a signal relayed to the heading indicator unit, which is also  remotely mounted. This signal operates a torque motor in  the heading indicator unit that processes the gyro unit until  it is aligned with the transmitter signal. The magnetic slaving  transmitter is connected electrically to the HSI.  There are a number of designs of the remote indicating  compass; therefore, only the basic features of the system are  covered here. Instrument pilots must become familiar with  the characteristics of the equipment in their aircraft.  As instrument panels become more crowded and the pilot’s  available scan time is reduced by a heavier flight deck  workload, instrument manufacturers have worked toward  combining instruments. One good example of this is the  RMI in  Figure 8-30.  The compass card is driven by signals  from the flux valve, and the two pointers are driven by an  automatic direction finder (ADF) and a very high frequency  (VHF) omni-directional radio range (VOR).  Heading indicators that do not have this automatic  northseeking capability are called “free” gyros and require  periodic adjustment. It is important to check the indications  frequently (approximately every 15 minutes) and reset the  heading indicator to align it with the magnetic compass  when required. Adjust the heading indicator to the magnetic  compass heading when the aircraft is straight and level at a  constant speed to avoid compass errors.  The bank and pitch limits of the heading indicator vary  with the particular design and make of instrument. On some  heading indicators found in light aircraft, the limits are  approximately 55° of pitch and 55° of bank. When either of  these attitude limits is exceeded, the instrument “tumbles”  or “spills” and no longer gives the correct indication until  reset. After spilling, it may be reset with the caging knob.  Many of the modern instruments used are designed in such  a manner so that they do not tumble.  An additional precession error may occur due to a gyro not  spinning fast enough to maintain its alignment. When the  vacuum system stops producing adequate suction to maintain  the gyro speed, the heading indicator and the attitude indicator  gyros begin to slow down. As they slow, they become more  susceptible to deflection from the plane of rotation. Some  aircraft have warning lights to indicate that a low vacuum  situation has occurred. Other aircraft may have only a vacuum  gauge that indicates the suction.  As the gyro spools up, make sure there are no abnormal  sounds. While taxiing, the instrument should indicate turns in  the correct direction, and precession should be normal. At idle  power settings, the gyroscopic instruments using the vacuum  system might not be up to operating speeds and precession  might occur more rapidly than during flight.  The purpose of an AOA indicator is to give the pilot better  situational awareness pertaining to the aerodynamic health  of the airfoil. This can also be referred to as stall margin  awareness. More simply explained, it is the margin that exists  between the current AOA that the airfoil is operating at, and  the AOA at which the airfoil will stall (critical AOA).  Speed by itself is not a reliable parameter to avoid a stall.  An airplane can stall at any speed. Angle of attack is a better  parameter to use to avoid a stall. For a given configuration,  the airplane always stalls at the same AOA, referred to as  the critical AOA. This critical AOA does not change with:  An AOA indicator can have several benefits when installed in  General Aviation aircraft, not the least of which is increased  situational awareness. Without an AOA indicator, the AOA  is “invisible” to pilots. These devices measure several  parameters simultaneously and determine the current AOA  providing a visual image to the pilot of the current AOA along  with representations of the proximity to the critical AOA.  [Figure 8-31]  These devices can give a visual representation  of the energy management state of the airplane. The energy  Figure 8-30.  Driven by signals from a flux valve, the compass card  in this RMI indicates the heading of the aircraft opposite the upper  center index mark. The green pointer is driven by the ADF.  8-22  Instrument Check  Angle of Attack Indicators  •  Weight  •  Bank Angle  •  Temperature  •  Density Altitude  •  Center of Gravity 