Incident Summary
While cruising at 6,500 feet, a private pilot with 404 hours of flight experience noticed a mist forming on the windscreen. Deciding to return to the departure airport, the pilot initiated a descent. During the descent, the oil pressure warning light illuminated and the oil temperature gauge indicated a very high reading. The pilot then elected to shut down the engine. Recognizing that the airport was out of reach, the pilot searched for a suitable landing field and attempted to declare an emergency. After selecting a field and turning toward it, the pilot observed power lines in the flight path and restarted the engine to avoid them. The engine responded with very little effect, but the pilot cleared the power lines and landed in the field. During landing, the nose landing gear collapsed and the airplane nosed over, coming to rest inverted.
Engine Examination
Post-accident examination of the engine revealed that the oil pressure switch (part number 83278) had failed internally, which allowed engine oil to vent overboard. No other pre-impact anomalies were noted with the engine. At the time of recovery, the engine oil sump contained approximately six quarts of engine oil.
Pilot Actions
The pilot, upon noticing the mist on the windscreen, chose to return to the airport. After the oil pressure light and high temperature indication, the pilot shut down the engine. The restart was performed specifically to avoid power lines, and although the engine responded weakly, it provided sufficient thrust to clear the obstacles.
Aircraft Manual Guidance
The Cessna 172R Skyhawk Information Manual dated December 2, 1996, page 3-20, states: "If a total loss of oil pressure is accompanied by a rise in oil temperature, there is good reason to suspect an engine failure is imminent. Reduce engine power immediately and select a suitable forced landing field. Use only the minimum power required to reach the desired touchdown spot."