The lateral reference datum, is usually located at the center of the helicopter. The location of the reference datums is established by the manufacturer and is defined in the rotorcraft flight manual. [Figure 7-4]
 
ARM—The horizontal distance from the datum to any component of the helicopter or to any object located within the helicopter is called the arm. Another term that can be used interchangeably with arm is station. If the component or object is located to the rear of the datum, it is measured as a positive number and usually is referred to as inches aft of the datum. Conversely, if the component or object is located forward of the datum, it is indicated as a negative number and is usually referred to as inches forward of the datum.
MOMENT—If the weight of an object is multiplied by its arm, the result is known as its moment. You may think of moment as a force that results from an object’s weight acting at a distance. Moment is also referred to as the tendency of an object to rotate or pivot about a point. The farther an object is from a pivotal point, the greater its force.
CENTER OF GRAVITY COMPUTATION—By totaling the weights and moments of all components and objects carried, you can determine the point where a loaded helicopter would balance. This point is known as the center of gravity.
WEIGHT AND BALANCE METHODS
Since weight and balance is so critical to the safe operation of a helicopter, it is important to know how to check this condition for each loading arrangement. Most helicopter manufacturers use one of two methods, or a combination of the methods, to check weight and balance conditions.
COMPUTATIONAL METHOD
With the computational method, you use simple mathematics to solve weight and balance problems. The first step is to look up the basic empty weight and total moment for the particular helicopter you fly. If the center of gravity is given, it should also be noted. The empty weight CG can be considered the arm of the empty helicopter. This should be the first item recorded on the weight and balance form. [Figure 7-5]
 Weight  (pounds)  Arm  (inches)  Moment� (lb/inches)
Basic Empty Weight�  1,700�  116.5�  198,050�
Oil�  12�  179.0�  2,148�
Pilot�  190�  65.0�  12,350�
Forward Passenger�  170�  65.0�  11,050�
Passengers Aft�  510�  104�  53,040�
Baggage�  40�  148�  5,920�
Fuel�  553�  120�  66,360�
Total�  3,175  �  348,918
CG   109.9  
Max Gross Weight = 3,200 lbs. CG Range 106.0 – 114.2 in.

Figure 7-5. In this example, the helicopter’s weight of 1,700 pounds is recorded in the first column, its CG or arm of 116.5 inches in the second, and its moment of 198,050 pound-inches in the last. Notice that the weight of the helicopter, multiplied by its CG, equals its moment.