What happened
On 14 April 2004, a Cessna F 177 RG, registration LN-ALK, was performing a private flight from Norway to Malmö Sturup Airport in Sweden. As the pilot prepared for arrival, they attempted to extend the landing gear, but the undercarriage failed to deploy as expected. The pilot attempted to use the manual hand pump to force the gear down, but the pump eventually failed to build any significant hydraulic pressure.
After receiving visual confirmation from the tower that the main wheels appeared to be down, the pilot proceeded with the landing. While the main wheels made contact with the runway, the nose gear failed to remain locked and retracted upon impact. This caused the aircraft to transition onto its nose, dragging along the runway and onto the adjacent grass. The impact resulted in limited damage to the propeller, the lower portion of the nose, and the nose gear door assemblies.
The investigation
Investigators examined the hydraulic system of the Cessna F 177 RG, which relies on an electric pump and hydraulic fluid to operate the landing gear. The investigation revealed that the hydraulic fluid reservoir was completely empty following the accident.
Upon inspecting the aircraft in a hangar, investigators identified significant abrasion on the hydraulic line serving the left down-lock actuator. This line had been worn down by contact with the gear mechanism of the right-hand undercarriage leg. Furthermore, the bracket intended to support this hydraulic line was found to be faulty, allowing the line to shift between two different positions: one safe position and one position where it rested directly against moving mechanical parts.
Findings
- The primary cause of the accident was a hydraulic line becoming displaced during recent maintenance work.
- The displacement of the line caused it to rub against the undercarriage gear, leading to a leak that eventually emptied the hydraulic reservoir.
- Because the system lacked sufficient fluid, there was enough pressure to lock the main gear via the down-lock actuators, but insufficient pressure to engage the nose wheel drag link into its locked position.
- The faulty support bracket allowed the hydraulic line to move into the path of the moving gear assembly.