Keeping Aging Planes Healthy
F
ew industries dwell more in the public eye than the airlines. Thus, when Aloha Flight 243 made an emergency landing at Maui's Kahului Airport on April 28, 1988, missing an 18-ft section of upper fuselage, the world took notice. The lost section, which formed the roof of the passenger compartment, had suddenly and violently ripped free while the 19-year-old Boeing 737 was traveling 330 mph at 24,000 ft. The rapid decompression threw a stewardess out of the plane and injured 61 passengers (1). The publicity surrounding the dramatic accident alerted the public to the fact that U.S. carriers are flying large numbers of older planes. The incident also warned federal agencies and Congress that it was time to reevaluate inspection and maintenance procedures. One immediate result of this heightened national concern has been greater support for developing new inspection procedures, especially those classified as nondestructive. The entire U.S. plane contingent now averages around 12 years in age. However, that value varies with individual carriers. Eastern's fleet, for instance, averages about 14 years, whereas Delta's aircraft average 8.4 years (2). Twenty-year-old planes such as Alo-
ha's Boeing 737 are not uncommon. Given the rising cost of new planes and the fact that more people are flying, U.S. carriers will probably continue to fly older planes. In contrast, many European carriers fly newer planes. West Germany's Lufthansa rolls out Boeing 747s that average 5.1 years in age, and British Airways fields a fleet averaging about 8 years of use (2). However, old planes are not the problem; it is the job of inspecting and maintaining their airworthiness that is difficult. In their review of the Aloha accident, the National Transportation Safety Board attributed the incident to metal fatigue and weakened adhesive bonding between fuselage sections. According to the board, Aloha and Boeing failed to properly maintain and correct problems on the 737. "Things break and cracks appear," says Stephen Bobo, a mechanical engineer with the Department of Transportation's Research and Special Programs Administration in Cambridge, MA. Airlines are responsible for careful inspections for three constant enemies: corrosion, cracking, and loss of adhesive bonding. (A significant amount of adhesive is used to glue down overlapping sections of aircraft). Visual inspection by trained technicians has al-
ways been essential in spotting damage, but nondestructive tests now also assume an important role. To understand the process of inspecting planes and the techniques being developed, it is important to understand the maintenance procedures airlines now follow. As regulated by the Federal Aviation Administration (FAA), airplanes now undergo a clearly defined and hierarchical series of inspections. The timing of these checks varies with each air carrier, the differ-
FOCUS ent types of planes it flies, the availability of maintenance facilities, and the numbers of technical personnel working. FAA enforces inspection schedules by random and unannounced checks on airline inspection facilities. "Basically, there are four categories of scheduled maintenance, commonly referred to as A through D checks," explains Fred Duvall, manager of FAA's Flight Standards Division Aging Fleet Office in Seattle. "An A check is a primary inspection looking at the general condition of the aircraft." Mainte-
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FOCUS nance personnel visually check the plane for obvious signs of damage. On the venerable Boeing 727, A checks or their equivalent are per formed, on a worldwide average, about every 149 h of flight time. Because most planes are in use approximately 8 h per day, an A check occurs almost every 20 days. "The Β check," continues Duvall, "is an intermediate check, an operational check for airworthiness." Switches and warning lights are tested, and oil filters might be changed. For 727s, this in spection occurs on average about every 612 h. At the same time the A check is repeated, just as the A and Β checks will be repeated during the C check. "You are constantly duplicating the lower checks," says Duvall. During the C inspection, access doors and panels are removed so that pumps, systems, and electronics can be tested against specified standards. For example, fuel pump low-pressure sys tems are tested, the autopilot is evalu ated, and the VHF radio is checked. On 727s, C checks average every 3000 h of flight time, or roughly once a year. "The D check is the most extensive, an intensified structural inspection," says Duvall. The airplane is partially dismantled and internal structures are carefully examined for structural in tegrity. Some interior cabin equipment is removed and most panels are opened. Depending on the size of the staff, D checks can last from 30 days to up to 3 months. Costs to the company can run well over half a million dollars. Engine inspections are carried out by another procedure, usually at the same time as the airframe inspections. The important parameter for jet engines is cycles that correspond to each round of start up through shutdown. Finally, FAA issues airworthiness directives whenever significant con cerns arise about the continued airwor thiness of an airplane fleet. One such directive after the Aloha incident man dated the immediate inspection of 737s. Most of the testing, which takes place during the D check or its equiva lent, is nondestructive. Three nonde structive methods are now widely used: eddy currents, ultrasonics, and X-ray analysis. These techniques can exam ine only small areas at a time and therefore are applied to points espe cially vulnerable to cracking and corro sion. For instance, X-rays in the 200300-kV range probe for cracks in the frame surrounding the pilot's sliding window, and the eddy current tech nique regularly checks tire rims. Eddy current testing uses a pencilshaped probe to search for cracks, es
pecially subsurface defects. The probe contains a coil through which a highfrequency alternating current runs. As sociated with the current is a magnetic field, which in turn induces a current in the test material. Therefore the probe never touches the test surface. Defects in the test section perturb the induced current, leading to a reduction in con ductivity. This affects the sample's im pedance, which is detected by a second coil in the probe. "The depth of penetration depends on the frequency," explains Stephen Bobo. Examination of the 0.039-in. (0.99 mm) thick metal skin of a Boeing aircraft or the 0.050-in. (1.3 mm) deep sections of a McDonnell Douglas DC-9 requires frequencies in the range of 20-
What seems to be needed are automated techniques that examine wide areas and detect various problems.·· 500 kHz. Practical considerations limit eddy current testing to a maximum depth of about 0.25 in. (6.4 mm). "Ultrasound," says Bobo, "is more useful since it is not limited by depth. It detects corrosion and, in some cases, adhesive dis-bonding." High-frequen cy pulses of sound are sent into the test site, and the rate of signal return is measured. Defects or thinning from corrosion send the signal back sooner. Following the Aloha incident, the U.S. government began reevaluating its inspection procedures. In 1988 Con gress passed the Aviation Safety Re search Act, which led to the establish ment of a National Aging Aircraft Re search Program by FAA. Congress specifically mandated that FAA use 15% of its research budget on long-term projects (3). The airline industry has responded with its own initiative, the Airworthi ness Assurance Task Force. This fo rum, which examines issues related to aging aircraft, is a who's who of the flight business. The task force includes representatives from American and foreign airlines, manufacturers, FAA and similar agencies in other countries, NASA, and the military. The group has already recommended an $800 million maintenance program for aging Boeing planes (4). NASA is also taking a more active
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role in the national overhaul of inspect ing older aircraft. Under the leadership of FAA, NASA is developing new test ing methods. "What seems to be need ed are automated techniques that ex amine wide areas and detect various problems," says NASA's Thomas Crooker, program manager for materi als in the Office of Aeronautics and Space Technology in Washington, DC. Airlines also want tests that are eco nomical and fast. Toward that end NASA plans to investigate a number of new nondestructive tests. One of the most promising is thermal analysis. There are several versions of this procedure. Essentially, a large section of the airframe is heated while an IR video camera sensitive to 0.1 °C tem perature differences images the test section. Anomalous heat flow indicates problems. Thermal analysis is particu larly useful for checking laminated sur faces for dis-bonding. Furthermore, as a "stand-off technique, it need not physically contact the plane. NASA al ready uses this technique to examine the solid rocket motors and carboncarbon brakes on its Space Shuttle. At NASA-Langley, researchers are refining this technique by what Joseph Heyman, head of the Nondestructive Measurement Science Branch, calls "inversion of the thermal data. Instead of just looking, we are telling it how to heat." Heyman's group models thermal dif fusion in large surfaces and determines which properties provide the highest contrast for defects. What they learn is how to collect and analyze the data. "No one cares if the plane is painted uniformly," he says. Instead, their ap proach provides a more quantitative assessment of the data, which in turn offers a clearer picture. Heyman also has some preliminary data suggesting that thermal analysis could detect cor rosion. Laser holography, another promis ing method under investigation, also looks at large sections for dis-bonding. A large-area laser scanner is used to measure displacements of a test section under stress. Uneven expansion could indicate structural problems. For example, by pressurizing the air craft's cabin, as if in flight, the fuselage can be forced to expand approximately 0.10 in. (2.5 mm). (This normal bulging of the hull is a major contributor to metal fatigue.) The laser technique now being developed can detect in- and out-of-plane displacements as small as 0.1 μτη in a surface with vibrations up to 50 kHz. This method could resolve tiny strains in the metal before they became a major problem, and early de tection would lower repair costs.
Other, more long-term approaches are also being pursued. Acoustic emission is a well-established technique for detecting cracks in simple devices such as pressure bottles or storage tanks. Changes in the internal pressure can cause cracks to form or grow. As the crack expands, it emits sounds that can be picked up by transducers for analysis. However, this type of analysis becomes difficult on an object as complicated as a jet plane. "We are looking at the next generation, which we feel will work on complex structures," says Heyman. Instead of using transducers that measure a very narrow acoustic range, NASA researchers are employing broad-band transducers to avoid missing important information. In addition, they have distributed the transducers across or have incorporated them into the test surface. They then collect sound waves before they propagate a great distance, aiding in the interpretation of the data. Finally, NASA is studying ways to obtain more information from data. An algorithm for ultrasound analysis, called minimum error deconvolution, works in real time and provides sharper resolution. The program not only examines the sound data being collected, but also factors in information about the transducer recording the signal. With this program ultrasound could examine laminated surfaces. NASA is also looking at fatigue testing. The agency has been evaluating proof testing, a controversial technique that deliberately overloads the plane to look for multisite problems. Planes that fail a test such as overpressurizing the fuselage are taken out of service. However, many workers worry that proof testing could actually introduce damage in planes certified as flightworthy. NASA plans to publicly release the results of its evaluation of proof testing in the near future. With Congress, federal agencies, and the public now keenly interested in proper maintenance of aging aircraft, it is apparent that the accident on Aloha Airlines marks a turning point for the airline industry. The changes instituted by the United States as a result of this accident will have worldwide ramifications. As Duvall summarized it, "[The U.S.] sets the marching order." Alan R. Newman
References (1) Time May 16,1988, pp. 62-65. (2) Aviation Week & Space Technology July 24,1989, p. 69. (3) Aviation Week & Space Technology July 24,1989, p. 60. (4) Aviation Week & Space Technology March 6, 1989, p. 64.
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