What happened
On April 11, 1971, a Cessna 421, registration HB-LDV, departed Geneva for a non-scheduled commercial flight to Zurich. The flight was conducted under IFR conditions with a pilot and one passenger on board. The landing at Zurich Airport proceeded normally, with the crew confirming the landing gear was properly extended and locked.
After the main gear touched down and the aircraft had traveled approximately 300 meters, the nose wheel was lowered. Shortly thereafter, the pilot experienced intense vibrations, which he initially suspected were caused by a flat nose tire. In an attempt to alleviate the pressure on the nose gear, the pilot tried to keep the nose elevated and notified ground control of the suspected issue while attempting to taxi onto taxiway 2a. However, the vibrations intensified, rendering the aircraft uncontrollable. The nose gear subsequently collapsed, causing the aircraft to skid for approximately 20 meters before coming to a halt near the intersection of taxiway 2a.
The investigation
The investigation focused on the mechanical integrity of the landing gear and the condition of the tire. The aircraft was found to be airworthy and properly maintained, with the landing gear settings having been adjusted five months prior to the event. The pilot was a highly experienced professional with no recent history of medical or operational issues.
Physical examination of the wreckage revealed that the nose gear linkage had suffered a forceful break due to bending stress. While the nose tire itself appeared undamaged, the inner tube was found in small fragments. Investigators ruled out pre-existing material fatigue or previous damage to the gear components. Additionally, no evidence of nose gear shimmy was detected.
Findings
- The primary cause of the accident was the release of the nose gear locking mechanism following landing.
- This release was triggered by excessive vibrations, which were likely initiated by a loss of pressure in the nose tire tube.
- Although the tire appeared intact, the destruction of the inner tube suggested a period of operation with a defect.
- The investigation concluded that the vibrations, potentially exacerbated by uneven runway surface joints, were sufficient to dislodge the nose gear lock, especially if the preload tension was insufficient.
- The pilot's attempt to relieve pressure on the nose gear was ineffective because the elevator becomes less responsive at the lower speeds encountered during the landing roll.