What happened
On November 19, 2023, at approximately 11:28 JST, a Diamond Aircraft HK36R Super Dimona, registration JA36HK, was performing a takeoff from Hanyu Glider Port in Saitama Prefecture. The flight was intended to tow another glider, but the pilot aborted the initial takeoff attempt after noticing a drop in engine rotation speed and insufficient airspeed.
Following the aborted takeoff, the pilot conducted a brief engine run-up, which showed no immediate anomalies. Believing the engine was functional, the pilot attempted a second takeoff. However, once the aircraft reached an altitude of approximately 10 meters, the engine power dropped abruptly. The pilot attempted a left turn to return to the runway, but as the altitude decreased, a forced landing on the grass beside the runway became necessary. During the maneuver, the left wingtip struck the ground, and the aircraft' and tail section sustained significant damage during the landing.
The investigation
The JTSB investigation focused on the fuel system and maintenance history of the aircraft. Investigators examined the fuel filter and the electric fuel pump, discovering that the fuel supply was obstructed by foreign objects, including metal powder and fibers. The investigation also revealed that the fuel pressure warning light was non-functional because the fuel pressure sensor had been removed due to incompatible wiring.
Furthermore, the investigation found that a critical orifice used to regulate fuel pressure had been removed and replaced with a non-genuine fitting. Regarding the source of the contamination, investigators noted that the pilot used unwoven fabric to cover the refueling nozzle and stored fuel in a corroded metal can. While maintenance records indicated that regular filter inspections had been performed, the investigation concluded that the filter had not actually been cleaned or inspected as required by the maintenance manual.
Findings
- The primary cause of the engine power loss was the clogging of the fuel filter by foreign objects, specifically fibers from the refueling nozzle cover and metal particles from a corroded fuel canister.
- The pilot failed to perform the required ground inspections mandated by the engine maintenance manual after the initial engine anomaly was detected during the first takeoff attempt.
- The fuel pressure warning light was inoperative because the fuel pressure sensor had been disconnected and removed, preventing the pilot from detecting the drop in fuel pressure.
- The fuel system was operating under improper pressure due to the unauthorized removal of a fuel manifold orifice and the use of non-standard fittings.
- Maintenance discrepancies existed, as the 100-hour inspection requirements for the fuel filter were not being physically executed despite being recorded as completed.