Qantas B744 near Singapore on May 9th 2011, engine shut down in flight
Last Update: December 19, 2012 / 12:04:29 GMT/Zulu time
Incident Facts
Date of incident
May 9, 2011
Classification
Incident
Airline
Qantas
Aircraft Type
Boeing 747-400
ICAO Type Designator
B744
A single intermediate-pressure turbine blade failed as a result of high-cycle fatigue cracking, producing high levels of vibration and the subsequent commanded shut-down of the No.4 engine.
Other safety factor: Wear of the interlocking shrouds of the intermediate-pressure turbine blades had the potential to reduce the dampening effects of the feature, and may have led to the development of conditions suitable for fatigue cracking of the IP turbine blades. [Minor safety issue]
Other key finding: The intermediate-pressure turbine blades were maintained on-condition and some blades had undergone multiple repairs in the region of the outer interlock shroud.
The ATSB reported the aircraft had been enroute at FL360 over Bali (Indonesia) when a step climb to FL380 was being initiated. The crew noticed that the EGT of engine #4 had rapidly increased to 850 degrees Centigrade, the thrust lever was retarded until the EGT returned to within limits. The crew then noticed the engine indicated vibrations at 3.5 units, well above normal operating level, and decided to shut the engine down. The crew informed ATC, descended the aircraft to FL340 and continued to Singapore, where a safe landing concluded the flight about 2 hours later.
Following landing a first inspection showed debris in the tail pipe of the engine, particles in the chip detectors of high pressure/intermediate pressure turbine and gear box, the chip detectors in the low pressure turbine and location bearings were clean. The low pressure turbine stator showed damage, the fan and low pressure turbine rotated freely.
A borescopic inspection revealed a single intermediate pressure turbine blade had fractured through the lower air foil section adjacent to the root section.
The engine was removed and sent for further analysis. Disassembly revealed only minor damage to internal componentry. The root of the fractured blade was removed and sent for laboratory analysis. The analysis revealed the blade had fractured as result of growth of a low stress/high cycle fatigue crack.
The engine manufacturer reviewed and examined all blades of the intermediate pressure turbine finding 64 new blades and 62 old refurbished blades. The refurbished/repaired blades showed evidence of thermal degradation of the microstructure, there was visible difference in the radii of the new and old blades, some repaired blades showed greater than expected quantities of weld (metal) on the shroud features, most of the repaired blades had lost material from the outer shroud interlock, the shrouds and interlocks of the new blades were all satisfactory.
The engine had accumulated 79,666 hours and 10,048 flight cycles since new, it's last refurbishment had been done in 2006, the engine had accumulated 18,944 hours and 2,017 cycles since.
The engine's last workshop visit was following an air return on Jan 25th 2011, see Incident: Qantas B744 near Bangkok on Jan 25th 2011, engine consumed too much fuel. The engine had been removed from VH-OJT, maintenance identified a high speed gear box bearing failure, the gear box was replaced and the engine returned to service as engine #4 on VH-OJH.
The ATSB analysed: "Blade failure stemmed from a low stress/high cycle fatigue crack that had initiated and propagated from the convex (suction) aerofoil surface. Metallurgically, the failed blade showed no localised discontinuities or material characteristics that could have contributed to the fatigue cracking sustained."
With regards to the shrouds the ATSB analysed: "Although work by the engine manufacturer to substantiate the effects of shroud interlock wear is continuing, it is known that the wear and loss of this feature can reduce the rigidity and damping effects of the shroud; potentially predisposing the blades to undesirable resonant behaviours and the onset of high-cycle fatigue cracking." and finished analysis stating: "The subject engine was last overhauled before the 2008 introduction of these new inspection criteria, and a current around-the-fleet inspection of the operatorÂ’s other engines overhauled before 2008 has not detected any further instances of unacceptable IP turbine blade shroud interlock wear."
The engine manufacturer released a service bulletin as result of the incident and investigation, the airline inspected all engines across the fleet with no further excessive wear identified.
Incident Facts
Date of incident
May 9, 2011
Classification
Incident
Airline
Qantas
Aircraft Type
Boeing 747-400
ICAO Type Designator
B744
This article is published under license from Avherald.com. © of text by Avherald.com.
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