Incident Overview
On 2 October 2020, an ATR 72 aircraft, registered SP-SPI, departed from Cologne/Bonn Airport (EDDK) on a scheduled flight to Ljubljana (LJLJ). Shortly after takeoff, at an altitude of approximately 1500 feet, the left engine (Pratt & Whitney PW127F) automatically shut down. The flight crew declared the situation and obtained clearance to return to Cologne, where the aircraft landed without further incident.
Post-Flight Inspection
During a subsequent ground inspection of the affected engine, maintenance personnel observed the following:
- Free rotation of the low-pressure compressor rotor and the power turbine rotor.
- No rotation possible of the high-pressure compressor rotor.
- Metallic particles present on the oil filter and the magnetic chip detector of the main oil system.
Engine Examination
The PW127F engine was sent to the manufacturer, Pratt & Whitney Canada, for detailed investigation. On 18 January 2021, the manufacturer issued an engine examination report, which attributed the shutdown to a fracture of the towershaft. This shaft connects the high-pressure compressor rotor shaft to the horizontal shaft of the accessory gearbox. The fracture interrupted the drive to the fuel pump, thereby cutting off fuel supply to the engine.
The fractured towershaft was subjected to metallographic laboratory analysis. However, due to extensive damage to the fracture surfaces, the direct cause of the shaft failure could not be determined. On a small area of the fracture surface that was not damaged, fatigue characteristics were detected.
Service Bulletin History
The manufacturer had previously identified towershaft failures and issued Service Bulletin No. 21936R2 in 2019. This bulletin recommends replacement of the towershaft and associated assembly components with a more rigid and wear-resistant design. Additionally, the assembly installation procedure was optimized, as the previous method was found to induce high stresses in the assembly. The service bulletin is to be applied during engine overhaul. In this specific case, the manufacturer performed the service bulletin modification to the engine under warranty repair after the incident.
Probable Cause
Państwowa Komisja Badania Wypadków Lotniczych (PKBWL) determined that the cause of the incident was a fracture of the towershaft connecting the high-pressure compressor rotor shaft to the horizontal shaft of the accessory gearbox. No contributing circumstances were identified.