TM 1-1500-204-23-4 Figure 4-20.  Compass Correction Card · After each engine change NOTE Check applicable aircraft special inspection to determine if the compass   check   and/or   calibration   is required after engine change. · After  any  modification/change  to  the  existing electrical system of major structural change · At  any  time  compass  is  suspected  of being in error · Whenever  the  flux  valve  has  loosened in    its    mounting,    has    been    removed during    troubleshooting,    or    has    been replaced with a new unit · At anytime the applicable aircraft technical manual specifies (b) Pre-compensation instructions. Accomplish the following procedures before compensating: 1 Ensure  that  aircraft  is  at  least  100 yards   from   any   steel   structure,   underground   cables, metal pipes, reinforced concrete or other aircraft. 2 Ensure    that    all    items    in    aircraft containing ferrous materials are located in the positions they will occupy during flight. 3 Remove  all  magnetic  articles  from one's person.  Use a nonmagnetic screwdriver. 4 Using   a   small   permanent   magnet, cause  compass  card  to  deflect  through  a  small  angle. Note  whether  the  card  rotates  freely  on  its  pivot,  and that its path of rotation is in a horizontal plane.  To avoid erroneous readings, read compass only directly in front of the lubber line. 5 Prior  to  compensation  of  magnetic compass,   have   aircraft   headed   successively   toward magnetic  North,  East,  South,  and  West  with  engines running  and  all  systems  operating.    Compass  readings must  read  within  20  of  deviation  noted  on  DD  Form 1613 (Pilot Compass Correction Card).  When reading is within   specified   tolerance,   compass   does   not   need compensating.    Transcribe  readings  from  old  DD  Form 1613  to  new  DD  Form  1613;  date  and  sign  form.    If magnetic compass is not within tolerance, proceed with next paragraph. 6 Remove one of the retaining screws and  loosen  the  other  from  the  compensator  assembly cover  plate,  and  rotate  the  plate  to  allow  access  to  the compensator   adjustment   screws.      Ensure   that   error compensator adjustment screws are set to neutral prior to realignment of compass transmitter. 7 Run the engines and turn on power to all electrical equipment which may have magnetic 4-26