What happened
On May 1, 1973, a pilot conducting the first training flight of the season departed from the Montricher airfield in a Ka 6 BR, registration HB-614, via aero-tow. While flying near Mont Tendre, the pilot encountered rising air but was subsequently ordered by ground control to return to the airfield due to a rapidly deteriorating weather front.
As the pilot approached the airfield, a cold front brought heavy rain, significant turbulence, and strong gusts. During the final approach for runway 21, the pilot initially deployed air brakes to manage altitude but retracted them following advice from an observer on the ground. During the short final phase, the pilot encountered wind speeds significantly higher than anticipated, alongside downdrafts. Consequently, the aircraft was unable to reach the runway threshold and struck a 4 to 5-meter-high hedge approximately 175 meters short of the runway, causing the glider to spin.
The investigation
The investigation examined the pilot's experience, the mechanical state of the Ka 6 BR, and the prevailing meteorological conditions. The pilot was a licensed glider pilot with over 161 hours of flight time. The aircraft, HB-614, was in an airworthy condition, and no mechanical failures were identified as contributing to the event.
Meteorological data revealed that a cold front had moved through the region, causing visibility to drop and wind gusts to reach between 46 and 50 knots. While a cautionary advisory had been issued by the Geneva-Cointrin meteorological service for nearby areas, this information was not communicated to the Montricher airfield, which lacks a teletype system and relies solely on telephone communications.
Findings
- The pilot's decision to retract the air brakes, based on ground advice, depleted the necessary altitude reserve needed to compensate for the wind.
- The pilot's estimation of the surface wind was accurate for average conditions but failed to account for the intensity of the sudden wind gusts.
- The presence of downdrafts and unexpected turbulence during the final approach contributed to the loss of flight path control.
- The lack of an anemometer at the airfield and the failure to disseminate the meteorological advisory to Montricher prevented the flight crew from being aware of the potential for 40-knot gusts.