Manufacturing Defects In Composites Do Not Worsen

Air Safety Week, Jan 14, 2002

Numerous sources were skeptical of the adequacy of visual inspections. For composites, one industry source characterized them as coming close to "a complete waste of time - and [they give] a false sense of security." Another source, with extensive experience testing composites for military application, said, "The tap test is very crude. You can get delamination that's invisible."

Dr. Bill Johnson, Ph.D., North and Latin America regional director for Lufthansa Technical Training GmbH, offered a more hopeful observation:

"Visual inspection is THE most common type of inspection in aviation. Human error in visual inspection is affected by such factors as clarity of the job card instructions, environmental lighting conditions, training/experience of inspection personnel, level of alertness of personnel, and such things. IF composite experts clearly specify the manner in which visual inspection must be conducted AND IF humans are trained and motivated properly, it would certainly be reasonable and safe to rely on visual inspection as one of the safety measures... Because of the accident, you can be sure that the human inspector would be at a very high level of alertness for this new type of visual inspection."

Lee Gaillard, an independent analyst who specializes in aviation and defense issues, said, "If ply separation within the composite structure has occurred, a sign of the problem may not appear on the surface." He advocated use of a transducer generating sound waves across a critical frequency range. Pressed against the composite skin, it would generate vibrations across the frequency range, inducing at one point sympathetic vibration between any detached plies. A sensor akin to a doctor's stethoscope would "listen" for the telltale buzz of two separated layers vibrating against each other. The process would first be applied to a part known to be in pristine condition to establish the benchmark sounds of a solid structure. Moreover, Gaillard suggested that manufacturers explore the option of some form of trans-ply or interlayer reinforcement that will further lock plies together after the epoxy has cured.

There clearly is a range of opinion. On the one hand, there is the view expressed by Baksh, who brings to the table 10 years experience in composite repair, to the effect that "I'd be very concerned" about a "naked eyes only" test. On the other side is the view expressed last week by Airbus: "We do not recommend NDT because it's very difficult to do in an airline environment in situ, and to get consistent results."

Visual inspection is considered adequate and does not include the tap test, according to Airbus officials.

Visual - verified adequate

The Airbus argument takes this form:

1. Every composite part is subjected to visual and NDT inspection during manufacture.

2. All detected defects are analyzed.

3. Those minor defects within allowable limits have been demonstrated through extensive testing not to reduce the structure's strength, and not to grow or propagate during subsequent years of service.