TM 55-1500-342-23 b. The  cg  (henceforth,  reference  to  cg  will  mean the  longitudinal  center  of  gravity)  is  not  necessarily  a fixed  point;  its  location  depends  on  the  distribution  of items  loaded  in  the  aircraft,  and  as  variable  load  items are  shifted  or  expended,  there  is  a  resultant  shift  in  cg location.  It should be realized that if mass center of an aircraft  is  displaced  too  far  forward  on  the  longitudinal axis,  a  nose  heavy  condition  will  result.    Conversely,  if the    mass    center    is    displaced    too    far    aft    on    the longitudinal axis, a tail heavy condition will result.  It is possible  that  an  unfavorable  location  of  the  cg  could produce  such  an  unstable  condition  that  the  pilot  could lose control of the aircraft. 2-7.  Principle of Moments.  To understand balance, it is   necessary   to   have   a   working   knowledge   of   the principle of moments.  For those unfamiliar with weight and balance terms, the word moment is the product of a force or weight, times a distance.  The distance used in calculating   a   moment   is   referred   to   as   the   arm   or moment  arm,  and  is  usually  expressed  in  inches.    To calculate a moment, a force (or weight) and a distance must  be  known.    The  distance  is  measured  from  some desired    known    point    (reference    point    or    reference datum)  to  the  point  through  which  the  force  acts.    A moment is meaningless unless the reference point about which the moment was calculated is specified. a. For  the  purpose  of  illustration,  an  aircraft  may be compared to a seesaw.  Like the seesaw, in order for an  aircraft  to  be  in  balance,  or  equilibrium,  the  sum  of the moments on each side of the balance point must be equal in magnitude.    For   example,   referring   to   figure   2-1,   the   moment produced about the fulcrum (reference point) by the 200 pound    weight    is    200    lb    x    50    in    =    10,000    in    lb counterclockwise.      The   moment   produced   about   the same reference point by the 100 pound weight is 100 lb x  100  in  =  10,000  in  lb  clockwise.    In  this  case,  the clockwise moment counterbalances the counterclockwise     moment,     and     the     system     is     in equilibrium.      This   example   illustrates   the   principle   of moments  which  is  as  follows:  For  a  system  to  be  in static  equilibrium,  the  sum  of  the  moments  about  any point must equal zero. b. As    illustrated    in    figure    2-1,    the    clockwise moment is arbitrarily given a positive (+) sign while the counterclockwise  moment  is  given  a  negative  (-)  sign. Therefore, the sum of the moments about the fulcrum = + 10,000 in lb (clockwise) -10,000 in lb (counterclockwise)  -0,  and  the  system  is  in  equilibrium. In determining balance of an aircraft, the fulcrum is the unknown,  and  the  problem  is  one  of  determining  the location of the fulcrum, or longitudinal center of gravity. 2-8.      Balance   Definitions.      Definitions   of   the   more important terms pertaining to balance and its relationship to aircraft weight distribution are as follows: a. Gross Weight Moment.  Gross weight moment is   the   sum   of   moments   of   all   items   making   up   the aircraft in the gross weight condition.  The gross weight moment  is  the  product  of  gross  weight  times  the  gross weight arm. b. Basic Arm.  Basic arm is the distance from the reference datum to the center of gravity of an aircraft in basic   condition.      It   is   obtained   by   dividing   the   basic moment by the basic weight. c. Gross  Weight  Arm.    Gross  weight  arm  is  the distance   from   the   reference   datum   to   the   cg   of   an aircraft  in  its  gross  weight  condition.    The  relationship between the gross weight, gross weight arm, and gross weight moment is as follow: gross  weight  arm  (in)  =  gross  weight  moments  (in  lb) gross weight (lb) d. Reference   Datum.      Reference   datum   is   an imaginary plane perpendicular to the longitudinal axis of the aircraft and is usually located at or near the nose of the  aircraft  to  eliminate  arms  with  a  minus  value.    If  a negative arm is encountered, the corresponding moment  will  also  be  negative.    Simplified  moment  is one   which   has   been   reduced   in   magnitude   through division by a constant.  For example, 3201 in lb/ 1000 is the  simplified  expression  of  3,200,893  divided  by  1000 and   rounded   off   to   the   nearest   whole   number.      The advantage  of  simplification  will  be  seen  in  application when   a   column   of   moments   is   added.      Inaccuracies resulting from rounding off figures tend to cancel. e. Aircraft Station.  An aircraft station is a position defined   by   a   plane   perpendicular   to   the   longitudinal aircraft  axis.    The  number  designation  of  this  station signifies   its   distance   from   the   reference   datum.      A station  forward  of  the  reference  datum  is  negative  (-) while a station aft of the reference datum is positive (+). 2-3