What happened
On October 16, 2011, an Issoire Aviation APM20 (registration F-GRRH) was performing a takeoff roll on the unpaved runway 27 at Salon Eyguières aerodrome during a flight training session. As the aircraft approached rotation speed, the instructor noted an unusual noise and elected to abort the takeoff. During the deceleration, the aircraft's nose pitched forward, causing the propeller to strike the ground. The aircraft came to a halt on the runway with significant damage to the airframe.
Upon inspection, the crew discovered that the nose gear strut had fractured at the attachment points for the shock absorber. This structural failure resulted in damage to both the fuselage and the horizontal stabilizer.
The investigation
The investigation focused on the structural integrity of the nose gear and the maintenance history of the aircraft. The club operating the aircraft utilized the APM20 primarily for instruction on unpaved surfaces, which subjects the landing gear to high stress. The specific nose gear on F-GRRH had been replaced in July 2011 following a previous hard landing and had accumulated approximately 150 hours and 700 cycles since installation.
Investigators noted that instructors had reported vibrations following takeoff for about a month prior to the accident. While maintenance personnel had checked the gear components for play, no significant anomalies were found, aside from slight ovalization of the tire. Fractographic examination of the broken strut revealed that the failure was caused by fatigue cracking originating from the four bores used to secure the shock absorber bolts. Although the edges of these bores had been smoothed, the cracks propagated through the material.
Findings
- The nose gear strut failed due to a fatigue cracking process.
- The fatigue cracks initiated at the bores of the shock absorber attachment points.
- The existing inspection protocols and the edge-smoothing procedures mandated by a 2009 service bulletin were insufficient to prevent this failure.
- The design of the nose gear was inadequate for the specific operational stresses encountered by the club, particularly when operating on unpaved runways.