BOEING 747 — Atlanta, GA, USA, 2014-09-21

NTSB summary

The airplane, a Boeing 747-451, had just taken off when the No. 4 engine, a Pratt & Whitney PW4056, lost power and then caught fire.  The crew had to discharge two fire bottles into the engine's nacelle before the fire was extinguished.  The airplane returned to the airport for a three-engine landing without further incident.  The disassembly of the engine revealed one 5th stage compressor blade was fractured transversely across the blade neck.  The metallurgical examination of the blade confirmed a fatigue fracture that had originated from the aft concave side corner of the blade neck.  The metallurgical examination also revealed evidence of grit blasting with embedded aluminum oxide media on the blade neck that, per the overhaul instructions, is a no grit blast area.  Grit blasting of the blade root's contact surfaces with aluminum oxide media is a preparatory step for the application of plasma spray material, which is one of the repairs that was accomplished to the fractured blade.  The blade was overhauled by Turbine Overhaul Services (TOS), Singapore.  TOS is an FAA-certificated Part 145 repair station for the overhaul and repair of turbine engine compressor blades and vanes.  An audit of TOS's PW4000 5th stage compressor blade overhaul process showed that it conformed to the requirements except for the grit blasting operation that showed the grit blasting gun was being held to close to the blade.  In addition, the masking process that was in use could result in no grit blast areas of the blade root being exposed to grit blasting.   The fire was caused by hydraulic fluid that leaked from the pressure line to pulse damper B-nut at the pump that loosened due to vibration following the compressor blade separation.

Probable cause

Turbine Overhaul Service's aggressive grit blasting and incomplete masking of the blade root neck that allowed the no grit blast area being exposed during the overhaul of the blade resulting in blasting media being embedded in the blade root from which a fatigue crack developed. The fatigue crack propagated until separation of the blade occurred that resulted in a complete loss of engine power. The fire was caused by the B-nut on the hydraulic line from the fuel pump and pulsation damper loosening from engine vibration after the compressor blade separated spraying high pressure fluid into the engine compartment that ignited on the hot engine cases.

Flight data recorder

Boeing 747-451 · N669US — 500,301 samples, 131 parameters, 17:46:57.0 to 19:38:23.0.

Sequence of events (FDR factual report)

Recorder documents in the docket

Docket: data.ntsb.gov/Docket/?NTSBNumber=ENG14IA027. This brief is an analytical aid assembled from public records; it is not an investigative finding.