This method uses a datum plate or scale mounted on the deck of a compartment. Provisions for hanging

Figure 9-44.Spirit leveling.

 the plumb bob are located directly above the datum plate. The aircraft is level when the plumb bob pointer is at 0 degrees on the datum plate. Figure 9-45 shows the plumb bob and datum plate method of aircraft leveling.

The alignment or symmetry check is made after the aircraft has been leveled. This check is made by measuring the distance between certain points on the aircraft. These points are selected because they are relatively static and because their location will best reflect any misalignment. Most manufacturers recommend that the measurements be taken directly from one specified point to another. Figures 9-46 and 9-47 show alignment checks.

On other types of aircraft, drop points are provided at various locations for use in checking the alignment. Plumb bobs are dropped from each of these points to the reference plane (floor) so that the pattern for measurement may be described. When you are using this method, the elevation check dimensions are measured from the drop points to the reference plane; in this case, the floor. The horizontal check dimensions are measured from one point (described by the plumb bob), along the reference plane (floor), to another point.

If the alignment check measurements exceed the tolerances listed in the aircraft structural repair manual, the aircraft must be considered non-airworthy until a special disposition can be made by higher authority.