History of Flight
On 4 September 2017, Pakistan International Airlines (PIAC) Flight PK-390, an ATR72-500 registered AP-BKY, was scheduled for a routine flight from Karachi to Sukkur. After taxi out, the crew reported a No 2 Engine Electronic Control (EEC) and Propeller Electronic Control (PEC) single channel fault with dashes on the torque indicator, followed by an engine flameout. The aircraft returned to the ramp without further incident. No injuries or damage were reported.
Findings
The investigation determined that the engine flameout resulted from a failure of the No 2 roller bearing. Post-occurrence inspection revealed heavy metal particles on the Magnetic Chip Detector (MCD) of the turbo-machinery. A review of the aircraft's maintenance history showed that on 3 September 2017, a defect of "Pressure Filter by-pass Indicator Popped Out" was reported. The filter element was replaced, but the required inspection of the MCD was omitted, contrary to maintenance manual instructions. This omission left the aircraft in an unsafe airworthiness status.
Analysis
The removed engine (S/N ED1104) was sent to Pratt & Whitney Canada for tear-down examination. The OEM's analysis indicated that the event was caused by deterioration of the No 2 roller bearing. Secondary damages included rubbing wear and fracture of the bearing air seal and inter-shaft rubbing. Continued operation led to significant component distress in the inner compressor case and loss of HP bevel gear contact, resulting in flameout. The OEM noted that the root cause of the No 2 bearing distress could not be conclusively established due to the advanced state of damage. Industry data from 2007 to 2018 show a low event rate for No 2 bearing distress (1 per 2.58 million hours), and the root cause remains unidentified.
Conclusion
The investigation concluded that the occurrence was caused by design inadequacies of the No 2 bearing and continued operation after bearing failure, compounded by the failure of PIAC maintenance personnel to inspect the MCD during the filter change on the previous night.
Safety Recommendations
The safety investigation board recommended that Pratt & Whitney Canada accelerate efforts to identify the intrinsic reasons for No 2 bearing failure and implement improvements. PIAC was advised to provide refresher training for maintenance engineers on proper troubleshooting per maintenance manuals.