If the aircraft requires a excessive amount of trim to maintain level flight, it should be written up and corrected. The Structural Repair Manual provides a chapter with asymmetery checks to verify any permanent set that the structure may have taken over years of flying. Addtionally, trim bias can be built into the aircraft. This is usually accomplished on the first test flight. However, if the structure is in fact bent, it could be accomplished again. To establish the trim bias the aircarft is rigged completly prior to flight, During the flight, after cruse altitude and still air are reached, the aircraft is trimmed to fly stright and level, using roll and yaw trim. When the aircraft is back on the ground the roll and yaw trim heads are removed and the head is set back to zero and the amount of bias in the system is recorded.