Abnormal engine indications Pilatus Aircraft PC-12, VH-FMC, 19 km SSE Port Augusta, 20 October 1998

While climbing through 7,000 ft the engine indicating system,
low oil pressure caution light, illuminated. With the oil pressure
indication fluctuating around 90 pounds per square inch, and the
caution light flickering, the pilot in command reduced power and
elected to return to Port Augusta. During descent the warning light
continued to flicker, changes in power setting having no effect. As
the aircraft turned on to base leg, the warning light became steady
and the oil pressure dropped to less than 60 pounds per square
inch. The aircraft made an uneventful landing. Shortly after
touchdown, the oil pressure returned to 100 pounds per square inch
and the caution/warning lights extinguished.

Investigation by maintenance personnel resulted in an oil filter
change due to suspected carbon contamination. The aircraft was then
returned to service. The operator raised the recurring oil pressure
fluctuation problems with the engine manufacturer.

The Pratt & Whitney PT6 engine model installed in the PC12
has experienced episodes of oil discolouration, and carbon
particles in the oil. The discolouration was traced to the coatings
in the magnesium gearbox, and the carbon was traced to the No. 2
bearing area.

Further investigation of the fluctuating oil pressure
indications revealed a fault in the main engine wiring loom.