What happened
On 28 August 2011, a Cessna U206E, registration SE-FMU, was conducting a parachute drop mission near Kumla, Sweden. During the ascent to 1,000 meters, the pilot experienced a loud bang followed by smoke filling the cabin and a significant loss of engine thrust. Following the pilot's instructions, five parachutists immediately exited the aircraft.
The pilot then performed an emergency landing in a ploughed field. While the initial landing run was successful, the aircraft nosed over after traveling approximately 30 meters, resulting in substantial damage to the engine, propeller, wings, and landing gear. The pilot sustained minor injuries and was able to exit the wreckage without assistance.
The investigation
The Swedish Accident Investigation Authority (SHK) examined the engine and the oil system to determine the cause of the power loss. The technical investigation focused on the presence of unusual deposits within the lubrication system.
Investigators discovered large quantities of lead bromide within the engine. This substance, a byproduct of Avgas 100LL combustion, had accumulated in the oil pan and eventually clogged the oil pump suction screen. This blockage interrupted the oil supply, leading to the engine seizing. The investigation also noted that the engine's inspection interval had been exceeded by 16 hours and that the aircraft lacked clear placards regarding proper fuel/air mixture settings.
Findings
- The engine seized due to a failure in the lubrication function.
- Accumulation of lead bromide in the oil system caused the suction screen to become blocked.
- The engine was operated using a fuel/air mixture richer than the flight manual recommended, which lowered combustion temperatures and allowed lead bromide to condense.
- The physical design of the oil pan and the position of the oil drain plug prevented the effective removal of these deposits during routine oil changes.
- The pilot's operational procedures for mixture settings differed from the official flight manual instructions.
Safety action
Following the investigation, recommendations were directed toward the FAA and EASA to update maintenance programs for engines with similar fuel injection systems, such as the Continental IO-520. The goal is to mandate internal inspections of the oil pan during oil changes to check for waste product deposits. Additionally, the Swedish Transport Agency was advised to remind operators of the importance of adhering to flight manual specifications for injection engines.