What happened
On June 13, 2014, an ATR 72-212A aircraft, registration VT-JCS, operated by Jet Airways, was involved in a serious incident at Bengaluru during the pushback phase of flight 9W-2731. The flight was scheduled to proceed from Bengaluru to Cochin.
During the pushback process on taxiway L3, the crew received a nacelle overheat warning. Following the shutdown of engine number two, the warning temporarily cleared, but the crew encountered further issues when attempting to restart the engine. After the second restart attempt, a mode selection auto fault appeared, and the crew began to notice a burning smell. This was quickly followed by heavy smoke filling both the cockpit and the passenger cabin.
In response to the deteriorating conditions, the pilots shut down both engines and requested fire services. The 64 passengers and 4 crew members were safely deplaned on the taxiway. The aircraft was subsequently towed back to the gate.
The investigation
An investigation by AAIB India focused on identifying the source of the smoke and the cause of the engine warnings. While no external fire was observed, inspections of the aircraft's air conditioning packs and air cycle machines revealed an oil smell in the ducts and hoses.
Subsequent boroscopic inspections of both engines revealed oil stains on the low-pressure impeller of the left engine and oil puddles on the high-pressure impeller vanes of both engines. Following a compressor wash and high-power ground runs, the aircraft was cleared for flight after the replacement of the air switching valves.
Detailed analysis of the removed components by the manufacturer revealed that the outer diameter of the air switching valve possessed a burr, and the inner diameter of the bushing showed wear beyond serviceable limits. This wear likely hindered the smooth movement of the valve within its housing.
Findings
- The primary cause of the smoke and oil smell was an oil leak resulting from the wear of the inner diameter of the air switching valve's inner valve.
- This mechanical wear disturbed the balance of the secondary sealing air to the bearing compartment, allowing oil to escape.
- The presence of a burr on the valve's outer diameter may have further restricted proper valve movement.
- The aircraft was being maintained according to approved programs, and all required airworthiness directives had been complied with.