Manufacturing Defects In Composites Do Not Worsen

Air Safety Week, Jan 14, 2002

Visual Inspections Are Adequate for Safety, Airbus Maintains

If damage or degradation is not seen on the surface of a composite material, there are no hidden flaws sufficient to compromise its structural strength. Thus, visual inspections provide adequate protection against structural failure and, in fact, other means of probing the depths of the material are not recommended if the surface is unscathed. If it's not seen, it's not a problem. This appears to be the overarching philosophy articulated by Airbus Industrie regarding the composite materials used in load-bearing structures on its aircraft.

At a recent briefing by Airbus officials, they asserted that "invisible damage cannot produce a significant sub-surface flaw." Company officials hastened to point out that they could not and would not discuss details of the fatal Nov. 12, 2001, crash of an American Airlines [AMR] A300 shortly after takeoff from New York's John F. Kennedy International Airport (JFK). The airplane's composite tail snapped off, and all 260 passengers and crew aboard were killed, along with five people on the ground (see ASW, Nov. 19, 2001) The case involves the first fatal Airbus crash in North America and is the first known failure of a primary composite structure in a transport-category aircraft. The U.S. National Transportation Safety Board (NTSB) is investigating; in addition to the pilots' control actions during a wake turbulence encounter during climb, investigators are closely scrutinizing the remains of the composite tail and its attachment fittings.

In the immediate aftermath of the tragedy, the U.S. Federal Aviation Administration (FAA) issued an emergency airworthiness directive (AD 2001-23-51) Nov. 16, 2001, four days after the accident, ordering operators to inspect the composite attachment fittings on all A300/A310 aircraft in U.S. registry. Similar action was mandated by the Direction Generale de l'Aviation (DGAC), the counterpart airworthiness authority in France.

The U.S. inspection directive ordered a one-time detailed visual inspection, the definition of which was fully explicated in the AD, "to prevent failure of the vertical stabilizer-to-fuselage attachment fittings." Operators were advised to look for cracks, surface damage, evidence of moisture damage, and for split or frayed fibers. It should be noted that this mandate would not identify an area of ply separation halfway up the fin, which might be vulnerable to spreading under repeated flex loads. However, the main point of concern is the breaking of the attachment points.

According to Allan Baksh, chief operating officer of Miami-based Aviation Composite Services, Inc., a composite repair facility, the specific wording of the AD, while not explicit on this point, clearly implied use of the so-called "tap test." Anybody doing the inspection per the FAA's emergency AD "would do the tap test," Baksh maintained. This test was described by another source as akin to "finding studs in a wall" by tapping along the surface. In the case of composites, a hollow sound could indicate separation of some of the many plies comprising the composite material. While Baksh believed a tap test would be appropriate, it was not explicitly part of the FAA's order. Other sources suggested that the tap test would require removal of the tailfin to perform properly.

Two cases, two approaches

No sooner was this AD on the street than an apparent disconnect was pointed out: after a composite structure failure in 1992, ultrasound inspections were ordered, but after this 2001 fatal accident, the FAA limited its mandate to a visual inspection. The first case involved an in-flight turbulence encounter involving an MD-11 with 265 passengers and crew on board for the flight from Taipei, Taiwan, to Anchorage, Alaska. Although portions of the left and right outboard elevator skins, manufactured of composite, separated from the airplane, the crew was able to land the airplane. Subsequent investigation uncovered a second similar incident. As a result, the NTSB urged nondestructive ultrasound scan inspections of composite elevators on all MD-11s "that are known to have been operated outside the design buffet boundary."

The FAA followed through, requiring the ultrasound inspections, and the NTSB closed the case with "Acceptable Response."

This is the disparity critics see: a higher-order nondestructive test (NDT) was decreed after non-fatal incidents, and a lower order visual inspection was conducted after a fatal accident in which failure of a primary composite structure was part of the deadly sequence of events.

An FAA official justified the mandate for visual inspections thusly: "The intent of the AD was twofold - to find any indications of a safety problem, and to gather initial data on the state of these A300/310 composite structures." In this respect, visual inspections may not satisfy either of these intended goals.

"The FAA has not ruled out inspections using ultrasound or other NDT techniques," the official added.