TM 55.1510-220-10 NOTE For maximum cooling on the ground,   turn   the   bleed   air   valve switches to ENVIRO OFF position. *3. AC/DC power Check for: a. AC frequency 394 to 406 Hz. b. AC voltage 104 to 124 VAC. c. DC load Check. d. DC voltage 28.0 to 28.5 VDC. WARNING Do  not  operate  radar  in  congested areas.          Injury     could     result     to personnel    in    close    proximity    to operating radar. CAUTION Do not operate the weather radar in an  area  where  the  nearest  effective surface  is  50  feet  or  less  from  the antenna  reflector.    Scanning  such surfaces    within    50    feet    of    the antenna reflector may damage receiver crystals. *4. Avionics master switch ON. 5. Mission panel Set and checked as required. 6. Electric  elevator  trim  and  autopilot/flight  director operation Check as follows: a. Pilot's  and  copilot's  PITCH  TRIM  switches Press     to     NOSE     UP     and     NOSE     DN positions,  singularly  and  in  pairs.    Check that  trim  wheel  moves  in  proper  direction and  operates  only  when  trim  switches  are pressed   in   pairs.      Any   deviation   requires that electric elevator trim be turned off and flight conducted using manual trim. b. DISC  TRIM  switch  Press  to  second  detent and verify that electric trim disconnects and ELEV  TRIM  annunciator  light  (in  pedestal) extinguishes. c. Flight   director   (FDI)   and   Radio   Magnetic Indicator (RMI) warning flags Check masked. NOTE Since  the  pressure  of  airflow  that normally    opposes    movement    of control   surfaces   is   absent   during preflight check, it is possible to get a hard over control surface deflection  if  an  autopilot  command is  allowed  to  remain  active  for  any appreciable  length  of  time.    Move turn   knob   and   pitch   thumbwheel only   enough   to   check   operation, then    return    them    to    the    center position. d. Autopilot  mode  selector  panel  Select  HDG mode. e. Horizontal    Situation    Indicator    (HSI)    Set heading marker under lubber line. f. Autopilot  Engage.    Check  that  controls  are stiff  and  that  AP  ENG,  HDG,  and  AIL  HI TORQUE annunciator lights illuminate. g. AIL    HI    TORQUE    test    switch    Engage. Check that AIL HI TORQUE light extinguishes. h. HSI heading marker Move 10°  left and right and   verify   that   flight   director   and   control wheels respond in the appropriate direction. i. Autopilot/yaw damp disengage switch (control  wheels)  Press  to  first  detent  and verify  that  autopilot  disengages  (AP  DISC annunciator    illuminates)    and    that    flight controls are free. j. Autopilot Engage. k. Autopilot pitch-turn control (pedestal extension) Command 5°  trim UP and verify that manual trim wheel moves nose up and AP trim light indicates UP trim. l. Pitch trim switch (control wheels) Command    nose    down    and    verify    that autopilot   disengages   and   AP   TRIM   FAIL and MASTER WARNING annunciator lights illuminate. 8-15