G S HDG  NA V 33  30  24  2I  I5  I2  6  3  Heading select bug  Compass warning flag  Heading select knob  NAV warning flag  Lubber line  Course select knob  Course deviation bar (CDI)  Course deviation scale  Course select pointer  Compass card  Symbolic aircraft  Glideslope deviation scale Dual glideslope pointers TO/FROM indicator  Figure 3-37. HSI  Figure 5-38. Horizontal situation indicator (HSI). Figure 5-37. Air data computer (Collins).  pressure, and temperature to calculate very precise altitude,  IAS, TAS, and air temperature. The ADC outputs this  information in a digital format that can be used by a variety  of aircraft systems including an EFIS. Modern ADCs are  small solid-state units. Increasingly, aircraft systems such as  autopilots, pressurization, and FMS utilize ADC information  for normal operations.  NOTE: In most modern general aviation systems, both the  AHRS and ADC are integrated within the electronic displays  themselves thereby reducing the number of units, reducing  weight, and providing simplification for installation resulting  in reduced costs. The HSI is a direction indicator that uses the output from  a flux valve to drive the dial, which acts as the compass  card. This instrument, shown in  Figure 5-38 , combines the  magnetic compass with navigation signals and a glideslope.  This gives the pilot an indication of the location of the aircraft  with relationship to the chosen course. In  Figure 5-38 , the aircraft heading displayed on the rotating  azimuth card under the upper lubber line is North or 360°.  The course-indicating arrowhead shown is set to 020; the  tail indicates the reciprocal, 200°. The course deviation bar  operates with a VOR/Localizer (VOR/LOC) navigation  receiver to indicate left or right deviations from the course  selected with the course-indicating arrow, operating in the  same manner that the angular movement of a conventional  VOR/LOC needle indicates deviation from course. The desired course is selected by rotating the course-indicating  arrow in relation to the azimuth card by means of the course  select knob. This gives the pilot a pictorial presentation: the  fixed aircraft symbol and course deviation bar display the  A Flight Director System (FDS) combines many instruments  into one display that provides an easily interpreted  understanding of the aircraft’s flightpath. The computed  solution furnishes the steering commands necessary to obtain  and hold a desired path.  Advances in attitude instrumentation combine the gyro  horizon with other instruments such as the HSI, thereby  reducing the number of separate instruments to which the  pilot must devote attention. The attitude direction indicator  (ADI) is an example of such technological advancement.  A flight director incorporates the ADI within its system,  which is further explained below (Flight Director System).  However, an ADI need not have command cues; however,  it is normally equipped with this feature. The glideslope deviation pointer indicates the relation of  the aircraft to the glideslope. When the pointer is below the  center position, the aircraft is above the glideslope, and an  increased rate of descent is required. In most installations,  the azimuth card is a remote indicating compass driven by  a fluxgate; however, in few installations where a fluxgate is  not installed, or in emergency operation, the heading must  be checked against the magnetic compass occasionally and  reset with the course select knob. aircraft relative to the selected course, as though the pilot were  above the aircraft looking down. The TO/FROM indicator is  a triangular pointer. When the indicator points to the head of  the course arrow, it shows that the course selected, if properly  intercepted and flown, takes the aircraft to the selected facility.  When the indicator points to the tail of the course arrow, it  shows that the course selected, if properly intercepted and  flown, takes the aircraft directly away from the selected facility. Analog Pictorial Displays Horizontal Situation Indicator (HSI)  Attitude Direction Indicator (ADI)  Flight Director System (FDS) 5-23