Figure 5-69 is an example of an inspection of a stabilator. The structure is an aluminum honeycomb sandwich
structure. Grids are marked on the surfaces to aid in maintaining transmitter/receiver alignment; mapping boundaries
of suspected flaws and assuring complete inspection coverage. The grid sizes are proportional to the critical flaw size
of the respective zones. During the inspection one search units is placed in the center of a grid square and the other is
manipulated to maximize the received signal. (See View B in Figure 5-70). Each square is inspected in turn. If the
through-transmission signal falls below 50 percent of saturation view C couplant and search unit alignment should be
checked. If there is a definable area where the signal is less than 50 percent, mark the boundary (at the centerline of
the receiving search unit) where the signal equals 50 percent according to the procedure in View D.
Figure 5-69. Typical Through-Transmission Inspection of a Stabilator.
Pulse Echo Method.
Figure 5-71 shows the basic principle of this method, which employs an angle beam search unit because straight beam
search units may produce multiple echo signals from the layers that would interfere with echo signals from the core.
This method is applicable only to honeycomb structures and is best applied to structures with single-layer skins (see
Figure 5-71, detail C) when the through-transmission method cannot be used. Straight beam search units may provide
better results on structures with multi-layer skins. This method should be used as a backup to methods associated with
dedicated bond inspection instruments discussed below. Angle beam search units producing refracted angles of 30 to
90 degrees may be used. The angle selected should be the one that produces the maximum signal response from the
back of the core.