Factual Information
On Monday, 3 July 2006, at 18:00 UTC, a Cessna 402B aircraft, registration EC-ERH, operated by Aerofan, was involved in an incident at Cuatro Vientos Airport in Madrid. The aircraft had departed at approximately 17:30 UTC for a local test flight carrying three persons: a pilot, a copilot, and a mechanic. The estimated flight duration was one and a half hours. After departure, the crew proceeded west of the airport to perform various maneuvers, including changes in flight regime and configurations. Thirty minutes into the flight, they contacted the tower to request touch-and-go landings, which were authorized.
After entering the traffic pattern, the crew conducted the necessary checks and verified the green indicator lights for landing gear down and locked on at least two occasions. They performed a smooth landing and began the takeoff roll, raising the flaps and reapplying takeoff power. At that moment, the pilot noticed the aircraft veering to the right and the right wing lowering. The pilot immediately cut the engines and warned the occupants of an impending runway excursion. Unable to control the aircraft's path, it turned right and came to a stop approximately halfway down the runway and about 15 meters off the right edge.
Once the aircraft was stopped and secured with engines off, magnetos and battery off, and fuel shutoff valves closed, the crew notified the tower of their evacuation. Firefighters arrived within a minute, and later the aircraft was moved about 50 meters away from the runway.
Injuries and Damage
The three occupants were uninjured. The aircraft sustained minor damage. The right main landing gear strut was collapsed. The lower right fuselage area was damaged, primarily affecting the gear door. The landing gear lock link was broken. The right propeller and right flap were also damaged. Additionally, the surfaces of the right aileron and the right horizontal stabilizer were deformed.
Investigation Findings
Visual inspection of the aircraft revealed that the bellcrank pivot bolt (bellcrank pivot bolt) securing the bellcrank to the trunnion had fractured into three pieces: the head and a small portion were missing, the central part remained lodged in the trunnion bushing, and the threaded end with the nut was recovered. The bellcrank itself had one of its two lugs broken at the connection to the overcenter mechanism via the adjusting screw.
Laboratory analysis of the bolt indicated that the failure was due to ductile overload, primarily from shear. The chemical composition corresponded to NAS 464 series bolts with a specified tensile strength of 160-180 KSI. Hardness tests (Rockwell-C and Vickers) gave a value of 195 KSI, exceeding the specified range, but prior deformation during the failure process may have increased the hardness from an initial value within the specification. The deformation direction on both ends of the fractured bolt indicated that the instantaneous load that caused the failure was a tensile force approximately along the direction of the bellcrank's reinforcement rib, applied at the junction of the adjusting screw and the side brace.
Additional investigation on an identical landing gear system from a Cessna A-310 showed that clearances and wear in the actuation mechanism were present but unlikely to cause disengagement of the overcenter lock. The procedure for adjusting the overcenter is straightforward: after releasing the side brace to rest on the stop, the length of the articulated bar attached to the lower half-brace is adjusted. The adjustment is considered correct when the mechanism returns to the locked position solely by gravity after being pushed out of lock (by pushing the lower bellcrank arm and the adjusting screw bar toward the fuselage). This unlocking operation allows the strut to rotate up to 6 inches toward the fuselage without moving the push-pull tube. The lower articulated arm of the bellcrank may become preloaded in compression if the adjustment forces it longer, or may have slack if forced shorter, allowing some oscillatory movement of the side brace.