What happened
On May 2, 2016, a Van's RV-4, registration LN-AAR, was performing a private flight from Agdenes to Ålesund Airport, Vigra, with one passenger on board. The pilot had selected an altitude of approximately 5,000 feet to avoid potential turbulence caused by winds from the southeast. As the aircraft approached runway 24, the pilot noted visible wave patterns on the sea, suggesting turbulent conditions.
During the approach, the pilot perceived the wind to be 180 degrees at 25 knots, which aligned with the windsock position. The pilot maintained a crab angle of 10 to 20 degrees to compensate for the crosswind. The landing itself initially appeared normal, with the aircraft bouncing slightly upon touchdown. However, during the landing roll, the aircraft began to rotate toward the left. Despite maximum right rudder application and braking, the rotation intensified. During this maneuver, the right landing gear failed, causing the right wing and propeller to strike the runway.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the structural failure of the aircraft and the environmental conditions at the time of the accident. While the pilot initially believed the wind was 25 knots, the air traffic controller reported a wind speed of 12 knots. The investigation focused on the mechanical cause of the gear collapse and the dynamics of the aircraft's rotation on the runway.
Findings
- The investigation determined that the right landing gear collapsed due to an overload of the combined engine mount and landing gear attachment.
- The rotation of the aircraft to the left created excessive lateral forces. Evidence from the runway suggests that as the rotation increased, the right main wheel became sideways to the direction of travel, placing extreme stress on the attachment point.
- While the pilot suggested that fatigue from previous landings on uneven grass surfaces might have contributed, the NSIA found no evidence of fatigue; instead, the failure was caused by the immediate overload during the ground loop.
- The investigation noted that tailwheel aircraft are inherently more difficult to control on the ground because the center of gravity is located behind the main wheels. This configuration causes any initial deviation in direction to be amplified, a phenomenon known as a ground loop.
Safety action
- The NSIA emphasized that tailwheel pilots must possess a strong theoretical understanding of how the center of gravity affects directional stability.
- The report highlights that pilots should avoid attempting to fly out of an established ground loop, as such actions can exacerbate the loss of control.