NTSB Factual Report

HISTORY OF FLIGHT

On October 10, 2007, at 1014 mountain standard time (MST), a Cessna 172L, N9897G, experienced a partial loss of engine power and made a forced landing near Williamson Valley Road at mile marker 6, near Prescott, Arizona. The pilot/owner operated the airplane under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The airplane sustained substantial damage to the fuselage. The pilot, the sole occupant, received minor injuries. Visual meteorological conditions prevailed, and no flight plan had been filed. The flight departed at 0900 from Phoenix Deer Valley Airport (DVT), Phoenix, Arizona, with a planned destination of Ernest A. Love Field Airport (PRC), Prescott.

The pilot reported that he was about 9 miles west of Prescott, in cruise flight, when the engine suddenly lost power and made "clanging noises." The engine would not run higher than 900-1,100 revolutions per minute (rpm). The pilot tried increasing the mixture, checked the fuel, and added carburetor heat. He turned the airplane towards PRC, and contacted the tower. After passing Granite Mountain, he encountered gusty wind conditions and decided he was not going to make it to the airport. He attempted to land in a horse pasture, but the wind was too strong. The pilot did not want to land on a paved road due to a blind curve and power lines. He instead landed beside the road, where the airplane struck a ditch and nosed over. The airplane came to rest inverted and "bent in half" across the fuselage. The pilot did not have memory of the touchdown or his exit from the airplane. The pilot recommended that he should have spent more time trying to prepare for a landing and less time trying to get the engine to operate during this occurrence.

TESTS AND RESEARCH

Visual examination of the engine by investigators revealed that the number 3 cylinder exhaust rocker arm had separated into two pieces. No other anomalies were found during the engine inspection.

A National Transportation Safety Board Materials Laboratory specialist examined all four exhaust rocker arms from the engine. The metallurgist reported that one arm was fractured at two locations through the central pivot hole. The inboard fracture went through the radial oiling hole. The outboard fracture was approximately diametrically opposed to the oiling hole. The outboard fracture had features typical of bending overstress.

The metallurgist noted that the inboard fracture was relatively flat and smooth. Both sides of the oiling hole displayed curved crack arrest marking indicative of fatigue progression. The crack arrest lines and radial fracture markings indicated that fatigue cracking initiated at opposite corners of the intersection of the oil hole and the outside surface. The fatigue progressed radially from these points through the wall of the pivot area to the bushing surface and toward the edges, but the fatigue regions did not reach the edges of the arm. The metallurgist measured the outer surface left fatigue region to be about 0.31 inches long and the outer surface right fatigue region to be 0.21 inches long. Both fatigue regions were stained and slightly darkened from the origins partially though to the propagation area. The remaining fracture surface displayed characteristics typical of overstress separation.

The metallurgist observed the corners of the oil hole at the fatigue origins to be sharp, and there were no indications of a radius or chamfer. They noted a raised lip or burr on the exterior edge around the oil hole. No evidence of deburring or hole edge machining was visible. The three intact rocker arms showed similar hole details at the outer surface.

AIRCRAFT INFORMATION

The pilot/owner reported that at the time of the accident, the 1971 single-engine Cessna 172L had a total time of 3,670 hours. The pilot reported that he had flown the airplane for about 2 hours since the field overhaul of the engine. The reason for the overhaul was due to a bent pushrod, stuck valve, and broken lifter for the number 3 cylinder.

The aircraft logbook entry for September 12, 2007, indicated that an annual inspection had been completed. An aircraft total time of 3,660.0 hours was recorded at the annual inspection. The engine logbook entry dated September 11, 2007, indicated that all connecting rods and rocker arms had been inspected, rebushed, overhauled, and certified by One Stop Aviation.

NTSB Probable Cause

The failure of maintenance personnel to detect fatigue cracks in the number 3 exhaust rocker arm, which lead to a partial loss of engine power.