Accident Details
On July 31, 2017, about 1230 Pacific daylight time, a Cessna 172S, registration N957TA, was substantially damaged in an accident approximately 5 nautical miles northwest of Seattle International Airport (SEA), Seattle, Washington. The flight instructor and student pilot receiving instruction sustained minor injuries. The airplane was operating as a Title 14 Code of Federal Regulations Part 91 instructional flight.
According to the flight instructor, after completing air work, they were returning to Boeing Field/King County International Airport (BFI) when the engine began running rough. During the descent, the engine roughened further and lost rpm. Shortly thereafter, the low oil pressure light illuminated, and within 10 to 15 seconds the engine quit. When the pilot attempted to restart, the propeller would not move. He elected to ditch the airplane in the water near the shoreline, resulting in deformation of the engine mount. The following day, the airplane was recovered to a secured facility for examination.
Examination Findings
A Federal Aviation Administration (FAA) aviation safety inspector present during the recovery observed that when the engine cowling was removed, the top bolt of the right magneto was loose and had backed off, creating a 3/8-inch gap between the magneto and its mounting pad.
An external visual examination revealed no catastrophic damage, but the engine was seized and would not rotate. The muffler was slightly damaged. The right magneto was canted aft from its mounting pad at the top right position, with the attachment nut loose. The lower mounting nut was also loose. Four accessory case bolts were missing from the accessory case (three from the right side bottom, one from the left bottom), and one bolt on the left side was protruding halfway out. When the right magneto was removed, no damage was observed to its gear, though metal flakes were found inside the magneto and its gasket was torn. The left magneto was tight in its mount, but metal flakes were detected on its gears.
Further accessory removal revealed metal flakes in and around them. No stress signatures were found on the threads of the backing-out right magneto bolt or on any other bolts. During removal of the accessory case bolts, more than half were found not tight. The manufacturer's maintenance manual requires proper torque and includes star lock washers but no safety wire. No stress was observed on bolt or mount threads, and each bolt had the appropriate washer.
When the engine case was disassembled, the crankshaft was found broken in half at the middle bearing, with a large section separated and located in the case bottom. The bearing had become welded to the shaft, and heat stress signatures were evident.
A representative of the engine manufacturer opined that the right magneto coming loose and backing out allowed all oil to exit at the magneto mounting hole.
Maintenance History
Maintenance records indicated that the zero-time rebuilt engine was installed on the airplane on December 29, 2015. At the time of the accident, the engine had accumulated 917.5 hours total, with 22.4 hours since its last annual inspection on July 25, 2017. Both left and right magnetos were removed and overhauled on November 3, 2016, during a 500-hour inspection—497 hours after engine installation and 420.5 hours before the engine failure. Approximately 122 hours before the accident, the accessory case was removed to replace the oil pump body, drive, and impeller; examination revealed no anomalies with these components.