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Examination of a Failed Fan Blade Rolls-Royce RB211 Trent 892 Turbofan Engine

Summary

On the early evening of 31 January 2001 at Melbourne International Airport, Boeing 777-300 A6-EMM aborted its take-off run at low speed as a result of a failure within the left (No.1) engine. Although the failure was associated with a large compressor surge within the engine, no subsequent fire developed and the aircraft was able to safely return to the terminal building on its remaining serviceable engine.

Failure of the RB211 Trent 892 engine as fitted to the aircraft was a result of the release of a single blade from the low-pressure compressor (fan) rotor disk. The blade release caused extensive damage to the remainder of the fan and the intake shroud, however the event was fully contained. The only escape of debris from the engine was small, low energy fragments, causing minor damage to the fuselage and the opposite engine.

Field and laboratory examination of the released blade found that progressive fatigue cracking had resulted in the loss of two major sections from the blade dovetail root. The remaining material was subsequently unable to carry the centrifugal loads associated with the accelerating engine and failed in ductile shear, allowing the release of the blade from the rotor slot. No defects or other anomalous material or manufacturing features were found to have contributed to the crack development.

The blades installed within the Trent 892-17 engine were an approved variant of the original design, incorporating an undercut radius between the dovetail faces and the blade body. The modification was developed in order to avoid 'edge of bedding' stresses that had been implicated in blade cracking on development engines. Cracking of the released blade had initiated within this undercut radius on both sides of the shear key slot; locations that had been identified by finite element techniques as areas of high localised stress. Extensive galling of the seating surfaces was also found on all blades, indicating the long-term inadequacy of the dry film lubricant applied to the blade dovetail faces. The galling and micro-welding damage can readily interfere with the distribution of loads across the seating surfaces, leading to elevated stresses within the blade root.

Blade failure was thus attributed to an interaction of the following -

  • Design - provided for areas of localised high tensile stresses arising from operating loads.
  • Operating Stresses - act on the blade to produce cracking in areas highlighted by the design. In the absence of defects predisposing the blade to failure, the development of cracking implies elevated operating stress levels.
  • Blade - Disk Connection Problems - galling of the dovetail surfaces indicates the potential for uneven load distribution through the connection, leading to increased stresses within the blade root and thus a greater disposition to fatigue cracking.
Type: Educational Fact Sheet
Publication date: 31 January 2001
 
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Last update 07 April 2014
 
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