Piper Aircraft Corp PA-31-350, VH-NPA

After establishing the Piper Chieftain in the cruise, the pilot
occupying the right seat commenced a daily trend monitor of the
engine operating parameters. During this procedure, the right
engine oil pressure was observed to be low. While monitoring the
oil pressure, the pilot noticed the right engine oil pressure
decreasing accompanied by an increase in engine oil temperature.
Shortly after, oil was observed trailing from the engine nacelle
vent and skin joints. The right propeller was feathered and the
engine shut down.

Inspection by a maintenance engineer found that the No. 3 piston
had sustained detonation damage. The piston and cylinder were
changed and the aircraft cleared for a ferry flight to Jandakot to
facilitate a bulk strip inspection of the engine carried out under
ATSB supervision. During this inspection further evidence of heat
stress was found necessitating shipment of the affected components
to the ATSB for a more in-depth examination. The components
inspected exhibited the typical effects of over-temperature
operation. Evidence of detonative combustion was also noted within
other cylinders of the engine.

The fuel injection unit was examined and its functions tested on
an approved component test bench. The unit was found to control
fuel/air mixture within normal limits without fault and the
automatic functions of the unit would not have induced processes
that lead to detonation.

The company's technique for leaning the engines in cruise
routinely allowed the exhaust gas temperature to exceed the maximum
permissible temperature for a short period of time, and set the
temperature at a higher level than would have been achieved by
following the aircraft manufacturer's procedures for leaning the
engines.

The right engine of a Piper Chieftain will normally run at a
higher temperature than the left because of factors such as the
counter rotation of the propeller inducing different cooling
airflow, and the presence of an airconditioning compressor in the
front intake area obstructing airflow. This aircraft was only
fitted with one cylinder head temperature (CHT) probe and one
exhaust gas temperature (EGT) probe per engine. The installation
has the number 6 cylinder instrumented for CHT and a single EGT
probe monitoring a combined gas stream. It would not be possible to
determine from the cockpit indications if any of the remaining five
cylinders of the engine were operating at or beyond the CHT and EGT
indicated to the pilot.

The company practice of leaning to peak EGT exceeded the pilot's
operating handbook recommendation of a maximum of 1650 degrees F
EGT. Exceeding the maximum permissible EGT on a regular basis, and
sustained operations at or very near peak EGT at the manifold air
pressure and engine revolutions per minute set as company policy,
most probably put one or more of the operating cylinders into the
detonation regime. This would have produced the heat stress and
contributed to the loss of material strength of components evident
within this engine.