NTSB Identification: LAX08LA009
14 CFR Part 91: General Aviation
Accident occurred Wednesday, October 10, 2007 in Prescott, AZ
Probable Cause Approval Date: 12/24/2008
Aircraft: Cessna 172L, registration: N9897G
Injuries: 1 Minor.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
While in cruise flight, the engine had a sudden partial loss of power and made "clanging" noises. Although the engine continued to operate at a maximum of 900-1,100 revolutions per minute (rpm), insufficient power was available to maintain altitude. The pilot contacted the tower of a nearby airport, but determined that he would not be able to reach the runway. Due to power lines and a blind curve on a paved road, the pilot opted to land adjacent to the road. The airplane impacted a ditch and came to rest inverted. The engine had been overhauled 2.0 flight hours before the accident. A field overhaul had been performed on the engine due to a bent pushrod, stuck valve, and broken lifter for the number 3 cylinder. During the field overhaul, all four of the rocker arms had been magnetic particle inspected. Examination of the engine revealed that the number 3 cylinder exhaust rocker arm had separated into two pieces. A metallurgical examination was performed on the rocker arms by a Safety Board metallurgist. The number 3 exhaust rocker arm had fractured around the central pivot hole due to fatigue. The fatigue originated at the outer surface of the oil hole. The corners of the hole were sharp and had not been deburred, which increases the likelihood of fatigue cracking. The outer surface of the fatigued area was 0.31 inches long on the left side of the oil hole, and 0.21 inches long on the right side. The fractured exhaust rocker arm had been magnetic particle inspected for cracks during the field overhaul. The Safety Board metallurgist also noted staining on the cracks from the solution used during the magnetic particle inspection. Based on the staining, the cracks would have been detectable in size (about 0.2 inches) at the time of the inspection.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: 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. Full narrative available
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