What happened
On March 16, 2015, at approximately 10:43, an S-6 COYOTE II ultralight aircraft crashed into the Laonwan Lagoon within the Laonwan National Scenic Area. The flight was part of a private recreational activity organized by the Ultralight Flight Development Association of the Republic of China.
The pilot had completed an initial flight with a different passenger and was performing a second flight carrying a different passenger. Witnesses observed the aircraft flying south at an altitude of approximately 500 feet. During a left turn toward the east, the aircraft's nose pitched down, and it entered a near-vertical descent into the lagoon. Eyewitnesses noted that the engine did not fail and actually appeared to increase in power during the descent.
Rescue efforts were initiated by local lifeguards and a nearby tour boat captain. Although the crew was extracted from the wreckage, both occupants sustained fatal head injuries and were pronounced dead at the hospital.
The investigation
The investigation examined the wreckage of the S-6 COYOTE II, the engine, and the flight conditions. Investigators found no evidence of structural failure, control surface malfunction, or engine failure. The control cables, ailerons, elevators, and rudder were all found to be in normal working order. The engine, a Rotax 532, was found at full throttle, and the propeller showed signs of impact consistent with rotation.
Investigators also reviewed the aircraft's weight and balance capabilities. The aircraft had been modified with new skin and a different fuel system. While the aircraft's empty weight and maximum takeoff weight were known, there were no records of the specific weight of the fuel or the passengers for this particular flight. Furthermore, the investigation noted that the pilot's communication equipment was switched off at the time of the accident.
Findings
- The primary contributing factor was the potential for a spiral stall or stall induced by weight and balance changes.
- There was no evidence of weight and balance calculations being performed for the flight, which is critical as the passenger weight varied significantly between the first and second flights (from 60 kg to 94 kg).
- The aircraft's configuration, including the use of 11-degree flaps during a turn, likely increased the stall speed and G-load, making a stall more likely.
- The pilot failed to reduce throttle power during the descent, which is the standard procedure for recovering from a spiral stall, thereby increasing the rate of descent.
- There was a lack of documented safety briefings or signed safety regulations for the passengers on this private flight.
- The aircraft's communication systems were inactive, preventing contact with ground management.