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Technical Analysis

Summary

Examination of a Failed Compressor Blisk - Saab Aircraft AB, SF-340B, VH-EKX

1. FACTUAL INFORMATION
1.1 Introduction

On 23 May 2001, the take-off of a Saab 340 aircraft (VH-EKX) was discontinued as a result of the right engine failing. To investigate the event, the Australian Transport Safety Bureau carried out an examination of the first stage compressor blisk (bladed disk) from the affected engine. An earlier disassembly of the engine had found the component damaged by the partial loss of a single aerofoil (blade) section. The examination attributed the downstream damage within the engine to the effects of the released aerofoil section.

1.2 Component history

The blisk component was installed within a General Electric CT7-9B turboprop engine; serial number 785492. At the time of failure, the engine had operated for a total of 13,931 hours and 16,226 flight cycles. The engine manufacturer indicated that the failed stage-one blisk (p/no. 6055T83G14, s/no. GLHG4996) had been fitted to the engine since new. A supplied specification indicated that the blisk was produced as a forging from a proprietary precipitation hardening stainless steel alloy, similar to UNS S35500 (AM355).

1.3 Visual examination and fractography

1.3.1 General condition
The leading edges and tips of the blisk aerofoil sections had been damaged by multiple impacts (figures 1 & 2). The single fractured aerofoil had separated transversely at a mid-span location, around 29 millimetres above the root transition (figure 3). The rear edges of the blisk hub (adjacent to the stage two compressor wheel coupling) showed appreciable erosion around the full circumference (figure 4). Heat tinting and other evidence indicating rubbing contact against the housing was present over approximately two-thirds of the blisk circumference (figure 5). The forward (convex) surfaces of the aerofoils were coated with an oily, black deposit which increased in density toward the blade tips (figure 6). Beneath the deposit, the surfaces exhibited a bright, lustrous finish, typical of a metallic coating (figure 7). Conversely, the rearward (convex) surfaces exhibited appreciable erosion; being most pronounced toward and along the trailing edges (figure 8). The metallic coating appeared to have appreciably eroded away on this side.

FIGURE 1

 
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