What happened
On April 12, 2024, an Aerospool WT9 Dynamic microlight, registration 67BVN, was conducting an instructional flight near Peynier, France. The flight, operated by Aix ULM, was intended to perform handling exercises, including steep turns and stalls, before proceeding to runway circuits.
During the stall exercise, the instructor took control of the aircraft to demonstrate the maneuver to the student-pilot. As the airspeed decreased toward the stall threshold, the aircraft suddenly entered a right-hand roll. The instructor, perceiving a potential structural failure due to an audible noise from the rear of the aircraft, was unable to regain control and activated the air/frame parachute. The aircraft descended under the parachute and struck a vineyard field at approximately 15:00. Both occupants evacuated the aircraft safely, though the microlight sustained substantial damage.
The investigation
The BEA examined wreckage, flight data from the aircraft's Dynon computer, and statements from the crew. The investigation confirmed that the flight controls were intact and that the airframe parachute deployed correctly and effectively.
Data analysis showed that the angle of attack reached 76% just before the aircraft began to roll. The investigation also noted that the aircraft was operating near its maximum takeoff weight. A key technical finding was that 67BVN was not equipped with stall strips, which are aerodynamic appendages designed to improve stall behavior by ensuring the wing root stalls first.
Findings
Several factors contributed to the loss of control:
- The absence of stall strips on the aircraft, which could have mitigated the tendency of the WT9 to roll or tip during a stall.
- The inherent tendency of the WT9 model, when not equipped with stall strips, to exhibit disconcerting roll behavior during stall maneuvers.
- Possible inappropriate control inputs by the crew, such as instinctive aileron movements to counter the initial roll, which can inadvertently deepen a spin.
Safety action
Following the accident, Aerospool published a Service Bulletin recommending the installation of stall strips on WT9 models that lack them. Additionally, the flight manual for the WT9 has been amended to instruct pilots to use the airframe parachute if an unintentional spin cannot be recovered or if the spin occurs below a specific altitude.