What happened
On February 1, 2006, a Cessna 172-RG, registration EC-HTL, was conducting a dual-instruction flight from Cuatro Vientos to Badajoz. The flight included an instructor and two students. During the final approach to Badajoz Airport, the crew noticed the nose gear unsafe light remained illuminated after the landing gear was deployed. To verify the gear position, the crew performed a low-altitude pass, during which air traffic control confirmed that the nose gear had failed to extend.
Despite following the established procedures for a gear extension failure, the nose gear remained retracted. The crew subsequently declared an emergency, prompting the mobilization of airport emergency services. The aircraft performed an emergency landing, which resulted in significant damage to the engine, propeller, and the lower front section of the fuselage. The three occupants of the aircraft were uninjured and evacuated the plane without incident.
The investigation
Investigators examined the wreckage at the site and analyzed maintenance records. The inspection revealed that as the nose gear actuator began to move, an element of the extension mechanism became hooked on one of the hinges of the right gear door. This caused the hinge to deform, forcing the door into the wheel well and physically blocking the nose gear from deploying.
Furthermore, the inspection found that the gear doors were heavily deformed from repeated impacts against the lower fuselage skin. These impacts appeared to have occurred during normal operations because the clearance between the doors and the fuselage was not being maintained according to the service manual. While the aircraft had been maintained according to its authorized program, investigators noted that the maintenance manual lacked specific dimensional tolerances for the mechanism components, providing only general functional guidelines.
Laboratory testing was conducted on the two springs responsible for regulating the door opening. Although they appeared in good condition, the springs were found to be 6 mm longer than the nominal design and possessed an elastic constant of only 0.11 Nw/mm, which was roughly 20% of the required design specification. This indicated that the installed springs did not match the specifications in the aircraft's component catalog.
Findings
- The primary cause of the incident was the blockage of the nose gear doors caused by a combination of malfunctioning opening springs and excessive wear within the extension and retraction assembly.
- The installed springs had significantly reduced performance characteristics, leading to a slower door opening process that contributed to the failure.
- The gear doors were weakened by excessive wear and repeated impacts against the fuselage, which facilitated the mechanical interference that blocked the gear.
- The maintenance manual did not explicitly require the verification of the springs' physical properties during standard inspections, and the springs had not been replaced during recent maintenance cycles.