What happened
On 26 March 2010, a Cessna T206H, registration HL1094, operated by Dong Seo GNI Co. Ltd., was conducting an aerial photography mission near the Muju area in South Korea. While flying at 7,500 feet, approximately 40 nautical miles west of Daegu International Airport, a sudden leak of engine oil occurred. The escaping oil sprayed across the cockpit windshield, completely obscuring the pilots' forward visibility.
Recognizing the imminent risk of engine failure, the pilots diverted the aircraft away from the nearby mountainous terrain and headed toward Daegu. As the aircraft descended, the engine eventually lost all power at an altitude of approximately 3,000 feet. Realizing they lacked the glide distance to reach the airport, the crew declared an emergency and executed procedures for an engine-out landing. The aircraft touched down in an open field near Seongju-gun, but the nose gear caught on agricultural covers, causing the aircraft to skid and eventually capgiize. The aircraft sustained substantial damage, resulting in a hull loss, though both pilots escaped without injury.
The investigation
The ARAIB conducted a teardown inspection of the Lycoming TIO-540-AJ1A engine to determine the source of the leak. Investigators examined the area between the engine crankshaft and the propeller hub, where they discovered that the crankshaft oil seal had been displaced. The investigation also reviewed the maintenance history of the aircraft and the performance of the crew during the emergency.
Findings
- The primary cause of the engine failure was that the crankshaft oil seal was pushed out of place, allowing all engine oil to leak out rapidly.
- The engine was missing a critical retaining plate that is designed to physically support the oil seal and prevent it from being displaced by internal pressure.
- The crankshaft oil seal is not a standard inspection item between major overhaul intervals; consequently, the seal had not been specifically checked since the engine's manufacture.
- The pilots' decision to divert toward the airport was appropriate given the loss of visibility and the need to avoid mountainous terrain, despite the eventual engine stoppage.