What happened
On 1 November 2006, an ATR 42, registration EI-CVS, was performing a scheduled public transport flight from Farranfore Airport to Dublin. While climbing through FL120, the crew experienced a loud bang and a physical jolt to the aircraft. Simultaneously, the Inter Turbine Temperature (ITT) on the No. 1 engine surged above 1200°C, and cabin crew observed smoke and flames emanating from the port engine.
In response, the captain immediately shut down the No. 1 engine and declared a "Mayday." The crew executed in-flight engine fire drills and diverted the aircraft to Shannon Airport. The aircraft completed a single-engine approach and landed safely on runway 2 and 4 at 15:11 hrs. Following the landing, the passengers disembarked without injury, and the aircraft was taxied to a stand.
The investigation
An investigation by the AAIU focused on the internal damage sustained by the PW120 engine. Post-flight inspections revealed that the No. 1 propeller could not be rotated and that the Low Pressure Turbine (LPT) was missing approximately 50% of its blades along its length. A detailed teardown at the manufacturer's facility in the UK and subsequent laboratory analysis in Canada established that two LPT blades had been released from the disc.
The investigation examined the history of the engine, noting that while the LPT stator and seal had been upgraded to a newer standard in 2005, several blades remained at an older configuration. Metallurgical analysis of the LPT disc revealed that a section of the disc's "fir tree" fixing had sheared, allowing the blades to escape.
Findings
- The primary cause of the engine failure was a fracture of the LPT disc fixing, which occurred via a mechanism of creep-assisted fatigue.
- The loss of the blades caused severe unbalance, leading to intense vibrations that caused the engine's shafts to rub against one another.
- This vibration resulted in the shearing of bearing housing bolts and oil tubes, which subsequently led to the No. 6 bearing overheating and fusing.
- The investigation noted that the engine's LPT disc had reached 91% of its total design life.
Safety action
- The manufacturer is recommended to increase the urgency of implementing Service Bulletin No. 21555 by upgrading its compliance category from 7 to 6.