What happened
On May 13, 2011, a DTA Combo ultralight, registration 68-RX, was performing circuit training at the Vieux-Ferrette airfield. Approximately thirty minutes into the flight, during the initial climb following a touch-and-go maneuver, the instructor noticed a loss of engine power followed by a complete engine failure. The instructor took control of the aircraft and performed an emergency landing in a plowed field. During the taxiing phase following the touchdown, the aircraft struck a fence, resulting in significant damage to the ultralight. The student pilot sustained minor injuries.
The investigation
Investigators examined the aircraft's Rotax 582 UL two-stroke engine and identified overheating on the connecting rod head of cylinder number one. Within the carburetor bowl for that cylinder, investigators discovered rubber residues. The investigation established that these residues likely clogged the carburetor jet, leading to an improper fuel-oil mixture. This lean condition caused the overheating and subsequent seizure of the connecting rod.
Further inquiry revealed that five weeks prior to the accident, the owner had noticed irregular engine performance. At that time, an inspection of the fuel system had uncovered significant rubber debris originating from the deterioration of the primer bulb. Although the owner replaced the hoses and the primer bulb and cleaned the fuel lines using compressed air, debris remained in the system.
Findings
- The primary cause of the engine failure was the failure to implement a manufacturer service bulletin.
- The owner had not applied Service Bulletin (BS) No. 14-2010, issued by the manufacturer in July 2010, which warned that alcohols in unleaded fuels could cause the primer bulb to dry out and degrade. The bulletin recommended replacing the bulb every 300 hours or three years.
- Because a general engine overhaul performed one month prior had shown no visible anomalies, the owner did not deem the bulletin necessary to follow. The problematic bulb had been installed in 2005 and had reached approximately 450 hours of use.
- While the fuel system was cleaned following the discovery of debris, the cleaning process was insufficient to remove all rubber particles from the circuit.
- The presence of ethanol in automotive unleaded fuels (such as E10 or E95) was identified as a contributing factor to the degradation of the rubber components.