Background
A certified flight instructor (CFI) and a second pilot were conducting an airplane checkout flight. The CFI authorized the second pilot, who was handling the flight controls, to land on a gravel road. Wind conditions at the time were calm.
Mishap Sequence
After touchdown, as the airplane decelerated during the landing rollout, the second pilot failed to maintain the airplane's alignment with the road. The right main landing gear wheel rolled over the road edge, causing a loss of directional control. The CFI attempted corrective action by applying rudder pressure, but the second pilot had already applied rudder input. By the time the CFI responded to correct the airplane's course, it had already drifted toward the edge of the road. The CFI then attempted to regain control by increasing engine power, but found that the second pilot had already opened the throttle. The airplane subsequently veered off the road, entered a ditch, and nosed over.