Incident Overview
The pilot was returning from a cross-country flight when the engine lost power. He ditched the airplane in the Pacific Ocean approximately 1/4 mile (about 300 yards) offshore. During the outbound leg, no problems were noted with the airplane or engine. Before departure on the return leg, the engine run up revealed no anomalies, and takeoff performance appeared normal.
After establishing cruise at 4,500 feet and 2,200 rpm, the pilot leaned the mixture. Immediately after, engine rpm dropped to 1,000. The pilot attempted to diagnose the issue but was unsuccessful. The engine then stopped, and the pilot turned toward shore. During the glide, a brief restart occurred, but the engine lost power again within two minutes, never exceeding 1,000 rpm. The pilot informed air traffic control that he would not reach the shore and ditched the airplane.
Emergency Response
The pilot and passengers exited the airplane before it sank in 70 feet of water. They stood on top of the airplane for about a minute before it submerged. Local authorities rescued the pilot and recovered one victim; one victim was not recovered and is presumed deceased.
Investigation Findings
Examination of the wreckage revealed no evidence of airframe or engine anomalies unrelated to the magneto. Investigators could not establish magneto timing or obtain spark. Disassembly of the magneto showed that the impulse coupling spring was fractured and separated at two places. After reassembly without the impulse coupling, one magneto fired on all four leads; the other fired intermittently on three leads due to salt water corrosion on the contacts.
Metallurgical examination of the fractured spring surfaces revealed micro features of corrosion, consistent with stress corrosion cracking. Optical and scanning electron microscopy identified extensive corrosion and corrosion pitting, particularly on the edges and flat faces of the spring. A broken spring could retard magneto timing to top dead center or after, whereas the specified timing is 25 degrees before top dead center.
The engine logbooks indicated compliance with Airworthiness Directive 96-12-07, which mandates magneto inspections. No other evidence of a contributing anomaly was found in the airframe or engine.
Wreckage Examination
The examination focused on the magneto assembly due to the reported loss of power. The fractured impulse coupling spring and corrosion observed on the spring surfaces were the only abnormalities identified that could explain the engine's failure.