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Summary

Summary

Hughes 269 helicopter was being ferried from the township of Ayr to a cattle property. The pilot reported that while cruising at about 1,000 feet above ground level, approximately 1 hour after departure, the engine suddenly began to run roughly and then lost all power. During the subsequent forced landing the helicopter impacted the rocky terrain heavily, resulting in substantial damage to all sections of the helicopter. The pilot sustained minor injuries.

Technical disassembly of the engine revealed extensive internal damage to the number four cylinder and piston. The exhaust valve head had separated from its stem section and was embedded in the ceiling of the cylinder head. The majority of the stem section was missing, probably having passed out through the exhaust port. Both of the valve retaining keys and the outer valve spring seat were loose in the rocker box. Broken fragments of the exhaust valve stem cap and a section of the outboard end for the exhaust push rod, including the ball-end, were also in the rocker box. A slight wear ring or indentation was found around the circumference of the inlet push rod near the outboard end. The investigation determined that during a previous maintenance action the intake valve rocker arm had been fitted to the exhaust valve position, and the exhaust valve rocker arm was fitted to the inlet valve position.

Examination of the maintenance documentation for the aircraft showed that the engine had completed approximately 200 hours time in service since it was last overhauled. It showed that the engine had also undergone significant repair work to rectify low power indications 26.5 operating hours prior to the accident. During that repair, all four cylinders were removed for inspection and several valve guides were replaced. The camshaft was also replaced. The maintenance engineer who completed the repair work and subsequent power checks reported that the engine was then indicating full rated power.

The incorrect positioning of the valve rocker arms resulted in a significant misalignment of both the rocker arms and push rods at the outboard (cylinder head) ends. That misalignment allowed the cup-edges of the rocker arms to make contact with the walls of both pushrods during operation. On the intake pushrod, the contact resulted only in a slight scoring of the pushrod wall. However, the contact on the exhaust pushrod was more severe and resulted in a circumferential thinning and weakening of the pushrod wall to the point where the wall collapsed and the ball end for the pushrod separated. After the pushrod collapsed the most likely sequence of events was that the valve stem cap came loose from the end of the exhaust valve stem due to the (now) excessive gap between the rocker arm and the outer valve seat. It then lodged sideways, or in an irregular manner, closing the gap between the two. The subsequent actuation of the pushrod against the rocker arm would have caused the rotor cap to impact upon the outer valve seat and valve retaining keys, in a manner that allowed the keys to fall out. The exhaust valve was then free to over-travel into the cylinder and impact with the upcoming piston.

It was reported that two experienced engineers were involved with the last repair work on the engine. One of them identified the parts that were to be re-fitted and the other carried out the actual re-fit.

 
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