2-2 Figure 2-3.  Balance lever datum located off the lever.$^{C = 200}$ Figure 2-2.  Balance point locations. Figure 2-1.  Balance lever. 1.  Measure the arm of each weight in inches from the  datum. • The  center of gravity (CG), or the point at which  all of the weight of the aircraft is considered to be  concentrated, must be maintained within the allowable  range for the operational weight of the aircraft. •  The total weight of the aircraft must be no than the maximum weight allowed by the FAA for  the make and model of the aircraft. 2.  Multiply each arm by its weight in pounds to determine  the moment in pound-inches of each weight. 3.  Determine the total of all weights and of all the  moments. (Disregard the weight of the lever). 4.  Divide the total moment by the total weight to  determine the balance point. Two elements are vital in the weight and balance considerations  of an aircraft. Consider these facts about the lever in  Figure 2-1 . The  100-pound weight A is located 50 inches to the left of the  fulcrum (the datum, in this instance), and it has a moment of 100 × –50 = –5,000 lb-in. The 200-pound weight B is located  25 inches to the right of the fulcrum, and its moment is  200 × +25 = +5,000 lb-in. In  Figure 2-2, the sum of moments is –5,000 + 5,000 = 0, and the lever is balanced. The  forces that try to rotate it clockwise have the same magnitude  as those that try to rotate it counterclockwise. If either weight  is moved or changed, the balance point or CG changes and  the lever becomes unbalanced. The arm is usually measured and expressed in inches and  refers to the horizontal distance between the CG of an item  or object and the datum, a point from where all measurements  are taken. Arms to the left of the datum are negative (–) and  those to the right of the datum are positive (+). The datum is an  imaginary vertical plane from which all horizontal distances  are measured for balance purposes. The position of the  reference datum varies by aircraft design and manufacturer.  When the datum is located off of the lever and to the left, all of  the arms are positive and computational errors are minimized.  Note: When the datum is established ahead of the aircraft, for  example at the aircraft nose, all of the arms are positive and  computational errors are minimized.  A moment is a force that tries to cause rotation and is the  product of the arm, in inches, and the weight, in pounds.  Moments are generally expressed in pound-inches (lb-in)  and may be either positive or negative.  Weight and balance problems are based on the physical law  of the lever. This law states that a lever is balanced when the  weight on one side of the fulcrum (a pivot point for the lever)  multiplied by its arm is equal to the weight on the opposite  side multiplied by its arm. In other words, the lever is balanced  when the sum of the moments about the fulcrum is zero. This  is the condition in which the positive moments (those that  try to rotate the lever clockwise) are equal to the negative  moments (those that try to rotate it counterclockwise). In an  aircraft, the balance point is referred to as the CG.  One of the easiest ways to understand weight and balance is  to consider a lever with weights placed at various locations.  The balance point or CG of the lever can be changed by either  moving the weights closer or farther from the fulcrum or by  increasing or decreasing the weights. The balance point or  CG of a lever may be determined by using these four steps: In  Figure 2-3,  the datum is located off the lever to the left  of weight A. Using the information provided in  Figure 2-3, The Law of the Lever Moment Arm  Weight and Balance Theory B = 200 A = 100 Arm B (+25") Arm A (−50") Fulcrum Datum − Moment + Moment − F o r c e s + F o r c e s Item Weight (lb) Arm (in) Moment (lb-in) −5,000 +5,000 0 Weight A Weight B 100 200 300 −50 +25 A = 100 B = 100 +50 +90 +110 +150 CG Datum