Busan B735 near Seoul on May 25th 2012, engine shut down in flight

An Air Busan Boeing 737-500, registration HL7232 performing flight BX-8803 from Seoul Gimpo to Busan (South Korea) with 121 passengers and 5 crew, had departed Gimpo Airport's runway 36L and was climbing through 11,500 feet accelerating through 294 KIAS when a loud bang was heard followed by the instruments of the right hand engine (CFM56) rapidly changing to abnormal readings. The first officer, pilot flying, handed controls to the captain, who declared emergency, shut the engine down and returned to Gimpo Airport for a safe landing on runway 36L about 20 minutes after the bang.

South Korea's ARAIB released their final report in Korean concluding the probable cause of the "semi accident" was:

inadequate engine maintenance, which did not properly connect the inlet guide vanes' (IGV) ring with the actuation assembly resulting in unusual high pressure at the high pressure compressor and unusual air flow inside the high pressure compressor causing vibrations, the separation of high compressor blades as result of fatigue and substantial engine downstream damage.

Contributing factor were inadequate procedures of quality control, that did not ensure the check of the lever arms being properly connecting IGV and actuating assembly.

The ARAIB reported that the engine had undergone high pressure compressor (HPC) maintenance at Lufthansa Technik in Ireland in December 2011, which was completed in January 2012.

Following the occurrence flight inspection of the engine revealed that the IGV lever arms had not been properly connected to the actuation ring, number of lever arms were missing and several lever arms had been bent and twisted as result. 2 HPC stage 1 blades had separated with substantial damage downstream.

The investigation determined that the IGV lever arms had been disconnected during the maintenance works in Ireland, and according to maintenance logs been reconnected upon completion of the works. There was no evidence of supervisory activity verifying the correct installation of the IGV lever arms however. The ARAIB reported that supervisor and workers at the maintenance facility were aware of the importance of that check however.

The ARAIB analysed that during the works maintaining the HPC of the right hand engine a mistake had occurred. The mistake remained undiscovered as there were no provisions in the procedures to verify the installation of the IGV lever arms.

Missing IGV lever arms (Photo: ARAIB):