What happened
On July 13, 2009, at approximately 11:15 AM, a Cosmos Chronos 16 pendular microlight, identified as 25-OB, was conducting a dual instruction flight near the Montbéliard aerodrome. The aircraft, operated by a private owner, took off from an unpaved runway. Witnesses on the ground observed that shortly after takeoff, the aircraft gained altitude, descended, and struck the runway surface before lifting off again.
At an altitude of roughly 30 meters, the aircraft began oscillating violently from side to side before performing a 18/0-degree left turn while descending. During this maneuver, the aircraft struck trees and ultimately crashed into the shallow waters of the Doubs river. The accident resulted in one fatality (the student pilot) and one injury (the instructor). The aircraft was destroyed in the crash.
The investigation
Investigators examined the structural integrity of the aircraft, specifically focusing on the connection between the rear mast and the base tube, as well as the seat roll bar. The examination revealed that the rear portion of the seat roll bar had fractured in two locations near its internal tubular reinforcement. The fracture surfaces showed characteristics of fatigue cracking that had developed prior to the accident.
Furthermore, the investigators found that the bores of the base tube and the triangular attachments had become ovalized. This condition indicated that the fastening bolts had been loosening over the course of several previous flights. The specific bolts involved in the failure were not recovered from the wreckage. The investigation also noted that while the instructor performed his own maintenance, the aircraft's maintenance manual required bolts to be replaced every 200 hours or every three years; however, no parts had been replaced on the carriage since the aircraft was purchased in 2001.
Findings
- The primary cause of the accident was a structural failure of the seat roll bar and the loss of mast foot fasteners during takeoff.
- Insufficient maintenance led to the failure to detect progressive fatigue cracking in the roll bar.
- The ovalization of the attachment points suggests that the fastening bolts had been loose for multiple flight cycles, contributing to the loss of structural integrity.
- The failure of these components likely caused the separation of the carriage elements, leading to the uncontrollable flight attitude observed by the instructor.