What happened
On October 2, 2020, an ATR 72, registration SP-SPI, departed from Cologne (EDDK) bound for Ljubljana (LJLJ). During the climb, at an altitude of approximately 1,500 ft, the left engine failed. The crew declared the emergency and received authorization to return to the departure airport. The aircraft landed safely without further incident.
Following the landing, a physical inspection of the engine revealed that while the low-pressure rotor and power turbine were rotating freely, the high-pressure rotor was unable to rotate. Additionally, metal particles were discovered on the main oil system filter and the magnetic plug.
The investigation
The PW127F engine was sent to the manufacturer, Pratt & Whitney Canada, for detailed analysis. The manufacturer's report, issued on January 18, 2021, determined that the engine shutdown was caused by a fracture in the towershaft, which connects the high-pressure rotor shaft to the propeller gearbox horizontal shaft. This fracture interrupted the drive to the fuel pump, resulting in a loss of fuel supply to the engine.
Metallographic testing was performed on the fractured shaft; however, due to extensive damage to the fracture surface, the specific origin of the failure could not be determined. Some fatigue characteristics were identified on a small area of the undamaged fracture surface.
Findings
- The primary cause of the engine failure was the fracture of the towershaft connecting the high-pressure rotor shaft to the propeller gearbox horizontal shaft.
- The fracture led to the interruption of the fuel pump drive, causing the engine to lose fuel supply.
- The manufacturer had previously issued Service Bulletin 21936R2 in 2019 to address similar historical shaft fractures by replacing the assembly with a stiffer, more wear-resistant version and optimizing assembly procedures to reduce stress. The engine involved in this incident had already undergone this replacement as part of a warranty repair.