Another Look at Visual Inspection
By Fred Workley
Visual inspection continues to be an important element of maintenance programs. Technology has made great strides in giving maintenance personnel a wide variety of Nondestructive Inspection (NDI) options in dye penetrant, eddy current and ultrasonic. But no matter how advanced and expensive the test equipment, the decision regarding serviceability is often made after you perform some visual task. This is done by the process of observing an object with your eyes that are either unaided or aided. Then you must make an assessment of the aircraft's condition based on all the information that is available to you.
Elements of inspection
There are some basic inspection principles and applications that are used with in-service airplane inspections. We will review these in this article and note how these basic visual inspection practices also support Nondestructive Inspection. Also, visual inspections become elements in inspection programs that are mandated by regulatory requirements, manufacturer's instructions for continued airworthiness and operators' FAA approved maintenance programs. Often maintenance personnel will not have available either FAA or manufacturer's approved processes and procedures for a particular application. In these cases, the person performing the visual inspection has to evaluate each individual inspection outcome and make determinations of airworthiness.
On general aviation aircraft, it is often true that almost all of the inspection is visual. Even transport aircraft requires maybe only 15 to 20 percent to be aided by technology. Visual inspection can often serve as the basis of documentation to adequately support the continued airworthiness of an aircraft. Often the first evidence of most defects are found in visual inspections. Thus, these visual inspections require a level of proficiency by the person performing them. To have anything less would be a concern for the safe operation of the aircraft.
The "eyes" have it
Let's look more closely at visual inspection and then how it supports NDI. Visual inspection is using the eyes either alone or with aids, such as sensing mechanisms, to determine the condition of the unit so that judgements can be made. Some examples of imaging devices are optical comparators, eddy current imaging, ultrasonic C-scans, and real-time X-rays. There are many advanced applications, which are also used: computed tomography in dye penetrant, automated eddy current scanning, computerized ultrasonic testing, acoustic emission, diagnostic X-ray, magneto-optic eddy current, coherent optics and advanced video imaging.* For our review, we will not consider the analysis of these displays as visual inspection. The visual inspection component of NDI is to validate the procedures and assure the calibration of appropriate standards. Visual inspection ensures that NDI equipment is maintained so that it functions well in the maintenance environment. The bottom line is that visual aids increase the probability of finding a defect or failure so that timely, corrective action can be initiated.
The power of observation
Visual inspection must provide an assessment of the overall condition of a component, system or structure. Defects must be identified early enough so that corrective action is taken before any progression causes a critical condition. Another use of visual inspection is to discover manufacturing or assembly errors. Of importance is to observe any condition of a component or structure that has a defect or malfunction that if not corrected, might lead to failure in flight. In all these situations, visual inspections will initiate the gathering of information upon which judgements are made. Airframe defects are generally of three types: cracks, disbonding and corrosion. On the other hand, visual inspection of aircraft components and systems finds wear, fatigue, interference, corrosion and accidental damage. Visual inspection of engines using borescopes finds nicks, pitting, spalling, burning, erosion, scoring, galling and cracks.