TM 55-1510-219-10 the course knob to select a magnetic     bearing     that     coincides with the desired VOR radial or localizer course.  Like the heading marker, the course arrow rotates with the heading dial to provide a continuous readout of course error to the computer.  When one of the NAV modes is selected, the vertical command bar in the flight director will display bank commands to intercept and maintain the selected radio source. Course deviation This bar represents the center- bar line of the selected VOR or lo- calizer course.  The aircraft sym- bol shows pictorially, actual aircraft position in relation to this selected course. Aircraft symbol A  fixed aircraft symbol  corre- sponds to the longitudinal axis of the aircraft and lubber line markings.  The symbol shows air- craft position and heading with respect to a radio course and the rotating dial. TO-FROM Two to-from pointers are situat- pointers ed 180 degrees apart.  The one which is visible points toward the station along the selected VOR radial. HDG control The  heading  marker  is posi- tioned on the rotating dial by the heading knob and displays preselected compass heading. The heading marker rotates with the heading dial so the differ- ence between the marker and the fore lubber line index is the amount of heading error applied to the flight director computer. In the heading mode, the flight director vertical command bar will display the required bank commands to bring the aircraft onto and maintain the selected heading. Course deviation In VOR operation, each dot rep- dots. resents  five  degrees  deviation from centerline.  In ILS opera- tion, each dot represents one de- gree deviation centerline.  In INS operation, each dot represents 3.75 NM deviation from center- line. GS flag Indicates that the information displayed by the glideslope pointer is invalid and should not be used. Glideslope Indicates deviation from correct pointer glideslope during ILS approach. Azimuth marks Azimuth marks are fixed at 45° bearings throughout 360 degrees of compass card for quick refer- ence. (2)  Copilot's VOR Switches (fig.  2-28). Copilot's Course Selects desired source of data for Indicator display on copilot's HSI and in- Selector Switch put to autopilot. VOR 1 Selects data from VOR 1 system. VOR 2 Selects data from VOR 2 system. TACAN Selects data from TACAN  sys- tem. INS Selects data from INS. Copilot's Selects desired source of magnet- compass No.  1/ ic heading data for display on No.  2 switch HSI compass card. No.  1 Selects data from No.  1 com- pass. No.  2 Selects data from No.  2 com- pass. 3-19. HORIZON REFERENCE INDICATOR. a. Description.      The   horizon   reference   indicator (HRI)  (fig.      3-12)   is   the   pilot's   basic   attitude   horizon indicator  and  the  attitude  direction  instrument  for  the automatic flight control system. b.  Controls and Indicators. CONTROL FUNCTION Crossed needles Display computed commands to autopilot. Lateral deviation Displays localizer deviation in- indicator formation from VOR  receiver. Vertical Displays glideslope deviation in- deviation formation from VOR  receiver. indicator Bank angle Indicates aircraft bank angle. pointer Bank angle index Reference indicating zero-degree bank. Bank angle scale Allows measurement of aircraft bank angle from zero to 60 de- grees. 3-22