Incident Overview
During the landing rollout on a gravel runway, the airplane encountered a deep water erosion depression in the runway's surface. Subsequently, the pilot lost directional control. The airplane veered off the runway, collided with a rock, and nosed over.
Runway Condition
The gravel runway was not maintained and had an uneven washboard-like surface. The depression was caused by water erosion.
Pilot Experience
During the previous 90 days, the pilot had landed at the airstrip approximately six times without experiencing any mishap.
Sequence of Events
The aircraft was landing on the gravel strip. During the rollout, it encountered a depression. The pilot then lost directional control, leading to the aircraft veering off the runway. The aircraft struck a rock and nosed over.