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Technical Analysis Report


Boeing 777-212ER, 9V-SRE

1.1 Introduction

During a flight from Brisbane to Singapore, the crew of the Boeing 777-212ER aircraft noticed the onset of abnormal vibration levels and several 'thumps' from the right engine. While continuing to monitor the engine, the vibration and thumps recurred and the engine was subsequently shut down after the oil and exhaust gas temperatures rapidly increased. Following a diversion to Darwin, the crew conducted an uneventful single engine landing.

On initial inspection, maintenance personnel found a single fractured first-stage variable stator vane (VSV) control lever (figure 1). Later internal boroscopic inspection of the engine found significant levels of mechanical damage within the intermediate and high-pressure compressor stages.

The fractured lever and a selection of other levers from the first-stage VSV assembly were removed from the engine for examination by the ATSB.

Fig 1
Fig. 1 External view of the stage-one variable stator vane actuator ring and the single fractured lever (arrowed).

1.2 Visual examination and fractography (failed lever)

Initial inspection in the ATSB laboratory found the VSV lever had fractured transversely through the end of the arm section (figure 2), at a location coincident with the riveted connection to the actuator pin. The fracture path followed a uniform arc, extending from one side of the arm to the opposite and intersecting the pin connection at the centre (figure 3).

Fig 2

Fig. 2 Underside of the VSV lever removed from the engine.

Fig 3

Fig. 3 Underside of the VSV lever showing the curved fracture path and the wear mark produced by in-service movement after failure.

A prominent track mark had developed on the underside of the arm where the relative movement between the separated arm and the pin flange had produced appreciable wear. The effects of wear extended to the fracture surfaces themselves, which were heavily eroded and all fracture surface detail obliterated (figures 4 & 5). Apart from the fracture, the arm had sustained little other mechanical damage and showed no evidence of deformation or distortion associated with the failure.

Fig 4

Fig. 4 Arm section fracture surface showing degree of wear and loss of detail.

Fig 5

Fig. 5 Pin section fracture - adjoins the surface shown in figure 4.

On close inspection, the fracture path appeared to intersect the bore of the rivet hole, with slight upward 'dishing' of the arm section beneath the rivet head (figure 6). A clearance or gap was not evident between the pin shaft and the bore of the arm hole through which the pin shaft was riveted.

Fig 6

Fig. 6 Gap between the pin flange and the control arm produced by upward 'dishing' of the arm beneath the rivet head.

The examination did not show any evidence of binding or excessive friction between the actuator pin and the mating bushing, nor did any other component show significant indications of miss-installation or anomalous operation.

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