What happened
On December 21, 2019, a privately owned Diamond Aircraft HK36R Super Dimona, registration JA36HK, departed from runway 14 at Matsuyama Airport, Ehime Prefecture. Shortly after takeoff, as the aircraft entered the climb phase approximately 50 feet above the runway, the engine power suddenly dropped and the aircraft began experiencing periodic vibrations.
The pilot attempted to activate the fuel pump to restore power, but the malfunction persisted. Due to the degraded performance, the pilot initiated a 180-degree left turn to return to the airport. During the circling maneuver, air traffic controllers recognized the emergency and cleared the aircraft for landing on runway 32. However, as the aircraft's altitude became insufficient to reach the runway, the pilot performed an emergency landing on a parallel taxiway. The impact was heavy, causing damage to the tail wheel and the left main wing tip. No injuries were reported to the pilot or the passenger.
The investigation
The Japan Transport Safety Board (JTSB) examined the engine, fuel system, and carburetor components. Investigation of the fuel showed no contamination, and spark plug inspections revealed no ignition-related failures.
Upon inspecting the engine's float-type carburetors, investigators discovered that one float in the left carburetor had sunk in the fuel. X-ray CT scans and microscopic analysis of this abnormal float revealed that the internal plastic insert was warped in an S-shape and featured cracks around the upper hole. Furthermore, a white substance, identified through chemical analysis as an adhesive containing carbon and oxygen, was found adhered to the float's liquid level bar and the insert holes. This substance was not part of any authorized maintenance procedure.
Findings
- The engine malfunction was caused by an inconsistent fuel supply from the left carburetor.
- The faulty motion of the float prevented the needle valve from regulating fuel levels correctly as the aircraft's attitude changed during climb.
- The deformation and cracking of the float insert, likely caused by improper handling or maintenance, restricted the smooth vertical movement of the float.
- The presence of an unauthorized adhesive on the float was likely an attempt to correct a previously noticed imbalance in float sinking during maintenance.
- The engine power and vibrations returned to normal once the aircraft was stationary, suggesting the malfunction was intermittent and triggered by the aircraft's pitch changes.