TM 1-1500-204-23-4 6 Return aircraft to the north magnetic heading.  Add coefficient C, shown below, algebraically to  compass  reading  on  that  heading  to  determine  what compass  should  read  when  compensated.    Adjust  MS compensating    screw    (of    polyplane    compensator)    or insert magnets in compensating drawer, which is at right value     angles     to     needle,     until     compass     indicates compensated value. Coefficient C= N-S = (+8) - (-4) = 12 = +6 2 2 2 7 Head  aircraft  east.    Add  coefficient B, as shown below, algebraically to compass reading on that heading.  Adjust EW compensating screw (or insert magnets   in   that   chamber   of   compensating   drawer   at right angles to needle) until compass indicates compensated value. Coefficient B= E-W = (-4) - (+8) = -12  =  -6 2 2 2 NOTE · The     spread     between     maximum positive    and    maximum    negative deviation after compensation shall not exceed 5 degrees. · Coefficient A represents total deviation   from   the   formula   given on  DD  Form  1613.    All    additions and   subtractions  are algebraic. Coefficient A = N + E + S +W = 4 (+8) + (-4) + (-4) + (+8)  = 8  =  2 4 4 8 After   adjustment   has   been   made, residual swing can begin on any heading.  Record actual heading (M) Column 3, and aircraft compass reading (c) Column  4;  repeat  every  45  degrees  around  the  circle. As indicated in the example in figure 4-22, C to M DEV is calculated by (3) - (4) or M to C by (4) - (3). NOTE The spread between maximum positive   deviation   and   the   maximum negative deviation after compensation shall not exceed 5 degrees. 9 Fill  in  back  of  the  DD  Form  1613. Detach heading correction card portion of the DD Form 1613  at  the  perforation  and  place  in  the  cockpit.    The location for the card may vary due to night vision goggle requirements.        See    applicable    aircraft    manual    for location of Compass Correction Card. 10 Place  the  balance  of  the  form  with the   aircraft   historical   record   as   a   record   of   compass swing   until   completion   of   the   next   compass   swing. Upon  transfer  of  aircraft,  this  portion  of  the  form  will accompany the aircraft with other records. (d) B16 master sighting compass method of compensating.  The compensator assembly is removed   from   the   compass   and   a   special   sighting device    fitting    is    substituted.        During    compensating procedure   with   this   instrument,   aircraft   is   turned   to desired  heading  as  indicated  by  aircraft  compass.    A sight is then taken by the swinging compass 2 points on the   fore-and-aft   axis   of   aircraft   being   lined   up   with hairline in collimator lens, as shown in figure 4-23.  The compass   used   for   this   purpose   will   be   painted   red, indicating  that  it  has  been  remagnetized  180  degrees from  normal.    This  permits  observer  to  read  magnetic heading of aircraft directly when sighting from in front of aircraft.  When a compass drawn from stock is used as a    swinging    sight,    180    degrees    must    be    added    to compass reading to obtain magnetic heading of aircraft. Corrections, as indicated by master compass, are made on  each  of  the  cardinal  headings  in  the  conventional manner.        In    every    other    respect,    compensation    is exactly the same as in the use of the compass rose. NOTE When    using    a    sight    compass    on helicopters,    line    up    with    any    two objects    that    are    parallel    with    the centerline of the aircraft. (e) Using AHRS and INS to compensate compasses.  If the aircraft is equipped with a  functional  Attitude  and  Heading  References  System (AHRS) or an Inertial Navigation System (INS) and the variation  is  known  to  within  0.    2  degree,  the  standby compass  system  may  be  swung  using  either  of  these systems as a heading reference in place of a compass rose. The standby compass system is then compensated by using the procedures in paragraph (c). Special   care   must   be   exercised   in   the   alignment   of aircraft and the use of nonmagnetic materials. 4-28