Pilot stalls Pilatus B4 in gusty thermal turn

An experienced glider pilot was fatally injured when a Pilatus B4 stalled in a gusty thermal turn and dove into a mountain slope; airframe destroyed. The pilot — ~685 h total, ~72 h on type but only 1 h 16 in the last 90 days, last B4 flight over two years earlier — was thermaling as the lowest of three gliders ~100 m above the ridge in gusty westerlies and moderate thermals. Witnesses saw the glider drop vertically; it struck the slope at ~60° nose-down. No technical faults; mass and CG in limits. Autopsy showed an acute brain inflammation that could have impaired control — incapacitation could not be ruled out.

1. Thermalling as low ship in gaggle: On a local practice flight from a Saanen gliding camp, the pilot was thermalling as the lowest of three gliders ~100 m above a ridge restaurant in gusty westerlies and moderate thermals.
2. Long lapse since last B4 flight: ~685 h total and ~72 h on the Pilatus B4 lifetime, but the last B4 flight was 1 h 20 over two years earlier; recent camp flying had been on a DG-300 high-performance glider.
3. Possible brainstem incapacitation: Autopsy showed acute brainstem inflammation that could have caused circulatory or respiratory disturbance; incapacitation just before the stall could not be ruled out.
4. Speed bleed in thermal turn: Possibly distracted by watching the two other gliders or affected medically, the pilot allowed airspeed to drop below the B4's ~61 km/h stall speed in the gusty air.
5. Stall and near-vertical dive: The glider dropped abruptly from the gaggle in a near-vertical attitude; the AFM expects ~80 m altitude loss if recovery is immediate, but no recovery action was apparent.
6. Slope impact — fatal: The glider struck the mountain slope ~50 m below the restaurant in a ~60° nose-down attitude, destroying the airframe. Accident was not survivable.