Incident Overview
An airplane departed the taxiway and came to rest in a concrete drainage ditch during night taxi operations. The pilot had maneuvered the airplane onto the taxiway after landing, operating under the assumption that the turnoff required a 90-degree turn.
Pilot's Awareness
The pilot reported being aware that the landing/taxi light on the airplane was inoperative prior to the incident. He stated that he flew the airplane after dark despite this known deficiency. Additionally, the pilot indicated there were no other mechanical anomalies with the airplane.
Sequence of Events
After landing, the pilot taxied without functional landing lights. While negotiating the taxiway, the airplane departed the paved surface and entered a concrete drainage ditch, where it came to rest. The pilot's assumption regarding the turn geometry was a contributing factor in the event.