The rate of turn indicator measures the rate of yaw (in deg/sec) of the aircraft around its vertical axis. The yaw axis is the vertical axis of the aircraft in level flight and the instrument will indicate a rate of turn only when this axis is fully vertical. However, the yaw axis is not vertical during a banked turn, it is tilted at an angle equal to the bank angle. Therefore, the instrument has been calibrated to indicate a correct turn rate at a chosen rate of turn (usually a low rate such as for a rate one turn), for accurate turns. When the needle indicates a rate one turn (3°/sec), it does not necessarily mean that the aircraft’s yaw rate is also 3°/sec.