What happened
On June 7, 2008, a privately operated Piper PA 22-160 Tri-Pacer, registration G-ARHP, was engaged in a cross-country flight from Saint-Etienne, France, toward Aix-les-Milles. The flight was part of a larger journey originating in Popham, United Kingdom, with stops in Orléans and Saint-Etienne, eventually intending to reach Albenga, Italy.
After departing Saint-Etienne at 14:52, the crew encountered unfavorable weather conditions. Based on a weather forecast received at Saint-Etellen, the crew altered their route to fly via Givors and follow the Rhône Valley. During the flight, the aircraft entered a rising valley where the cloud ceiling prevented further ascent. Finding themselves in a position where a turnaround was impossible, the pilot executed a forced landing in a forest of fir trees. The aircraft struck the trees at an altitude of approximately 12 meters and came to rest 10 meters further along the valley floor. The impact resulted in three injuries (the pilot and two passengers) and the destruction of the aircraft.
The investigation
The investigation focused on the crew's navigation methods and their adherence to the flight plan. The aircraft was equipped with two portable GPS units and ICAO aeronautical charts. While the crew had manually marked a route toward Givors on a 1/500,000 scale paper chart, the investigation revealed a discrepancy in how the flight was actually conducted.
Analysis of the flight path showed that after takeoff, the pilot activated a GPS unit and followed a pre-programmed route intended for a different flight segment (Saint-Etienne to Aix-les-Milles) rather than the manually plotted route on the paper chart. Shortly before the collision with the trees, the aircraft deviated from this GPS-programmed track as the pilot attempted to avoid terrain obscured by clouds.