What happened
On June 4, 2006, at approximately 09:30 UTC, a Cosmos, Phase III ultralight aircraft, registration 54 AEK, crashed in a valley within the Serra do Marão mountain range, near Ansiães, Portugal. The aircraft was participating in a cross-country flight event organized by a French association, traveling from Espinho to Vila Real.
While flying at an altitude of approximately 5,000 feet, the pilot encountered severe turbulence caused by strong easterly winds (exceeding 55 km/h) interacting with the complex mountain terrain. The pilot, who had limited experience in mountain flying, was unable to maintain control of the aircraft as it entered a zone of atmospheric instability. The aircraft struck the ground in a deep valley at an altitude of roughly 2,100 feet, causing the death of the pilot and the total destruction of the aircraft.
During the rescue attempt, a spark ignited fuel vapors that had accumulated from ruptured tanks. This explosion triggered a massive forest fire that consumed approximately 280 hectares of woodland and destroyed all remaining aircraft wreckage.
The investigation
The GPIAAF investigation was significantly hindered by the intense forest fire, which prevented investigators from examining the wreckage or recovering specific components. The investigation relied heavily on witness testimony from a forest ranger and emergency responders.
Investigators noted that the aircraft lacked a transponder, meaning its position could not be monitored by air traffic control radar. Furthermore, the investigation faced difficulties obtaining technical documentation for the aircraft and engine from the French owners, as cooperation from the event organizers was minimal. The investigation focused on the orographic effects of the Serra do $\text{Marão}$ mountains and the meteorological conditions present at the time of the accident.