What happened
On the evening of May 25, 2009, a Schleicher ASK-13 glider, registration PH-664, was performing a local training flight at Midden-Zeeland Airport (EHMZ). The aircraft was occupied by a student and an instructor. During the final approach leg of the landing circuit, the glider entered a right-hand spin at an altitude of approximately 50 meters. The aircraft struck the ground with its nose and right wingtip, subsequently flipping over due to the wind. The accident resulted in one fatality (the student) and one serious injury (the instructor). The aircraft was destroyed.
The investigation
The investigation examined the meteorological conditions, the flight sequence, and the decision-making processes of the flight crew. At the time, the glider club was conducting its final operations for the day, intending to move remaining aircraft to the hangars before a forecasted storm arrived. Investigators reviewed KNMI weather data, which indicated a sudden and significant shift in wind direction and intensity. While the wind had been easterly, it shifted to the west and increased sharply to approximately 28 knots with gusts up to 36 knots and moderate to heavy turbulence just before the impact. Witnesses and flight personnel provided statements regarding the visibility of the approaching storm front and the wind conditions observed from the ground.
Findings
Technical analysis determined that the asymmetric stall of the right wing was the direct cause of the spin. This stall was triggered by a low airspeed during the turn onto the final approach leg, exacerbated by the sudden onset of strong tailwinds and turbulence. The investigation identified several contributing factors:
- The flight crew underestimated the speed and intensity of the approaching storm front and the accompanying change in wind direction.
- The decision to perform additional winch launches while a thunderstorm was approaching created unfavorable flying conditions.
- The instructor failed to adjust the landing circuit to account for the new wind direction, resulting in a landing approach performed with a significant tailwind.
- The rapid shift from an easterly wind to a 25-knot westerly wind significantly reduced the margin for error during the low-altitude maneuver.