What happened
On 1 December 2004, a Cessna 560 XL, registration SE-DYX, was performing a commercial taxi flight from Stockholm-Bromma to Plovdiv, Bulgaria, and back to Sweden. During the initial leg of the trip, the crew encountered an extremely bumpy runway in Plovdiv, characterized by concrete blocks. While no immediate damage was visible after that landing, the crew noted the severe vibrations.
On the return flight to Stockholm, the crew attempted to minimize impact by using a static takeoff technique. However, after takeoff, the aircraft's cockpit indicators signaled that the nose gear had failed to retract and that the stabilizer trim was malfunctioning. Despite attempting various landing gear extension procedures, including the use of the emergency air pressure system, the green indication light for the nose gear remained unlit.
During the approach to Stockholm-Arlanda, the crew declared a MAYDAY. As the aircraft touched down on runway 26, the nose gear failed to support the aircraft's weight. At approximately 50 knots, the nose dropped, and the aircraft slid on its underside for about 200 meters before coming to a halt. All six occupants survived the incident.
The investigation
The Swedish Accident Investigation Board (SHK) examined the aircraft's mechanical components and flight data. The investigation focused on the nose gear retraction mechanism and the impact of the previous landing in Bulgaria. Technical testing of the dismounted nose gear revealed a nitrogen gas leak caused by a defective o-ring in the lower landing gear strut. Investigators determined that the intense vibrations and stress experienced on the rough runway in Plovdiv likely caused the pressure loss in the strut.
Findings
- The primary cause of the nose gear collapse was the failure of the nose gear strut to spring out during extension, which caused the gear to become stuck between the hinge arms of the landing gear doors.
- The underlying cause of the mechanical failure was a defective o-ring in the lower strut, which led to nitrogen leakage.
- The severe vibrations and physical stress from operating on a rough, uneven runway in Plovdiv contributed to the pressure loss in the nose gear.
- An unsuccessful design of the nose gear retraction mechanism on this aircraft type acted as a contributing factor to the gear becoming stuck.
Safety action
Following the investigation, the SHK recommended that aviation authorities implement better classifications for runway surface quality and smoothness. Additionally, the board suggested that information regarding the specific surface covering of runways should be included as standard in landing charts within Route Manuals.