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3. Turn and Bank Indicator
a) The turn needle indicates both direction and rate of turn. When the needle is left of centre, with the ball centred, the helicopter is turning left in co-ordinated flight. If the needle is right of centre, ball centred, the helicopter is turning right in co-ordinated flight. There is usually a mark on the face of the instrument to show the correct needle position for a rate one turn. Cyclic is used to initiate, maintain, and recover from turns. In turbulent conditions needle oscillations must be averaged to detect a turn.
b) The ball functions as a result of gravity and centrifugal force. The needle and  ball are interpreted together, the ball indicating if the helicopter is slipping or skidding. A slip is indicated when the ball is off centre toward the inside of the turn, and a skid if the ball is off centre toward the outside of the turn. If the ball is off centre it should be adjusted by pedal. A simple rule to remember is to step on the ball, ie. if the ball is off to the left press left pedal to centre it and vice versa. The ball helps in achieving correct co-ordination. Using the cyclic and pedals correctly in relation to each other realize accurate co-ordination in helicopters.
4. Turn Co-ordinator
This instrument displays the movement of the helicopter on the roll and yaw axis.  It should be clearly understood that only the rate of roll and rate of turn are shown, not the actual bank angle.
TRIM
The inherent instability of helicopters requires the pilot to maintain the helicopter in trim as accurately as possible to minimize fatigue and reduce pilot workload. This is achieved by continual cross checking of the instruments using a radial scan. The yaw axis is the unstable in a single rotor helicopter and demands special attention from the pilot.
THE RADIAL SCAN
Performance must change before the performance instruments can react. The pilot must interpret any lag in instrument indications. If the performance instruments indicate that the heading or altitude is not as required, attention is directed to the control instruments while adjusting attitude and power sufficiently to make a correction. The pilot then confirms that  the required correction is in progress by referring again to the performance instruments.
While flying with reference to instruments, the pilot continuously extracts information from the performance instruments, and makes control adjustments as required while referring to the control instruments. In extracting this information, the pilot focuses long enough on each instrument to read it. A split second is often long enough to extract the information and then the scan moves on to the next instrument. The attitude indicator is the central instrument. It becomes the hub of the scan so that the pilot’s attention usually returns to it after looking at a particular performance instrument. A diagram depicting the path of the pilot’s viewing pattern would show lines between the attitude indicator to the other instruments, just as spokes radiate from the hub of a wheel. This pattern of viewing the instruments is called a radial scan.
As part of using a radial scan, the pilot frequently scans those instruments that provide the information most needed, given the task at hand. The other instruments are scanned less frequently for backup information. A radial scan that uses particular instruments to get the information necessary to carry out a particular task is called the selective radial scan. For example, when the task is to fly straight and level, the most important performance instruments are the heading indicator and the altimeter. Therefore, the pilot would use  the selective radial scan to focus on these two instruments more frequently. As shown in  Fig. 30-2, the viewing pattern moves from the attitude indicator to the heading indicator,  back to the attitude indicator, and from there to the altimeter.
Of the remaining performance instruments, the airspeed indicator and the vertical speed indicator give information to confirm the attitude; the turn co-ordinator and magnetic compass confirm direction. The selective radial scan uses the same, but less frequent, pattern for these instruments as it used for the altimeter and heading indicator.
　
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