TM 55-1500-342-23 reaction points.  When measuring these distances, it is necessary that the tape be parallel to aircraft centerline. Measurements made from the main reaction points are taken perpendicularly to the chalk line joining these two points.  These measurements may be made quickly by placing one end of the tape on the point in question and swinging  the  other  end  of  the  tape  across  the  line  in  a small arc.  Notice the point at which the tape crosses the chalk line which shows a shorter distance than any other along the line.  This is the shortest distance between the line   and   the   point   in   question   and,   therefore,   is   the perpendicular distance from the point to the line.  When fuselage   and   wing   jack   points   are   used   as   reaction points   in   weighing   the   aircraft,   it   is   unnecessary   to measure dimension.  These points will remain fixed with respect to the reference datum and their moment arms may  be  found  in  Chart  E  data.    When  measuring  is necessary, the required dimensions should be recorded on  DD  Form  365-2  as  soon  as  the  measurements  are taken. 3-5.      Aircraft   Weighing   Area.      Procedures   outlined herein are general in nature, since methods of weighing vary with each type aircraft. a.    Weigh    aircraft    in    closed    hangars    to    avoid aircraft  vibrations  which  would  otherwise  be  caused  by air currents flowing over lifting surfaces.  This vibration would  result  in  fluctuating  scale  readings  and  increase the possibility of error. b.    Insure that aircraft is thoroughly dry before it is weighed.   Never  weigh  aircraft  immediately  after  it  has been washed. c.    Set electronic weighing cells oil their respective jacks, using proper jack and jack pad adapters.  Be sure that jack adapter is fully threaded into cell assembly.  If a ring-type adapter is used, see that it is centered flush on  ram  applying  a  partial  load  to  it  before  tightening setscrews. CAUTION Use   proper   adapters   to   prevent jacks   from   slipping   or   buckling. Damage  to  aircraft  or  inaccurate weight    readings    may    result    if improper adapters are used. Never  apply  loads  to  the  rim  of  a weighing cell. d.    Prepare    electronic    weighing    kit    for    use    by following instructions furnished with kit.  Warm up cells mounted on jacks to be used for jacking. e.    Actuate  all  jacks  simultaneously  until  weighing cells are in contact with aircraft jack pads.  Continue to jack  aircraft,  insuring  that  aircraft  is  kept  level.    When aircraft  is  supported  at  weighing  reaction  points  only, and is in level position, scale readings may be obtained. Weight and balance personnel must be alert for possible errors  in  scale  readings  (e.g.,  side  loads  or  misaligned jack and cell, etc.).  To insure accuracy of results, take several independent readings (e.g., for beam scales by upsetting  the  beams  of  all  scales  between  readings  or completely    unloading    the    electronic    load    cells    and rejacking).  A minimum of two weighings shall be made. If  the  first  two  weighings  are  within  one  quarter  of  one percent in weight i.e.  (divide the less weight reading by the  greater  reading,  subtract  the  results  from  1.00  to obtain  the  percentage)  and  0.1  inch  in  c.g.,  additional weighings are unnecessary.  If these constraints are not met,  additional  weighings  shall  be  made  until  they  are satisfied.    Enter  average  weight  and  c.g.    of  the  two suitable weighings on aircraft weighing form (DD 365-2). If  variations  in  scale  indications  for  any  reaction  point are greater than that prescribed in the technical manual covering  the  kit,  reweigh  aircraft  with  another  weighing kit. f.    Before    final    lowering    of    the    aircraft,    make certain   that   all   necessary   measurements   and   scale readings have been obtained and recorded. g.    Rotary  wing  aircraft  are  weighed  in  the  same manner    as    conventional    aircraft,    except    that    four reaction   points   are   frequently   used   instead   of   three. When   four   reaction   points   are   employed,   it   will   be necessary   to   use   two   weighing   kits,   since   each   kit contains  only  three  cell  assemblies.    If  a  second  kit  is not available, it is permissible to weigh the aircraft using three reaction points.  The cell assemblies will be placed on  the  two  front  jacks  and  the  right  rear  jack.    The  left rear  jack  is  used  while  raising  the  aircraft  to  weighing position   to   maintain   proper   attitude.      When   the   full aircraft weight is bearing on the jacks, and the aircraft is level,  stop  jacking  and  activate  locking  devices  on  the two front jacks and right rear jack.  Slowly lower the left rear  jack/reaction  point  by  ½   inch,  at  which  time  the aircraft's weight can be recorded.  Raise the fourth jack to again bear the weight of the  aircraft,  then  lower  all four  jacks simultaneously to return the aircraft to rest on the hangar floor. Change 1  3-5