What happened
On 21 December 2022, a Saab 340B operated by Regional Express departed Perth Airport, Western Australia, bound for Albany. While the aircraft was in the cruise phase of the flight, the right engine failed. Following the malfunction, the crew successfully returned the aircraft to Perth Airport.
The investigation
Technical examinations of the right engine focused on the hydro-mechanical unit (HMU) and the accessory gearbox (AGB). Investigators identified significant wear and deformation on the splines of the HMU drive shaft and the internal splines of the AGB shouldered shaft. Evidence, including polished O-ring lands and specific wear marks, indicated that the drive shaft had decoupled and was spinning independently within the shouldered shaft during flight.
Measurements revealed that the internal diameter of the AGB shouldered shaft receptacle was larger than the maximum diameter of the HMU spline, making it physically impossible for the AGB to drive the HMU. Furthermore, wear marks on the inner surface of the V-band coupling suggested that the flanges or the coupling itself were misaligned during installation. Damage to the threads on the fuel line connected to the HMU also pointed toward a misaligned installation. While functional testing of the HMU and high-pressure fuel pump showed no faults, investigators concluded that the test stand's shaft likely masked the issue by driving the HMU correctly.
Findings
- The HMU was incorrectly seated and misaligned during installation.
- This misalignment caused uneven loading on the HMU drive shaft and the AGB shouldered shaft splines.
- The resulting wear and damage led to the decoupling of the HMU drive shaft from the accessory gearbox.
- The loss of connection caused the fuel pump and metered fuel output to fail, resulting in the engine failure.