making straight flight and precision turns to headings difficult  to accomplish, particularly in turbulent air. A heading  indicator, however, is not affected by the forces that make  the magnetic compass difficult to interpret.  [Figure 8-25]  The operation of the heading indicator depends upon the  principle of rigidity in space. The rotor turns in a vertical  plane and fixed to the rotor is a compass card. Since the rotor  remains rigid in space, the points on the card hold the same  position in space relative to the vertical plane of the gyro. The  aircraft actually rotates around the rotating gyro, not the other  way around. As the instrument case and the aircraft revolve  around the vertical axis of the gyro, the card provides clear  and accurate heading information.  Because of precession caused by friction, the heading  indicator creeps or drifts from its set position. Among  other factors, the amount of drift depends largely upon the  condition of the instrument. If the bearings are worn, dirty,  or improperly lubricated, the drift may be excessive. Another  error in the heading indicator is caused by the fact that the  gyro is oriented in space, and the Earth rotates in space at a  rate of 15° in 1 hour. Thus, discounting precession caused  by friction, the heading indicator may indicate as much as  15° error per every hour of operation.  Some heading indicators referred to as horizontal situation  indicators (HSI) receive a magnetic north reference from  a magnetic slaving transmitter and generally need no  adjustment. The magnetic slaving transmitter is called  a magnetometer.  The flux gate compass that drives slaved gyros uses the  characteristic of current induction. The flux valve is a small,  segmented ring, like the one in  Figure 8-27,  made of soft iron  that readily accepts lines of magnetic flux. An electrical coil  is wound around each of the three legs to accept the current  induced in this ring by the Earth’s magnetic field. A coil  wound around the iron spacer in the center of the frame has  400 Hz alternating current (AC) flowing through it. During  the times when this current reaches its peak, twice during  each cycle, there is so much magnetism produced by this  coil that the frame cannot accept the lines of flux from the  Earth’s field.  As mentioned earlier, the lines of flux in the Earth’s magnetic  field have two basic characteristics: a magnet aligns with  them, and an electrical current is induced, or generated, in  any wire crossed by them.  The AHRS sends attitude information to the PFD in order  to generate the pitch and bank information of the attitude  indicator. The heading information is derived from a  magnetometer that senses the earth’s lines of magnetic flux.  This information is then processed and sent out to the PFD  to generate the heading display.  [Figure 8-26]  Electronic flight displays have replaced free-spinning gyros  with solid-state laser systems that are capable of flight at  any attitude without tumbling. This capability is the result  of the development of the Attitude and Heading Reference  System (AHRS).  Figure 8-25.  A heading indicator displays headings based on a 360°  azimuth, with the final zero omitted. For example, “6” represents  060°, while “21” indicates 210°. The adjustment knob is used to  align the heading indicator with the magnetic compass.  Figure 8-26.  Attitude and heading reference system (AHRS).  8-20  Attitude and Heading Reference System (AHRS)  The Flux Gate Compass System  30 24 33  2I I5 I2 6  3  Compass card gear  Adjustment gears Gimbal  Main drive gear  Gyro  Adjustment knob  Gimbal rotation 