What happened
On August 18, 2011, at approximately 18:45 local time, a DTA Dynamic ultralight (identified as 14-JS) was conducting a paid scenic flight from the Colleville-Montgomery microlight platform. The pilot was accompanied by one passenger when the flight encountered heavy sea fog near the cliffs of Commes. Due to the reduced visibility, the pilot performed an unplanned landing on the plateau above the cliffs to wait for the weather to clear.
After roughly ten minutes, the fog dissipated. Observing what appeared to be a light surface wind, the pilot attempted a takeoff into the wind. Shortly after rotation, at an estimated altitude of 15 to 20 meters, the aircraft was struck by a powerful downdraft, forcing it into nearby trees. The impact resulted in one passenger seriously injured, one pilot slightly injured, and the destruction of the aircraft.
The investigation
The investigation focused on the aerodynamic conditions present at the cliffs of Commes during the accident. Investigators analyzed meteorological data from Météo France, which confirmed the presence of sea mist (stratus clouds) and significant turbulence on the leeward side of the cliffs. The data indicated a north wind of 6 to 9 knots, with gusts reaching 10 to 15 knots.
Investigators examined the specific topography of the area, noting that the wind follows the upward slope of the cliffs and accelerates at the top due to the Venturi effect. This phenomenon can create wave patterns, intense turbulence, and sudden downdrafts just dozens of meters above the ground, even when surface winds appear calm.
Findings
- The primary cause of the accident was the insufficient consideration of the specific aerodynamic phenomena that can occur on the leeward side of coastal cliffs.
- While surface winds appeared manageable, the interaction between the wind and the cliff topography generated sudden downdrafts and turbulent wave patterns.
- The investigation noted that while current regulations regarding unauthorized landings and takeoffs primarily apply to certified aircraft, had similar protocols applied to ultralights, the pilot might have sought a safety evaluation of the conditions before attempting the departure.