What happened
On November 14, 2015, a prototype ultralight aircraft, identified as HKW-01, departed from Strasbourg-Entzheim airport to conduct a flight test. Approximately six minutes after takeoff, while flying at an altitude of roughly 1,000 feet, the pilot reduced engine power. This reduction in power led to an aerodynamic stall. In an attempt to recover, the pilot applied full power and pushed the control stick forward. However, the aircraft entered a right-hand spin. The pilot attempted to counter the rotation using left aileron and rudder inputs, followed by a rapid back pressure on the stick, but the aircraft continued in a spin until it struck the ground. The accident resulted in one fatality and the destruction of the aircraft.
The investigation
The investigation focused on the flight dynamics and the pilot's recovery attempts, aided by onboard video footage. The aircraft was equipped with two cameras: one inside the cockpit to record instrument readings and pilot actions, and another mounted on the wingtip to monitor control surface positions and airflow via tufts of yarn attached to the wing and fuselage. The aircraft, a metal-construction HKW ultralight powered by a Rotax 503 engine, was in a pre-production testing phase. Investigators noted that while the aircraft featured an emergency parachute, it was not equipped with a stall warning system.
Findings
Investigators determined that the stall occurred shortly after the power reduction. While the initial forward stick input briefly helped reduce the angle of attack, the immediate application of full engine power induced a rotation around the roll axis, leading to the spin. The pilot's subsequent recovery inputs—specifically the left rudder and aileron movements—created differential drag that sustained the spin. Furthermore, the rapid application of full back elevator pressure contributed to the loss of control. The investigation concluded that the pilot's actions may have been influenced by the high-stress environment of the low-altitude encounter. The report emphasized that maintaining sufficient altitude during technical flight tests is critical to providing the margin necessary to recover control or deploy a parachute.