NTSB Factual Report

On January 13, 2004, approximately 1515 Pacific standard time, a Cessna 172RG, N64ML, impacted the terrain during a forced landing about five miles north of Enumclaw, Washington. The certified flight instructor and his student, a private pilot, were not injured, but the aircraft, which is owned and operated by Galvin Flying Service, sustained substantial damage. The 14 CFR Part 91 commercial maneuvers instructional flight, which departed Boeing Field, Seattle, Washington, about 30 minutes prior to the accident, was being operated in visual meteorological conditions. No flight plan had been filed.

According to the instructor pilot, after arriving in the practice area, the student completed one power-on stall sequence and was just starting a second when he heard a bang and the engine lost a significant amount of power. Soon thereafter, he (the instructor pilot) took control of the aircraft, unsuccessfully tried to get the engine to return to full power, and then, after noting that the oil pressure was at zero, ultimately decided that he would have to execute a forced landing. Since there was no suitable landing area directly below the area where the power loss occurred, he headed to the south and attempted to land on what appeared to be the only available reasonably safe terrain. In order to stretch his glide to the chosen landing area, he increased the propeller pitch and kept the gear retracted until he reached the edge of the field, and then, based upon his quick evaluation of the landing site, chose to leave the gear fully retracted. During the landing sequence, the initial touchdown was uneventful, but soon thereafter the aircraft contacted the rough and uneven terrain in a manner that led to substantial damage to the airframe.

A post-accident inspection of the engine revealed that the top crankcase mating flange was cracked in an area adjacent to the number one and number two cylinder intake valve cam followers, and that a one inch by one inch section of the case flange had broken out adjacent to the number three cylinder exhaust valve cam follower. In addition, it was noted that the number three cylinder push rod and push rod tube were bent away from the surface of the cylinder at their midpoint. There was no evidence of external impact on the subject pushrod tube, and both it and the pushrod were bent in a manner consistent with compression of the pushrod from its ends.

The crankcase was separated for inspection, and it was noted that the heads of six of the eight cam followers had fractured, and that the crankcase surface behind all but two of the follower heads was dented and gouged. The crankcase surface behind two of the heads had small chunks of fractured steel imbedded/crushed into the metal of the case. Removal of the oil pan revealed about a dozen pieces of metal that were clearly identifiable as sections of the cam follower heads. Removal and inspection of the valves from cylinder number three could not establish clear evidence of valve sticking during this event, and the initiating event leading to the fracturing of the cam followers could not be determined.

NTSB Probable Cause

A loss of engine power while maneuvering due to the failure of the heads of six of the eight cam followers. Factors include the pilot's decision to keep the gear retracted during the forced landing, and the rough/uneven terrain.