The fan is actually the first stage of compression as well as the major thrust producer. Because the fan assembly constitutes the largest rotating mass in the engine, an out of balance condition will most likely result in noise and vibration. Out of balance conditions can be caused by a variety of situations. Remember that the original assembly optimized the placement of each blade. If foreign object damage necessitates the change of a blade, the balance condition may change drastically. Blades are normally, but not always, changed in matched pairs. The damaged blade is removed along with its symmetrically opposing blade and the two are replaced with the matched pair. Blade erosion, FOD damage and repair, spinner rotation, and normal wear can also change the condition of the balance. In any of these cases, the balance of the fan will change. To top off the problem, the fan rotates at speeds that resonate in the cabin. Dynamic Fan Trim balancing is the technic that provides extremely accurate solutions for reducing the out of balance condition to an acceptable level.