Lufthansa Technik is responsible for keeping the Lufthansa fleet in shape. It was facing a huge challenge due to required fuselage inspections of all Boeing 737s prescribed by Boeing as a consequence of the “Aloha” accident in 1998. The inspections were demanding monumental man-hours and cost to perform effectively.
The hull of a Boeing commercial jet is manufactured using a variety of different materials and fasteners, including aluminum and rivets, carbon fiber reinforced plastics, honeycomb structures, and other composites. Due to constant temperature changes, pressurization and de-pressurization, vibration, and high wind loads, material fatigue occurs. Typical damages include de-riveting, disbonding, cracking, and delamination. Water inclusion in composite materials can also cause cracks at high altitudes as a result of expansion when turning into ice.
Automation Technology (AT) GmbH has developed a thermographic-based technology that was used in other industrial areas for similar fault detection. After meeting with Lufthansa at an air show in 2002, Michael Wandelt, AT Managing Director, said that a subsequent test of its nondestructive testing (NDT) process proved that larger fuselage areas could be easily examined, and disbonding and other defects could be precisely located.
“However, to be able to use this in our maintenance program, it first needed the blessing of Boeing and of the FAA,” said Peter Feddern, Dipl. Engineer, Lufthansa Technik AG.
Therefore, in 2003, AT and Lufthansa organized a presentation in Berlin, Germany, where leading members of Boeing’s technical staff were invited to a demonstration.
“Within one day, the complete fuselage of a B-737 was inspected, work which would have required around 1100 man-hours using classic inspection techniques. After this presentation, Boeing was convinced,” said Cristian Ferber, AT Senior Applications Engineer. “The remaining steps were certification of the system by the FAA and by other aviation administrations worldwide.”
With the support of AT, Lufthansa Technik wrote the inspection procedure and submitted all required documentation for certification by Boeing and the FAA. “This process took about three years; the main reason for the delay was due to the fact that they recognized that this IR-NDT system can be also used in a much wider range of aircraft inspection tasks than just for the inspection of the fuselage,” Wandelt said.
Marcus Tarin, President of MoviMED, AT’s U.S. partner, explained that the development of the LockIn Thermograph technology was tied directly into the concurrent availability of reasonably priced IR cameras from FLIR. This allowed the design of a quality thermograph-based NDT system that was both highly effective and affordable.
The technology uses thermal wave propagation algorithms within a proprietary computerized system. Custom-designed power electronics generate a heat source, and the FLIR IR camera records the thermal modulation generated from the area being inspected. These thermal images and data are fed back to a standard Windows-based PC containing a custom-designed PCI board with frame grabber that pinpoints any defects.
Tarin said that this technology can effectively inspect large sections of the fuselage in only minutes. “It is also possible to locate defects at different depths below the surface by varying the amplitude of the stimulus wave,” he said.
LockIn Thermography was certified by the FAA in December 2005, followed by the aviation administration of Germany in 2006.
“It was then integrated into the NDT procedures for all B-737s by Boeing. At Lufthansa Technik, the system has been in use since May 2007,” Wandelt added. “It is used for the inspection of the bondings between the molding skin and the inner structures of the fuselage. These bondings are very important to assure that no cracks will appear that could affect the safety of the airplane,” he said.
Today, NDT inspection of a Boeing 737 fuselage at Lufthansa Technik, comprising the complete “pressure-cell” one complete inspection, takes about 100 man-hours. When compared to the old procedure, this represents a time-saving of about 1000 h.
Feddern said that while using the new technology saves Lufthansa Technik time and money during maintenance/overhaul, it also provides the ability to perform tests that could not be performed previously. “We are very satisfied with the inspection results.”