What happened
On May 23, 2006, at 21:10 local time, a SOCATA Rallye 150ST, registration EC-DDI, was performing a local training flight at Alicante Airport. The flight, operated by Air Sureste, was being conducted by a pilot instructor with a student under supervision. The flight consisted of various maneuvers, including simulated engine failures and landing circuits.
During the landing on runway 10, the aircraft was in a clean configuration (flaps retracted). The pilot reported a touchdown speed of approximately 140 km/h, which exceeded the manufacturer's recommended touchdown speed of 100 km/h. As the aircraft decelerated and the nose wheel made contact with the runway, significant shimmy oscillations began to develop. The pilot attempted to alleviate pressure on the nose gear by pulling back on the controls, but after hearing two distinct impacts, the pilot took control of the aircraft. The nose gear assembly subsequently detached from the strut, causing the nose of the aircraft to drag along the runway until it came to a stop. The two occupants of the aircraft were uninjured.
The investigation
The CIAIAC investigation focused on the structural failure of the nose gear assembly and the history of the aircraft's maintenance. Investigators examined the wreckage and found that the nose wheel and its attachment arm had separated from the strut. The investigation revealed that the break occurred at the section of the bolt hole in the nose gear arm stem.
Laboratory analysis of the broken component showed that the stem had been modified with an undocumented 14.7 mm diameter insert placed inside a 15 mm diameter hole. This modification was not recorded in the aircraft's official documentation. Furthermore, investigators found evidence of progressive wear and ovalization of the bolt hole, likely caused by torsional movement between the bolt and the stem during shimmy events. The investigation also noted that the nose tire had existing damage that could have contributed to the instability.
Findings
- The landing was performed at a speed of 140 km/h, significantly higher than the 100 km/h limit specified in the flight manual.
- The aircraft experienced intense shimmy oscillations, a phenomenon that the pilot noted had occurred frequently during previous landings but had not been reported to maintenance.
- The primary cause of the failure was the breakage of the nose gear arm stem head due to an overload caused by severe shimmy oscillations.
- The shimmy was exacerbated by the high landing speed, an unbalanced nose wheel, and existing looseness in the nose gear assembly.
- An undocumented and unreferenced structural modification—the installation of a metal insert within the stem—likely weakened the component and accelerated the development of the shimmy phenomenon.