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Final Report

Summary

At about 0930 Western Standard Time (WST), a Cessna 208B, registered VH‑LNH, departed Kununurra, Western Australia, on a scenic flight to the Bungle Bungle Range. On board were the pilot and 12 passengers. Soon after take-off, oil began leaking from the engine area, forming an opaque film on the windscreen. The pilot also found that there was insufficient thrust to maintain level flight. Despite substantially limited forward visibility due to the film of oil on the windscreen, the pilot was able to return to Kununurra and land. Subsequent inspection of the aircraft showed that the propeller had feathered and seized. Aside from the apparent damage to the engine/propeller system, there was no damage to the aircraft, and no injuries.

Inspection of the engine showed that an oil leak had developed from a join between the oil transfer elbow and oil transfer tube at the forward end of the engine. One of the oil transfer elbow mounting lugs (that secured the elbow to engine assembly) was found to be fractured, allowing the oil transfer elbow to move and unseat the seal (O-ring) between the elbow and the tube.

Analysis of the fractured lug and securing assembly by the engine manufacturer indicated that the securing bolt was sitting askew in the bore of the flange to which the elbow mounting lug was secured. The analysis report commented that the wear pattern was consistent with a lack of pre-load on the bolt. The lack of pre-load was considered to be the initiating factor that led to fracture of the mounting lug, however the reason for the lack of pre‑load could not be ascertained.

Notwithstanding their ongoing consideration of relevant information, CASA did not believe that there was any conclusive evidence of a lack of pre-load on the bolt. CASA also commented to the effect that incorrect installation of the oil transfer elbow itself, rather than incorrect installation of the mounting lug bolt, may have been the origin of the problem, and that this is a scenario that is known to have occurred in practice. At the time this report was prepared, both the engine manufacturer and CASA were considering the provision of additional information and advice regarding the oil transfer elbow to relevant organisations.

In response to this incident, the operator was planning to expand existing periodic inspections to include inspection of the internal radius of the mounting lug for signs of crack development.

The manner in which the pilot handled a very difficult set of circumstances provides some positive examples for other pilots to consider, particularly with respect to prioritisation and use of available resources for assistance. The incident highlights the importance of care and attention to detail when working on aircraft engines and accessories.

 

Aviation Short Investigations Bulletin - Issue 48

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