Use of Composite Material for Airplane Parts Scrutinized

A modern material that has replaced metal in many aircraft structures has become a central focus of the probe into the crash of American Airlines Flight 587, raising questions about possible problems in hundreds of other jets.

The material, used in the tail fin that broke away from Flight 587, is a composite made of many layers of carbon fibers embedded in a special resin and molded together under heat and pressure. However, the material can develop internal flaws, causing it to weaken and come apart.

On Friday, the Federal Aviation Administration ordered emergency inspections of about 140 Airbus jets with a similar tail fin and rudder construction, saying “a potential unsafe condition may exist.”

Aviation industry experts say that even if a failure of the composite material is found to have contributed to Monday’s disaster, they do not expect the widening use of composites to come to a halt. However, “we may have to do a better job of maintaining these parts of the aircraft and inspect composite-type material more closely,” said Lee Dickinson, an engineer and failure analysis consultant.

The A300 and A310 planes covered in Friday’s FAA order are owned by American, FedEx and United Parcel Service. However, foreign aviation authorities are also expected to require inspections of the planes, and the total number affected around the world may exceed 400. The FAA gave a 15-day deadline for making the checks in the United States.

Flight 587 was an A300, while the A310 is a shorter version of the same wide-body design. Both planes are built by Airbus Industrie, a European consortium based in France. The FAA ordered a thorough visual inspection, but several experts said special techniques that include the use of ultrasound are needed to detect potentially serious interior defects in composites. For that reason, they suggested, more extensive inspections may be required later.

Tail fins, like wings, should be able to withstand very heavy stress. The first close-up photographs of the failed tail fin were posted by the National Transportation Safety Board on its Web site (https://www.ntsb.gov). In one frame, a part of the tail fin that attaches to a fitting on the main fuselage looks like it was bitten off by a shark. The layered composite material can be clearly seen.

Other frames show ragged, broken pieces of the attachments still bolted into their fittings on the fuselage. There are six such sets of attachments and fittings on the A300.

Composites have become increasingly common in aviation because they are lighter and stronger than many metals and not susceptible to corrosion. But they have the potential for a failure known as “delamination,” when the internal layers come apart and lose their strength.

“A failure can occur on the order of a thousandth of an inch,” said Paul Lagace, director of the advanced composites lab at MIT. “Failure in a composite can be hidden to the eye and can occur between the layers.”

Delamination of a composite material can occur for many reasons, including damage in flight or on the ground, fatigue, a flaw in manufacturing or a shortcoming in design. A problem could exist on one plane or in an entire design family.

“The various layers separate like pages of a book,” said a structural engineer who has worked extensively with composites in aircraft. He asked not to be identified because of his ties to the industry. “It’s caused by stress--something pulls them apart. It is not a chemical decomposition.”

The plane that crashed Monday had flown through very severe turbulence in 1994, injuring 47 people aboard. Previously, when it was delivered to American Airlines in 1988, delamination was found in one of the six tail fin attachments.

However, inspections after the 1994 incident revealed no serious damage. And the NTSB said records show that the 1988 problem with the tail fin attachment was corrected according to approved procedures before the plane was put into regular service.

Peter Goelz, former managing director of the NTSB, said it’s possible that the 1988 repair could have shifted unusual loads onto some of the other attachments and fittings. “Sometimes if you repair something, you can make it too strong,” Goelz said. “And that can have unintended consequences.”

Dickinson said the photographs released by the NTSB appear to show some discoloration of the composite material. Although no conclusions can be reached from looking at pictures, he said, “I’d want to take a closer look at that.”

The military has made extensive use of composite materials in high-performance and stealth aircraft, but manufacturers of commercial jets traditionally limited their use of composites to less-critical interior components.

Any sign of a problem in other planes could have serious consequences for Airbus Industrie. Airbus pioneered the use of composites in major structural components of passenger airplanes. Rudders and tail fins were first, in the mid-1980s. Some manufacturers now use composites for horizontal stabilizers, elevators and portions of wings. Boeing’s new 777 passenger jet has a composite tail fin.

While the NTSB’s metallurgists are among the best in the business, the science of composites is new territory for the agency. “The board has extensive experience with metals,” said Tom Haueter, a senior investigator on the Flight 587 crash. “It has less experience with composites. That’s why we’re bringing in more experts.”

Unlike metals, which usually develop cracks on the surface, weaknesses in composites tend to begin on the inside, and special inspection techniques are required to detect them.

“When metal starts to fail, it happens on the outside and it’s usually associated with rivets,” said Chris Pastore, a materials engineer who worked with composites for NASA. “With composites, damage can separate some of the layers inside the part, and that will never show up on the surface.”

Pastore said the information released by the NTSB thus far “suggests there was something wrong with the composite”--a preexisting condition that could have been caused by prior damage or some other problem.

In addition to ultrasound, a special thermal technology can be used to inspect the interior layers of composites, Pastore said.

The FAA ordered airlines to look for outward signs, such as cracked paint and surface damage, that may signal a deeper problem.

Flight 587 crashed into a residential neighborhood in the Queens borough of New York on Monday, less than three minutes after takeoff from John F. Kennedy International Airport. All 260 aboard were killed, as were five on the ground.

Investigators are also seeking to explain how and why Flight 587’s engines separated from the wings. Adding to the puzzle is the fact that the left engine was found to the right side of the main wreckage, and the right engine landed to the left.

“It means that the plane was in contortions at the end,” Goelz said. “It was probably upside down and twisting when those engines came off.”