TM 55-1510-219-10 b.  Controls and Indicators - Pilot's HSI. (1)  Pilot's HSI (fig.  3-10). CONTROL/ FUNCTION INDICATOR Rotating heading The rotating heading dial, which dial rotates with the aircraft through- out 360 degrees, displays gyro stabilized magnetic compass in- formation.  The azimuth ring is graduated in five-degree incre- ments. Heading marker Indicates  heading  selected  by HDG knob. Lubber line This pointed fixed index mark aligns with the aircraft symbol to indicate aircraft magnetic head- ing on the compass card. Course arrow Indicates VOR or TACAN radi- al course selected by COURSE knob. HEADING flag Indicates that the heading infor- mation displayed is invalid and should not be used. COURSE Provides a digital readout of the indicator selected course. 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/TACAN radial. NAV flag The NAV flag indicates that the information derived from the se- lected navigational beacon is in- valid and should not be used. COURSE The yellow course arrow is posi- control tioned on the heading dial by the course knob to select a mag- netic bearing that coincides with the desired VOR/TACAN radial or localizer course.  Like the heading marker, the course ar- row rotates with the heading dial to provide a continuous readout of course error to the computer. When one of the radio modes is selected, the vertical command bar in the flight director will dis- play bank commands to inter- cept and maintain the selected radio source. 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. 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. 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 com- puter.  In the heading mode, the flight director vertical command bar will display the required bank commands to bring the air- craft onto and maintain the se- lected 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 from centerline. In INS position, each dot repre- sents 3.75 NM linear deviation. 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)  Pilot's VOR Switches (fig.  2-28). CONTROL FUNCTION Pilot's course Selects desired source of data for indicator selector display on pilot's HSI and input switch to autopilot flight computer. VOR I Selects data from VOR I system. VOR 2 Selects data from VOR 2 system. TACAN Selects data from TACAN sys- tem. INS Selects data from INS. 3-20