France

The collision with the ground was due to the deliberate and planned action of the copilot who decided to commit suicide while alone in the cockpit. The process for medical certification of pilots, in particular self-reporting in case of decrease in medical fitness between two periodic medical evaluations, did not succeed in preventing the copilot, who was experiencing mental disorder with psychotic symptoms, from exercising the privilege of his licence. The following factors may have contributed to the failure of this principle:
-The copilot’s probable fear of losing his ability to fly as a professional pilot if he had reported his decrease in medical fitness to an AME,
-The potential financial consequences generated by the lack of specific insurance covering the risks of loss of income in case of unfitness to fly,
- The lack of clear guidelines in German regulations on when a threat to public safety outweighs the requirements of medical confidentiality.
Security requirements led to cockpit doors designed to resist forcible intrusion by unauthorized persons. This made it impossible to enter the flight compartment before the aircraft impacted the terrain in the French Alps.

Due to the absence of any flight data recorder, investigations were unable to determine with certainty the exact cause and circumstances of the accident.

The accident was the consequence of the decision of the pilot to continue an unstabilized approach, resulting in a hard landing and the rupture of the left main gear upon touchdown.

During the flare the captain detected that the landing would be hard and in an emergency action, he quickly pulled the nose up without announcing his intention to the first officer who was the pilot flying. This lack of coordination within the flight crew caused a double controls and successive and opposite actions on the flight controls during the bouncing management.

Investigation did not reveal any technical element that could have contributed to the accident. However, considering the fact that the aircraft was totally destroyed upon impact, it was not possible to carry out all the examinations generally carried out on a wreck. It is possible the aircraft was flying in moderate icing conditions. Investigation could not determine if the deicing systems were activated. However, analysis of the flight path shows that the cruising speed was stable until the descent, which tends to indicate an absence of icing of the aircraft in normal cruise. A rapid and heavy icing of the aircraft during the descent making the aircraft to be difficult to control seems unlikely given the icing conditions predicted by Météo France. Investigations were unable to determine the reasons for the loss of control. Maybe it occurred during an unusual situation or any failure. Whatever the reasons, the lack of experience of the pilot on TBM-700, especially in the absence of visual references, may increase his workload beyond his capabilities, not allowing him to regain control of the aircraft. Once the loss of control occurred, given the weather conditions, it is very likely that the pilot did not recover any visual references until the collision with the ground.

The n°2 propeller blade root on the left engine failed due to fatigue, resulting in separation from the propeller hub and then interaction with the n°1 blade and its disconnection from the propeller hub. The imbalance created by the loss of these two blades led to the front part of the engine being torn off. The cause of the fatigue cracking could not be determined with certainty. The following may have contributed to the fatigue fracture of the propeller blade root:
- Insufficient preloading of the propeller, increasing the stress exerted on it. The lack of maintenance documentation made it impossible to determine the preload values of the bearings during the last general overhaul;
- The presence of manganese sulphide in a heavily charged area of the propeller. The presence of this sulphide may have generated a significant stress concentration factor, raising the local stress level.
The tests and research carried out as part of this investigation show that the propeller blade root is made of a steel whose microstructure and composition are not optimal for fatigue resistance. However, the uniqueness of the rupture more than 50 years after commissioning makes it unlikely that the rate of inclusions, their distribution, size, or sulphur content of the propeller is a contributing factor in the accident.

The accident probably occurred as a result of an asymmetrical flight starting from the rotation that the pilot was not able to control. As technical examinations and observations from the wreckage could not give any conclusive malfunction of the engines or systems, the initial cause is most likely an improper adjustment of the steering trim before takeoff. The poor experience of the pilot on this high powered and complex aircraft as well as the low height reached did not allow the pilot to understand and manage the situation quickly and avoid the loss of control.