A plumb bob is a heavy metal object, cylinder or cone shape, with a sharp point at one end and a string attached to the other end. If the string is attached to a given point on an aircraft, and the plumb bob is allowed to hang down so the tip just touches the ground, the point where the tip touches will be perpendicular to where the string is attached. An example of the use of a plumb bob would be measuring the distance from an aircraft’s datum to the center of the main landing gear axle. If the leading edge of the wing was the datum, a plumb bob could be dropped from the leading edge and a chalk mark made on the hangar floor. The plumb bob could also be dropped from the center of the axle on the main landing gear, and a chalk mark made on the floor. With a tape measure, the distance between the two chalk marks could be determined, and the arm for the main landing gear would be known. Plumb bobs can also be used to level an aircraft. Figure 4-17 shows a plumb bob being dropped from the leading edge of an aircraft wing.
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