PANEL SUMMARY: Aging Aircraft

In its fourth day, the NTSB public hearing into the explosion and crash of TWA flight 800 turned to the issue of maintenance of aging aircraft. The plane that crashed was a 25-year-old Boeing 747 that had logged 90,000 hours of flight in 18,000 cycles of takeoffs and landings.

Robert Swaim, an NTSB aircraft systems investigator, said the loss of part of the fuselage of an aging Aloha Airlines Boeing 737 in 1988 was the catalyst for major changes in how aging airplanes are inspected and maintained. But he said most aging-aircraft efforts concentrate on the structure – the metal skin and internal framework -- and not on the systems. Airplanes contain electrical, fuel and other systems, and these have been among the subjects of the week-long NTSB hearing.

The NTSB has determined that the explosion of flight 800 was in the center fuel tank, but has not determined what touched off that explosion. Board investigators have looked into the wiring in and around the center fuel tank, which lead to various pumps and fuel-gauging components. Swaim said that many of the potential ignition sources may be age-related.

Dr. Gregory Dunn, a member of the FAA Transport Standards staff, said that overseeing the airworthiness of older planes is a joint function of the FAA, the plane manufacturer and the airline. Dunn said FAA regulators require the plane-maker to provide instructions to maintain airworthiness. This, he said, amounts to providing maintenance instructions for the airline – recommended periods for parts to be cleaned, inspected, adjusted, tested and lubricated.

Robert Vannoy, Boeing’s chief of 747 fleet support, said the industry’s extensive aging-aircraft program for planes flying beyond their original designed service objective began in 1983 and was expanded in later years. Vannoy said the age-related programs include both the structural fatigue cracking and the plane’s operating systems. But he added that emphasis has been on the structure because of the Aloha accident, and that there had been no reason to give specific attention to the plane’s systems.

Vannoy said there is no life limit on 747s, although older ones need additional costly maintenance. He said the first 747s entered service in 1970 and had a design life of 20 years, 20,000 flight cycles and 60,000 hours, but while airlines are still using the planes, they need extensive maintenance after the 20-year mark. Vannoy said 380 of the planes have over 60,000 hours, 240 have been in service over 20 years and 95 have more than 20,000 flight cycles.

The accident 747 had 18,000 flight cycles and 90,000 hours. It had scheduled inspections at 13,000 and 16,000 cycles and was due for another at 19,000 cycles. James Wildey, an NTSB metallurgist, said there is no evidence of any fatigue cracking in the accident 747.

Vannoy said the industry’s aging-aircraft program, under which structural damage is detected, has resulted in 20 reports that prompted FAA airworthiness directives. He said maintenance recommendations have also been issued concerning system items when a safety problem is uncovered and could also result in an airworthiness directive. Vannoy said that in the inspections there have been no significant findings related to systems, and Boeing handled minor ones with maintenance recommendations. He said that the systems performance of the older planes is continuously monitored and that the goal of system design and maintenance practices ensured safety through redundancy, system indication and self monitoring and scheduled maintenance. In fleet surveys, visual inspections showed no problems were found in electrical systems.

Vannoy described a fatigue testing program Boeing had implemented based on work with a retired 747 that had 20,000 cycles. Through mechanical means, he said the plane was aged the equivalent of another 20,000 cycles. The body structure was monitored, and the result was a new fuselage inspection program, announced through an alert service bulletin to 747 operators.

NTSB Chairman Jim Hall asked if there was any reason Boeing did not look as closely at the aircraft systems as well as the plane’s structure. Vannoy said there was no specific reason. He said that at that time Boeing was focused on structure.

Chairman Hall also asked if there was an economic design life for wire. Vannoy replied that Boeing had not established any life in the design requirement and wires should last as long as the airplane does.

Swaim said there was a feeling by some pilots and aircraft machinists that in the highly competitive field of commercial aviation some airlines were cutting back on maintenance to save money. Ken Craycraft, a TWA maintenance engineer, said he did not know the cost of maintenance programs. But Vannoy said that the monthly "A" checks put a plane out of service one day, the annual "C" check puts a plane out of service one week and the heavy maintenance program, the "D" check, takes a month or more every five years. And, he added, the cost of a "D" check on planes 15 years old takes about 30,000 man-hours and for planes 25 years old, the man-hours needed could double or triple.

Chairman Hall stressed that the cause of the TWA 800 accident is not yet known, so it is not clear that these issues are involved. But to do a thorough job and consider all possibilities, he continued, the investigation must consider the age of the aircraft. He noted Boeing’s effort to study the structure of aging aircraft and questioned whether a similar effort should be made into the systems of aging aircraft.

Dr. Bernard Loeb, director of the NTSB’s Office of Aviation Safety, addressed a similar question to the FAA officials testifying. Dunn replied that the issue is under study and by mid-1998 the agency will decide whether to mandate an aging-aircraft program for systems similar to the one focused on structure.

George Slenski, an electronic-materials engineer with the U.S. Air Force (USAF), discussed the wiring in aircraft and the problems that can be associated with it. In contrast to earlier testimony from witnesses involved with civil aviation, Slenski said the Air Force considers wiring a system itself, not a component of a system. He showed photographs that revealed the masses of wire to be found in military aircraft. A wire bundle can be like a tree trunk, he said, and one problem is that the process of inspecting for damage can actually cause damage.

As Slenski demonstrated, aircraft wiring can have prolonged exposure to hydraulic fluid, jet fuel and water. It can be subject to chemical degradation, temperature extremes and mechanical damage. Because of the latter, the Air Force has an anti-chafing program. Alex Taylor, an engineer and wire expert with Boeing, said there is no equivalent program on the civil side, and such chafing-related damage is supposed to be prevented by proper design.

Slenski echoed the earlier response from Boeing officials that wire is not assigned a separate design lifespan, that it is can theoretically last as long as the airframe.

Wire integrity can be compromised by hot-stamp marking, a means of labeling the insulation, Slenski reported, but the process is acceptable if performed correctly. He also said arc tracking (current passing over insulation surface between wires) is rare but has occurred in some military applications. Both Taylor of Boeing and Craycraft of TWA said they have seen no evidence of it on 747s.

Dunn and Bill Crow, both of FAA, underscored the agency’s commitment to regulating the civil air fleet so that it is maintained at a high level of airworthiness. But Crow said the agency also wants to make sure it does not order unwarranted inspection and maintenance procedures that could themselves cause damage. He cited the analogy of a surgeon who looks for unexpected signs of trouble in the vicinity of a surgical site but who would not undertake the risk of an operation without cause.

TWA’s Craycraft, Boeing’s Vannoy and Swaim discussed a malfunctioning of flight 800’s volumetric fueling shutoff system, designed to shut off automatically when the tank is full. In this case, it shut off before the full mark, and a mechanic reported that a circuit breaker was pulled and the pressure fueling was continued. Swaim said the NTSB was investigating this because some of the wiring for the shutoff system shared a common routing with the fuel flow line, and there might be a problem there.

Debra Eckrote, a staff member of the NTSB’s Northwest regional office in Seattle, said that TWA has not yet acted on a service bulletin alert from Boeing directing the testing of the pump wiring insulation resistance, to make sure that no conductive corrosion was in the wire terminal assembly. According to Eckrote, TWA said it is in the process of completing a modification in preparation for an FAA airworthiness directive that is being reviewed. Craycraft said that it is TWA policy to treat all alert service bulletins as FAA directives and to comply with them. Chairman Hall said that in the factual materials compiled by the NTSB no concerns were cited about maintenance of the flight 800 aircraft.