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Cases › ENG14IA027 · Part 121 airline

BOEING 747Delta Air Lines

Atlanta, GA, USA · 2014-09-21·N669US

0
fatal
0
serious
No injury
highest injury
—
damage
Replay the flight data500,301 samples · 131 parameters17:46:57.0 – 19:38:23.0AnalyseThe analystOne timeline · precedents · ask the docket · what the record saysGenerateInvestigation briefFacts, cause, recorder extremes, sequence, sourcesSourceNTSB docket6 documents on data.ntsb.gov

What happened

NTSB summary, verbatim

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.

Photographs

1the aircraft, and what the investigators found
The aircraft before the accident
Delta Air Lines B747-451, N669US, as DL 72 ATL-AMS (17229638640)
Charles from Port Chester, New York · CC BY 2.0 · Wikimedia Commons

Probable cause

the Board's determination
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.

Occurrence sequence

3 stepsNTSB coding · CICTT taxonomy
  1. 1 · Initial climb
    Loss of engine power (total)
    defining event
  2. 2 · Initial climb
    Powerplant sys/comp malf/fail
  3. 3 · Initial climb
    Fire/smoke (non-impact)

Findings

1 causal · 0 contributing
  • CAUSE
    Personnel issues › Task performance › Use of equip/info › Use of equip/system — Maintenance personnel

Sequence of events

5 timed eventsfrom the FDR factual report · claude-sonnet-5
  1. 18:04:47 UTCinitial climbp.3
    engine 4 N1 decreased from 96.38% RPM to 61.13% RPM, approximately 19 seconds after takeoff
  2. 18:05:52 UTCinitial climbp.3
    engine 4 fire warning transitioned from normal to fire warning
  3. 18:06:44 UTCclimbp.3
    engine 4 N1 decreased to 0% RPM and remained at 0% RPM for the rest of the flight
  4. 18:09:05 UTCclimbp.3
    engine 4 fire warning transitioned back to normal and remained at normal for the rest of the flight
  5. 19:23:48 UTClandingp.3
    gear weight on wheels transitioned from air to ground indicating the aircraft touched down

NTSB analysis

from the final report

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.