the V-band coupling demonstrated that the flanges, or the V-band coupling, were misaligned during installation.
Also noted during post-occurrence inspections was damage to the threads on the fuel line that connected to the HMU. This line was connected as part of the HMU installation, and the presence of damage to the threads was a possible further sign of the fuel line being connected to a misaligned HMU.
The ATSB determined that the misalignment of the HMU, considering the HMU drive shaft was not bent, likely translated to uneven loading of the splines of the HMU drive shaft and the AGB shouldered shaft receptacle during operation. This in turn probably caused wear, damage, and subsequent decoupling of the HMU drive shaft during operation.
Safety Actions
The engine manufacturer, GE Aerospace, intends to share the learnings of this occurrence with customers and maintenance facilities and to remind them of the correct installation and maintenance of the hydro-mechanical unit to prevent premature wear of the drive shaft and accessory gearbox internal shouldered shaft. Regional Express reported taking several safety actions, including the dissemination of a technical notice to its engineers highlighting the importance of correct hydro-mechanical unit installation and the addition of V-band coupling installation to the list of critical control system maintenance items. The operator has also commenced a fleetwide inspection of its aircraft to confirm correct V-band coupling installation. As of 24 July 2024, 50% of the fleet had been inspected with no incorrect installations identified.