What happened
On February 7, 1972, a Cessna 421, registration HB-LFQ, was performing a commercial IFR flight from Zurich to Samedan. The flight included the pilot and two passengers, one of whom was an experienced aviator observing from the right seat. After transitioning to VFR flight rules over the Albulapass, the pilot configured the aircraft for landing on runway 21, confirming that all three green landing gear indicator lights were illuminated.
The aircraft touched down approximately 200 meters past the runway threshold. Shortly after the throttles were brought to idle, the landing gear warning horn began to sound intermittently. The pilot noted that the green indicator light for the right main landing gear was flickering before eventually extinguishing. Approximately 400 meters into the rollout, the right main gear collapsed. The pilot responded by feathering both propellers, shutting down the engines, and using asymmetric braking and full left aileron to keep the aircraft on the runway and prevent the right propeller from striking the ground. The aircraft came to a stop about 1000 meters from the threshold without catching fire.
The investigation
The SUST investigation focused on the mechanical state of the landing gear and the circumstances of the touchdown. Investigators examined the damaged HB-LFQ and found that the right main landing gear had unlocked and collapsed under load. While the main strut itself was intact, the locking linkage showed significant damage caused by forceful impact while in the collapsed position. No signs of fatigue, corrosion, or cracks were found in the components.
The investigation also noted that the aircraft had undergone a 1200-hour inspection just one week prior, which included a revision of the landing gear actuation and locking system. The weather at the time of the accident was clear with moderate visibility and no precipitation.
Findings
- The pilot's actions during the emergency were appropriate and effective in preventing further damage.
- The landing itself was not hard, but rather a normal, positive touchdown.
- The primary cause was the insufficient locking of the right main gear's main strut, which led to the subsequent failure of the locking linkage.
- It was observed that the landing gear actuation and locking system is prone to changes in its preload settings due to load shifts within the airframe, particularly at the wing.
- Investigators concluded that a load shift during the landing likely caused the right main gear locking mechanism to unintentionally disengage.