What happened
On July 11, 1965, an S-18 III glider, registration HB-353, departed from Mollis airfield via winch launch. After initially reaching an altitude of approximately 1,000 meters above the airfield, the pilot began flying toward Netstal. During the flight, the pilot noted a descent rate of 1 m/s. As the altitude dropped to 400 meters, the pilot decided to return to the airfield for landing.
During the initial approach, the pilot deployed the airbrake flaps. Upon retracting them, the aircraft experienced a significant increase in sink rate, rising from 1 m/s to as much as 5 m/s while maintaining a speed of roughly 85 km/h. Realizing the airfield was no longer reachable, the pilot attempted an emergency landing in a built-up area near the Netstal railway station. To avoid obstacles, the pilot executed a sharp left turn involving significant nose-down input. During this maneuver, the aircraft lost critical altitude and the left wing struck the flat roof of a low-rise factory building. The S-18 III then fell into the front garden of a nearby residence.
The investigation
The investigation examined the flight path, weather conditions, and the pilot's decision-making process. Investigators analyzed the aircraft's performance following the deployment of the airbrake flaps and evaluated the impact of local atmospheric conditions. The investigation also considered the altitude of the aircraft during the final maneuvers, noting discrepancies between the pilot's account and observations from other glider pilots on the ground.
Findings
- The aircraft encountered strong downdrafts during the approach, which significantly increased the sink rate.
- The pilot's decision to use airbrakes at the start of the approach contributed to a rapid loss of altitude.
- The pilot identified the need for an emergency landing too late, leaving insufficient altitude to maneuver safely over the built-up area.
- The execution of a steep turn at low airspeed and low altitude resulted in the wing strike.
- The pilot was flying at a very low speed in an attempt to extend the glide, which necessitated a heavy nose-down input that further depleted altitude.