NTSB Identification: ANC00FA056.
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Accident occurred Tuesday, May 09, 2000 in ANCHORAGE, AK
Probable Cause Approval Date: 07/10/2001
Aircraft: Beech B24R, registration: N6951R
Injuries: 2 Uninjured.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The first pilot, a certificated flight instructor, was providing flight instruction to the second pilot, a certificated private pilot. While returning to the airport, during the initial descent, the engine began to run rough, and lose power. The roughness improved momentarily, followed by a severe engine vibration, a loud bang, and complete loss of engine power. During a forced landing in a densely populated neighborhood, the airplane struck a power line, veered to the right, collided with the street, and came to rest in the front yard of a daycare center. Examination of the engine revealed the number 3 cylinder was broken free at the crankcase-mounting flange. A metallurgical examination of the number 4 position crankcase thru-bolt showed signs of fatigue cracking that emanated from the thread root. In addition, the cylinder flange mating surface displayed signs of fretting damage consistent with relative movement between the cylinder and the crankcase. The operator's director of maintenance stated that about 40 hours before the accident, he repaired a substantial oil leak between the number 3 cylinder, and the engine crankcase. He said that in the process of the repair he removed the number 4 position crankcase thru-bolt nut, applied an engine sealant, reinstalled the through bolt nut, and torqued the nut in accordance with the Lycoming overhaul manual. There was no engine log book entry pertaining to the engine total time, aircraft total time, or date when this repair was accomplished.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: A fatigue failure of the crankcase through bolt, the separation of number 3 cylinder assembly, and the inadequate maintenance/service of the airplane by company maintenance personnel. Full narrative available
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