What happened
On 21 December 2022, a Regional Express Saab 340B, registered VH-RXE, was operating a scheduled domestic passenger flight from Perth to Albany, Western Australia. While the aircraft was cruising at 15,000 feet, the flight crew experienced two distinct vibrations followed by a noticeable yaw.
Upon monitoring the engine parameters, the crew identified a failure of the right engine as the RPM began to drop. The crew performed the necessary emergency checklists and secured the engine. The aircraft was subsequently returned to Perth Airport, landing without further incident. There were no injuries to the 32 passengers or the 3 crew members on board.
The investigation
The investigation focused on the right engine's GE Aerospace CT7-9B turboprop engine and its accessory gearbox. Post-occurrence inspections of the engine revealed a significant amount of oil pooled in the exhaust.
Detailed analysis of the engine components by GE Aerospace and investigators revealed that the hydro-mechanical unit (HMU) was not properly seated. Specifically, measurements showed that the HMU drive shaft's maximum diameter was smaller than the minimum diameter of the receptacle splines in the accessory gearbox, making a proper connection physically impossible. Furthermore, wear patterns on the V-band coupling indicated that the flanges were misaligned during installation, preventing the necessary clearance known as the apex gap.
Findings
- The primary cause of the engine failure was the incorrect seating and misalignment of the hydro-mechanical unit.
- This misalignment caused significant wear on both the HMU drive shaft and the accessory gearbox's internal shouldered shaft.
- The resulting wear eventually led to the decoupling of the HMU drive shaft from the engine accessory gearbox.
- The decoupling caused a loss of fuel pump function, which interrupted the metered fuel output and led to the engine failure.