What happened
On April 2, 1973, a Cessna 421 B, registration D-IAFO, was conducting a private flight from Neheim-Hüsten, Germany, to Ascona, Switzerland. The flight, carrying a pilot, a co-pilot, and five passengers, initially proceeded under IFR via Milan-Malpensa before transitioning to VFR for the final approach.
Upon arrival at Ascona, the aircraft was cleared to land on runway 35, which had a light 2-knot tailwind. The crew performed all landing preparations, including the extension and verification of the landing gear and flaps. However, the aircraft touched down approximately 69 meters before the paved runway threshold. Shortly after passing the left side of the runway threshold, the left main landing gear began to collapse. The aircraft continued to travel for about 450 meters before veering 40 degrees to the left and coming to a halt just outside the left edge of the runway.
The investigation
The investigation focused on the mechanical state of the landing gear and the flight parameters during the landing phase. Investigators examined the wreckage of the D-IAFO and found that the left main landing gear had unlocked and collapsed under the aircraft's weight. The locking strut of the left gear showed multiple fractures that occurred after the initial collapse.
While the pilots had verified the gear position multiple times during the approach and the cockpit warning system remained functional, the investigation looked into the stability of the gear locking mechanism. It was noted that the design of the gear locking system was susceptible to changes in preload tension due to structural load shifts in the airframe. The investigation also reviewed the landing technique, noting that the crew had intentionally performed a short landing.
Findings
- The crew performed an intentional short landing, which was deemed inappropriate given that the dry runway was sufficiently long even with a light tailwind.
- The aircraft touched down on the unpaved surface before the runway threshold, which introduced additional risks.
- The left main landing gear locking strut failed due to insufficiently secured locking tension, likely triggered by the impact of the aircraft rolling over an uneven section of the runway edge.
- There was no evidence of pilot impairment or mechanical failure of the gear extension system prior to the landing impact.