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.