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.
As a result of investigations into this and several other related occurrences, the Australian Transport Safety Bureau issued the following recommendation on 9 November 2000 to the Civil Aviation Safety Authority: [ R20000250 ]
The Australian Transport Safety Bureau recommends that the Civil Aviation Safety Authority alert operators of aircraft equipped with turbo-charged engines to the potential risks of engine damage associated with detonation, and encourage the adoption of conservative fuel mixture leaning practices.
|Date:||10 August 2000||Investigation status:||Completed|
|Time:||0615 hours WST|
|Location:||111 km NE Port Hedland, (NDB)|
|State:||Western Australia||Occurrence type:||Abnormal engine indications|
|Release date:||22 November 2000||Occurrence class:||Technical|
|Report status:||Final||Occurrence category:||Incident|
|Highest injury level:||None|
|Aircraft manufacturer||Piper Aircraft Corp|
|Type of operation||Charter|
|Damage to aircraft||Nil|
|Departure point||Port Hedland, WA|
|Departure time||0555 hours WST|
|Role||Class of licence||Hours on type||Hours total|