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.