AIRSPEED INDICATOR BANK CONTROL ATTITUDE INDICATOR 12-9 Figure 12-13. The flight instruments used for bank control are the attitude, heading, and turn indicators. The attitude indicator gives a direct indication of the bank attitude of the helicopter. For instrument flight, The bank attitude of a helicopter is the angular relation of its lateral axis and the natural horizon. To maintain a straight course in visual flight, you must keep the  lateral axis of the helicopter level with the natural hori- zon. Assuming the helicopter is in coordinated flight, any deviation from a laterally level attitude produces a turn. [Figure 12-13] Occasionally, the vertical speed indicator may be slightly out of calibration. This could result in the instrument indicating a slight climb or descent even when the helicopter is in level flight. If it cannot be readjusted properly, this error must be taken into con- sideration when using the vertical speed indicator for pitch control. For example, if the vertical speed indica- tor showed a descent of 100 f.p.m. when the helicopter was in level flight, you would have to use that indica- tion as level flight. Any deviation from that reading would indicate a change in attitude. The airspeed indicator gives an indirect indication of helicopter pitch attitude. With a given power setting and pitch attitude, the airspeed remains constant. If the airspeed increases, the nose is too low and should be raised. If the airspeed decreases, the nose is too high and should be lowered. A rapid change in airspeed indi- cates a large change in pitch attitude, and a slow change in airspeed indicates a small change in pitch attitude. There is very little lag in the indications of the airspeed indicator. If, while making attitude changes, you notice some lag between control application and change of airspeed, it is most likely due to cyclic control lag. Generally, a departure from the desired airspeed, due to an inadvertent pitch attitude change, also results in a change in altitude. For example, an increase in airspeed due to a low pitch attitude results in a decrease in alti- tude. A correction in the pitch attitude regains both air- speed and altitude.  The initial movement of the vertical speed needle is instantaneous and indicates the trend of the vertical movement of the helicopter. It must be realized that a period of time is necessary for the vertical speed indicator to reach its maximum point of deflection after a correction has been made. This time element is commonly referred to as “lag.” The proportional to the speed and magnitude of the pitch change. If you employ smooth control techniques and make small adjustments in pitch attitude, lag is minimized, and the vertical speed indicator is easy to interpret. Overcontrolling can be minimized by first neutralizing the controls and allowing the pitch attitude to stabilize; then readjusting the pitch atti- tude by noting the indications of the other pitch instruments. vertical speed indicator immediately, the results will show on the altimeter as a gain or loss of altitude. BANK CONTR BAN K CO NTROL OL