Summary
On 25 April 2012 at 13:02 local time, a Cessna Reims F337F Super Skymaker, registration OY-BYZ, was completing an ornithological survey flight over Estonian coastal waters. After landing on runway 03 at Pärnu Airport, the pilot began a left turn onto taxiway C at a speed of 13–15 knots. During the turn, the pilot felt a sudden intense vibration from the nose wheel. The aircraft became uncontrollable and veered to the right, leaving the paved taxiway surface. The nose wheel and right main wheel sank into soft ground. The aircraft continued forward and struck the concrete foundation of a stop bar light, fracturing the nose wheel rearward. The forward momentum caused the aircraft to pitch down, damaging the forward propeller and the composite gondola under the fuselage. The pilot and three bird counters on board were not injured.
Aircraft Information
The aircraft was a 1970 Cessna Reims F337F Super Skymaker, twin-engine, tandem-seat, retractable landing gear, serial number F3370025, registration OY-BYZ. Its maximum takeoff weight was 1,996 kg. The last maintenance was performed on 20 April 2012, 12.5 flight hours before the incident. The nose wheel shimmy damper was inspected during that maintenance by checking resistance when moving the nose wheel with the aircraft on jacks.
Sequence of Events
After landing, the pilot initiated a left turn onto taxiway C at approximately 13–15 knots. The pilot initially overshot the centerline slightly and corrected with increased nose wheel steering. Halfway through the turn, a strong vibration started in the nose wheel. Pulling back on the control yoke to unload the nose wheel did not stop the vibration. Brakes were not used during taxi. The vibration rendered the aircraft unsteerable despite firm rudder pedal inputs. The aircraft departed the asphalt to the right, about 3 meters from the taxiway edge. Later braking proved insufficient to stop the aircraft, and it struck the concrete foundation of the stop bar light. Although the light fixture was designed with a frangible coupling to minimize damage, the nose wheel had sunk approximately 15 cm into soft ground and hit the light's embedded concrete base.
Investigation
The Estonian Safety Investigation Bureau (Ohutusjuurdluse Keskus) conducted the investigation with participation from the Danish Accident Investigation Board (Havarikommissionen). The nose wheel landing gear assembly, steering, and retraction mechanisms were removed for examination. The shimmy damper was externally undamaged and moved freely. Upon disassembly, it contained no hydraulic fluid, but approximately 3–4 cm² of condensed water was present. The piston seals and forward end cap seals were in good condition. The inner seal on the rear end cap showed wear, with a flattened contact surface against the piston rod. The investigators concluded that hydraulic fluid had leaked past this worn seal over time, allowing air and moisture to enter the damper, rendering it ineffective.
Findings
The investigation found that the pilot held valid licenses and medical certificate. No piloting errors were identified. The aircraft's nose wheel shimmy damper was technically unserviceable due to a worn seal that allowed hydraulic fluid to escape. Otherwise, the aircraft was airworthy. The last maintenance of the nose wheel, performed 12.5 flight hours prior, did not adequately detect the fluid loss. Additionally, the stop bar lights on taxiway C were positioned less than 3 meters from the runway edge, not complying with ICAO Annex 14 requirements. The taxiway shoulder soil lacked sufficient load-bearing capacity, as it was soft and wet, contributing to the aircraft sinking and striking the light foundation.
Probable Cause
The investigation identified the cause of the serious incident as a failure of the aircraft's nose wheel shimmy damper.