the imaginary point at which all the weight is concentrated.  To provide the necessary balance between longitudinal  stability and elevator control, the CG is usually located  slightly forward of the center of lift. This loading condition  causes a nose-down tendency in flight, which is desirable  during flight at a high AOA and slow speeds.  As mentioned earlier, a safe zone within which the balance  point (CG) must fall is called the CG range. The extremities  of the range are called the forward CG limits and aft CG  limits. These limits are usually specified in inches, along the  longitudinal axis of the airplane, measured from a reference  point called a datum reference. The datum is an arbitrary  point, established by aircraft designers that may vary in  location between different aircraft.  [Figure 10-2]  The distance from the datum to any component part or any  object loaded on the aircraft is called the arm. When the  object or component is located aft of the datum, it is measured  in positive inches; if located forward of the datum, it is  measured as negative inches or minus inches. The location  of the object or part is often referred to as the station. If  the weight of any object or component is multiplied by the  distance from the datum (arm), the product is the moment.  The moment is the measurement of the gravitational force  that causes a tendency of the weight to rotate about a point  or axis and is expressed in inch-pounds (in-lb).  To illustrate, assume a weight of 50 pounds is placed on  the board at a station or point 100 inches from the datum.  The downward force of the weight can be determined by  multiplying 50 pounds by 100 inches, which produces a  moment of 5,000 in-lb.  [Figure 10-3]  To establish a balance, a total of 5,000 in-lb must be applied  to the other end of the board. Any combination of weight  and distance which, when multiplied, produces a 5,000 in- lb moment will balance the board. For example (illustrated  in  Figure 10-4) , if a 100-pound weight is placed at a point  (station) 25 inches from the datum, and another 50-pound  weight is placed at a point (station) 50 inches from the datum,  the sum of the product of the two weights and their distances  total a moment of 5,000 in-lb, which will balance the board.  An aircraft’s weight and balance restrictions should be  closely followed. The loading conditions and empty weight  of a particular aircraft may differ from that found in the  AFM/POH because modifications or equipment changes  may have been made. Sample loading problems in the  AFM/POH are intended for guidance only; therefore, each  aircraft must be treated separately. Although an aircraft is  certified for a specified maximum gross takeoff weight, it  may not safely take off at this weight under all conditions.  Conditions that affect takeoff and climb performance, such as  high elevations, high temperatures, and high humidity (high  density altitudes), may require a reduction in weight before  flight is attempted. Other factors to consider when computing  Figure 10-3.  Determining moment.  Weight and Balance Restrictions  Figure 10-2.  Weight and balance.  Figure 10-4.  Establishing a balance.  10-6  50  lb  100"  Moment = 5,000 in-lb  Fulcrum  Datum  Wt x Arm = Moment  (lb) x (in) = (in-lb)  50 x 100 = 5,000 Note: The datum is assumed to be  located at the fulcrum.  Aft limit Fwd limit  Datum  CG  range  ( + )  Arm  ( – )  Arm  ( + ) Arm 70"  Moment = 700 in-lb  Sta 0  Sta 70  10 lb  100  lb 50  lb  50  lb  25"  50"  100"  2,500  in-lb  2,500  in-lb  5,000  in-lb  Fulcrum  Datum  100 x 25 = 2,500  50 x 50 = 2,500  Total = 5,000  Wt x Arm = Moment  (lb) x (in) = (in-lb) 