What happened
On July 16, 2015, a student pilot was performing a solo thermal flight training mission at Krosno Airport (EPKR) using an SZD-22 Mucha Std. glider, registration SP-2316. The launch was conducted via a winch using a synthetic rope equipped with a BZ-4 release fuse.
During the final phase of the climb, at an altitude of approximately 500 meters, the glider entered a strong thermal column and encountered significant turbulence. The pilot maintained a relatively flat climb angle of approximately 45 degrees, with speeds fluctuating between 90 and 120 km/h. This speed exceeded the 85 km/h limit specified in the aircraft's operating manual for winch launches in turbulent air.
While climbing, a loud noise was heard, followed by a piece of the keel bar striking the left side of the cockpit canopy, shattering the glazing. Although the pilot was initially disoriented, they managed to stabilize the airspeed. An instructor on the ground attempted to signal a release via radio, but the pilot was initially unable to operate the release handle. The towline was eventually released by the winch operator. Despite the structural damage, the pilot confirmed control effectiveness and performed a standard circuit to land safely. There were no injuries to the pilot.
The investigation
The PKBWL examined the aircraft's maintenance records and the physical damage to the airframe. The investigation included a technical assessment by specialists in wooden glider repairs. The inspection revealed that the keel bar had undergone repairs in the past using AR glue. While the glue bond itself was strong, the wooden structure showed signs of aging and reduced strength. Specifically, the investigators found that a longitudinal member of the keel bar was not properly glued, and a section of the plywood near the landing gear box was also inadequately bonded. These defects were difficult to detect during standard inspections.
Findings
- The primary cause of the incident was the glider entering an area of strong turbulence during the final phase of the winch launch.
- The pilot maintained an excessive airspeed during the launch, exceeding the limits prescribed for turbulent conditions.
- An incorrect release fuse (BZ-4) was installed on the winch line; a BZ-2 fuse should have been used according to the aircraft's operating manual.
- An improper previous repair left a portion of the keel bar unglued, which led to the structural failure.
- The use of a higher-strength fuse (850 daN instead of the required 690 daN) contributed to the inability to release the glider promptly during the emergency.
Safety action
- A safety recommendation was issued to the manufacturer, Zakład Szybowcowy "Jeżów" Henryk Mynarski, to update the operating instructions for wooden gliders regarding the selection of release fuses, ensuring that operational limitations are strictly accounted for.