What happened
On June 14, 2020, a Schempp-Hirth Ventus B glider, registration OY-HXB, was undergoing a winch launch at Hammer Airfield (EKHM) when it experienced a sudden loss of lift. During the launch, the aircraft's acceleration was insufficient, leading to a prolonged ground run at low speeds. As the glider reached a low altitude, it encountered turbulent wind conditions that caused the aircraft to bounce and subsequently roll heavily to the left. At approximately 10 meters above the ground, the pilot released the winch cable, but the aircraft continued to roll up to 60 degrees before impacting the runway. The impact caused the tail boom to break and resulted in structural damage to the forward fuselage and instrument panel. There were no fatalities and no injuries to the pilot.
The investigation
The investigation examined the winch operation, the aircraft's configuration, and meteorological conditions. Investigators noted that the winch operator, who had limited proficiency in Danish, may have misunderstood the pilot's radio instructions regarding acceleration profiles. The investigation also reviewed the aircraft's weight and balance, noting the presence of 50 kg of water ballast. Analysis of flight data from the onboard FLARM unit revealed a low acceleration rate, reaching only 57 km/h during the initial phase of the launch. Furthermore, the investigation looked into the impact of wind gusts and the aerodynamic characteristics of the glider at low speeds.
Findings
Several contributing factors were identified in the sequence of events:
- A language barrier between the pilot and the winch operator likely led to a misunderable instruction regarding the desired acceleration.
- The winch operator's limited training level may have affected the precision of the winch control.
- The aircraft experienced a lack of sufficient acceleration, resulting in a takeoff at a speed too low for stable control.
- Highly variable wind components and gusts from shifting directions likely caused the glider to lose lift unexpectedly.
- The glider may have suffered from lateral imbalance, as water ballast may not have been evenly distributed in the wings prior to takeoff.
- The pilot did not release the winch cable early enough to prevent the loss of control.
- The specific aerodynamic design of the Schempp-Hirth Ventus B provides reduced aileron effectiveness at low airspeeds, making recovery difficult.