On January 14, 2003, at 1733 central standard time, a Robinson R22B, N530DB, operated by W.W. Helicopters, LLC, as a rental helicopter, impacted terrain during a hover turn at Wittman Regional Airport (OSH), Oshkosh, Wisconsin. Dark night visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 instructional flight was not operating on a flight plan. The commercial pilot and a non-rated pilot received minor injuries. The flight departed from Sheboygan County Regional Airport, Sheboygan, Wisconsin, at 1700, en route to OSH. The right seat pilot stated that he was a student trying to log hours in order to obtain a private pilot certificate with a helicopter rating. He stated that they had flown from East Troy to Sheboygan and then to OSH where they were going to refuel. The left seat pilot stated that he works out of W.W. Helicopters LLC, which is a flight school. The left seat pilot stated that he was flying with the right seat pilot so that the right seat pilot could log hours in order to obtain his helicopter rating. The left seat pilot stated that they had missed a turn to a taxiway leading to a fixed base operator at the airport. They then performed a right 180-degree turn to taxi back to the taxiway. About 45-75 degrees into the turn, the turn began to accelerate rapidly. The left seat pilot attempted to regain control, but the helicopter continued to the right at a faster rate even with left pedal applied. He could not get the helicopter under control. The helicopter went back up and rolled right and struck the ground about 15 feet from the taxiway onto the grass area adjacent to it. The left seat pilot stated that the accident could have been prevented by being, "more attentive when helicopter began turn - covered controls." The left seat pilot held a commercial pilot certificate with a rotorcraft-helicopter rating and a private pilot certificate with a single engine land rating. He accumulated a total flight time of 303.9 hours in the make and model helicopter, of which 7.7 hours were in the last 90 days and 3.0 hours were in the last 30 days. The right seat pilot held an airline transport pilot certificate with an airplane multiengine land rating, CE-500 type rating, and DC-9 type rating. He held a commercial pilot certificate with airplane single engine land, airplane single engine sea, and airplane multiengine sea ratings. He held a certified flight instructor rating with airplane single engine and multiengine land ratings.