REX minus prop at SYD, photo Grahame Hutchinson

Regional airline REX appears was blindsided by a previously unknown flaw in an engine gearbox when a propeller came off one of its SAAB 340 turbo-prop aircraft while it was approaching Sydney Airport with 19 people on board on March 17.

None of the maintenance requirements REX was following in servicing a particular sub-set of its SAAB 340 fleet even required it to look at the component that failed, according to a preliminary report just released by the ATSB.

But that is about to change, with the ATSB warning similarly at-risk-airlines world wide of the fault.

The propeller that came off the aircraft when it was over the Macarthur area not only missed hitting vital control surfaces on the wing or tail of the small commuter aircraft, but then crashed to earth near houses when it came down in a bushland reserve in the Revesby area.

The ATSB says that the cracks and corrosion that were apparent in photos of the aircraft after it landed safely at Sydney Airport started within the so-called mounting flange of the propeller’s gear box, before spreading to a shaft section.

The propeller shaft had then fractured, leading to the separation of the propeller.

It found corrosion and pitting in a dowel pin bore in the engine, and describes the process in considerable detail by which this then led to the ultimate breaking free of the propeller.

The ATSB report says this is the first known critical failure of this type initiating within the propeller hub flange of a GE Aviation CT7-9B engine. It points out that the same propeller gear box is fitted to the widely used CASA CN-235 utility turbo-prop. It warns that any corrosion or cracking within the bore may go undetected until it progresses to the surface of the flange.

The safety investigator has outlined extensive additional work that it will have to conduct before it can issue a final report.

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