Boeing 747 451 N668US incident near Tokyo, JP

Probable cause

The PW4056 engine lost power due to the fatigue fracture of a 5th stage compressor blade. The cause of the fatigue fracture could not be determined.

NTSB analysis

The disassembly and examination of the engine revealed a 5th stage compressor blade was fractured transversely across the airfoil directly above the blade root platform. Unfortunately, that fractured blade was lost during the shipment of the engine parts from Delta Air Lines to P&W's Material and Processes Engineering Laboratory. However, the similarity of the elliptical-shaped fracture surface on that 5th stage compressor blade to the elliptical-shaped fracture surfaces on 6th, 7th, 8th, and 9th compressor blades that were confirmed as fatigue fractures would indicate the 5th stage compressor blade also fractured due to a fatigue crack. The PN 5th stage blade that fractured in this engine and the compressor blade that it had superseded had a combined total of 38 airfoil fractures that would suggest some subtle interrelationship that would cause a resonance leading to a fracture. A detailed examination of the HPC did not identify anything that could have triggered an airflow disturbance. The fractured blade 5th stage compressor blade was subsequently superseded with a new blade in 2011 that has not experienced any fractures suggesting that whatever was causing the fractures was designed out.

The engine's turbine sections sustained extensive thermal and mechanical damage. When the 5th stage compressor blade fractured, the cascade of damage through the HPC disabled the compressor's ability to pump air. The FDR showed that following the loss of power after the 5th stage compressor blade fractured, the fuel flow dropped to zero but then recovered to a sub idle level probably from the electronic control's auto relight feature activating the igniter plugs. Although the fuel flow was less than what was seen when the airplane taxied out for departure, with little to no airflow through the engine, the combustion flame would stagnate around the turbine vanes and blades burning off the early stages and the liberated pieces causing the mechanical damage to the trailing stages.

This incident was initially reported as an uncontained event because of the 360° circumferential split in the LPT case. The examination of the split showed that the edges were not pursed outward indicating there was not a ballistic challenge of liberated blade to the case. The examination of the LPT case showed that the case's walls were not bulged radially outward indicating that there was not a ball up of material whirling around the case. The examination did show that the edges of the cut in the case matched the edges of the circumferential breaks in the 6th stage blade outer airseal segments that both matched the geometry of the 6th stage blade tip shroud indicating that a tip shroud was dragged around by one of the near full length blades as the engine wind milled during the return to NRT. So although the LPT case was cut in half, this was not an uncontained event.

Text from the NTSB. Open the NTSB docket for ENG17IA026 for the full record.