A Canadair CL-215-1A10 firefighting airplane, registration EC-HET, was destroyed when it impacted hilly terrain during a firefighting operation close to the Lindoso reservoir near the Portuguese/Spanish border. The aircraft had departed from its base in Castelo Branco, Portugal, at 08:15 together with another CL-215. On approaching the area of the fire, the two aircraft used the Lindoso reservoir to load water. The direction of loading was southwest and the area where they were dropping to water load was on a northeast-facing slope. After dropping some loads, the Portuguese Fire Coordinator asked the crew of EC-HET to go and drop the water in an area further south. The aircraft made a turn to a heading of 100ø and entered a narrow valley in which they were stuck without being able to turn to either side, so they continued their flight up the valley. During the climb up the valley the aircraft had its flaps deployed in the 10ø position and had a tailwind of 3 kt. Prior to impact, the flaps were fully deployed. When the aircraft was 150 m away from the crest, it hit the ground with the tail, causing the tail assembly to detach. After the first impact, the aircraft touched the ground with the lower fuselage and tilted on the lower fuselage raising its rear end to a near vertical position and the cockpit impacted the ground and crushed. The commander was able to exit the aircraft under his own power and pulled the co-pilot out of the aircraft. The impact caused a fire around the aircraft which was extinguished by some of the aerial resources that were in the area and by the components of a forest fire fighting brigade that was transported by one of the helicopters on its way to the area and which was landed near the site of the accident. The co-pilot died approximately one hour after the impact, close to the crash site, while being treated by the emergency services. The Commander was transferred by helicopter to a hospital in Portugal. Causes/Contributing factors: The investigation has determined that the accident was due to an erroneous assessment of the aircraft’s ability to climb over the mountain.