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.
ALIGNMENT CHECK
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.
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.
WING TWIST CHECK
With the aircraft leveled and the wings folded, it
is possible to check the wings for twist. One
checkpoint is provided on each wing. Clinometer
readings taken at these points, when compared to the
fuselage longitudinal clinometer readings, will enable
you to determine the condition of each wing. This is
possible because there is a definite relationship
between the fuselage longitudinal and wing reference
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
Figure 9-45.—Plumb bob and datum plate leveling.
9-56
