ASK 23 stalls in ridge-lift windshear and lands in treetops
A pilot new to ridge soaring the Hasenmatt in an ASK 23 dropped below the stall speed, mushed into the trees and slid to the ground; the glider was destroyed but the pilot was unhurt. In weak, turbulent ridge lift he flew figure-eights, turned 180° off the slope and straight back toward it, and airspeed collapsed to about 50 km/h. With the CG at the aft limit the glider was pitch-sensitive, and in the sheared air a 10 kt gust was enough to stall it. Unable to regain speed, he committed to the tree landing and opened the airbrakes. Factors: too little speed reserve for the turbulence, very little ridge experience, and the aft CG.
- Ridge training flight: During the second afternoon training flight after aerotow to 2,000 m, the pilot flew ridge soaring along the Hasenmatt between about 1,200 and 1,300 m, in short figure-eight patterns.
- Low ridge experience: The pilot had very limited overall gliding experience and only a brief prior ridge-flying checkout, so he was not familiar with windshear hazards in ridge soaring.
- Aft CG and turbulence: The glider was loaded with the centre of gravity aft in the allowable range, reducing stability, over ridge terrain known for turbulent airflow with only weak lift.
- Turn away from slope: While flying figure-eights along the ridge, the pilot made a 180° left turn away from the slope and then immediately turned right, during which airspeed dropped rapidly below minimum.
- Stall and height loss: The glider entered a mush/stall with airspeed reported down to about 50 km/h, losing height quickly despite the pilot applying full control inputs to turn right and away from the slope.
- Emergency landing attempt: Recognizing that he could not regain sufficient airspeed or control, the pilot committed to an emergency landing and extended the airbrakes.
- Crash - no injury: The glider touched down in treetops on the slope, slid down the trees to the ground, was destroyed, and the pilot exited uninjured.