TM 1-1510-223-10 3-50           Change 3 21.   YD   ENGAGE   push-button   switch-indicator (autopilot controller) - Depress while holding rudder boost/yaw control test switch in TEST. Yaw damper should not engage. 22.   RUDDER   BOOST/YAW   CONTROL   TEST switch - RUDDER BOOST. Check RUDDER BOOST annunciator extinguished. 23.   Electric elevator trim - Check. a.   ELEV TRIM switch - ON. b.   Pilot and copilot trim switches - Check op- eration. WARNING Operation of the electric trim system should oc- cur only by movement of pairs of switches. Any movement of the elevator trim wheel while de- pressing  only  one  switch  element  indicates  a trim  system  malfunction.  The  electric  elevator trim control switch must then be turned OFF and flight conducted by operating the elevator trim wheel manually. Do no use autopilot. c.   Pilot and copilot trim switches - Check in- dividual element for no movement of trim, then check proper operation of both ele- ments. d.   Pilot   trim   switches   -   Check   that   pilot switches  override  copilot  switches  while trimming in opposite directions, and trim moves in the direction commanded by pi- lot. e.   Pilot or copilot trim switches - Check trim disconnects while activating pilot or copi- lot trim disconnect switches. f.   ELEV TRIM switch - OFF then ON (ELEV TRIM OFF annunciator extinguishes). p.   Digital Flight Control System Modes of Operation. (1)    Heading hold mode (wings level). The basic lateral mode of the autopilot is heading hold. Heading hold mode is defined as: Autopilot engaged No lateral flight director mode selected Bank angle less than 6 degrees If the above conditions are satisfied, the autopilot will roll the aircraft to a wings level attitude. When the air- craft’s bank angle is less than 3 degrees for 10 seconds, the heading hold mode is automatically engaged. The heading hold mode is not annunciated on the EADI as it is the default lateral mode when no other lateral steering mode is selected. Autopilot engagement is annunciated on the EADI. (2)    Roll hold mode. The autopilot will recognize the roll hold mode as being operational when: No lateral flight director mode has been selected The aircraft’s bank angle is greater than 6 degrees, but less than 35 degrees The autopilot is engaged When the above conditions have been satisfied, the autopilot will maintain the desired bank angle. With the autopilot engaged, bank angles up to 35 degrees can be established using the turn knob. If touch control steering (TCS) was used to initiate the roll, bank angles up to 35 degrees can be maintained and TCS ENG will be annun- ciated on the EADI. (3)    Heading  select  mode.  The  heading  select mode is used to intercept and maintain a magnetic head- ing. The mode is selected with the HDG switch-indicator on  the  flight  director  mode  selector  panel  (fig.  3-22). Heading mode selection is annunciated by a green HDG annunciator on the EADI. The heading marker is set on the heading dial of the EHSI by means of the instrument remote controller (pedestal extension, fig. 2-12). When operating in the heading mode the flight control computer provides inputs to the pitch and roll command cue on the EADI to command a turn to the heading selected by the heading  marker  located  on  the  EHSI.  Heading  mode signal gain is a function of airspeed. Bank angle limit is 25 degrees in the heading select mode. Heading select mode is cancelled by: Capture of any other lateral steering mode