Operators are seeking better, less expensive ways to evaluate ageing airframe condition

Graham Warwick/ATLANTA

While cost-conscious carriers are keeping aircraft in service longer, they want to minimise the maintenance burden of ageing airliners. New inspection techniques provide one answer.

The goal of NASA's Airframe Structural Integrity Programme (NASIP), launched in 1990, is to use non-destructive evaluation (NDE) instead of a tear-down inspection to assess aircraft condition. The goal is "still a few years away", says Edward Generazio, head of the Non-Destructive Evaluation Sciences Branch at NASA Langley, in Virginia, but several promising technologies are in the process of being commercialised.

Technologies being developed under the NASIP are designed to be easily portable, so that aircraft can be inspected on the ramp, rather than in a hangar, Generazio says. Most approaches being pursued provide real-time imagery of the aircraft structure, to enable location and extent of damage to be assessed.

NASA is commercialising four NDE technologies based on advanced thermographic, radiographic, ultrasonic and eddy-current inspection techniques. Field trials with airlines, Boeing and McDonnell Douglas have been completed, and probability-of-detection testing is under way to determine how effective the systems will be in everyday use.


Developing technology

Thermography is a technique for inspecting bonded structures, metallic or composite, by measuring the way heat is conducted across the bonds. NASA is working on thermal diffusivity, an enhancement of thermography able to detect disbonds in bonded lap-joints. The aircraft skin is heated with a flash or quartz lamp, and an infra-red camera is used to observe the area as it cools. Generazio says that thermal diffusivity "...is an excellent means of wide-area evaluation".

Another technology under development is reverse-geometry X-ray. "This is the opposite of radiography," Generazio explains, in that the X-ray source is located outside the structure and the detector inside. The point detector can be located anywhere inside the aircraft and the point source scanned across the surface of the area to be inspected.

The technology is safer and cheaper than conventional radiography, he says. The inspection can be conducted safely without the need for a lead-lined room. Expensive X-ray film is not required, as the scanning system produces a real-time video output.

Instead of the typical "shadow" image produced by radiography, reverse-geometry X-ray generates a "laminagraphic", or layered, image similar to a tomographic body-scan, Generazio explains. This allows the system to be used to determine the location and thickness of corrosion within a structure, he says.

Ultrasound is an established technique for inspecting metallic and composite structures for defects and damage. NASA has developed ultrasonic technology which allows quantitative measurement of skin corrosion. The technique is capable of measuring the percentage change in skin thickness to within 3%, Generazio says.

The system can also distinguish between disbonds and corrosion. A neural-network processor, which is "trained" to recognise the different defects, provides "close to 100% classification accuracy, even through paint", he says. The system is hand-scanned over the aircraft's skin, "like a paint brush", and uses a pulse-echo ultrasonic transducer.

The first of the NDE technologies to be licensed to industry for commercial application is advanced eddy-current inspection. This is being used initially to detect surface-breaking fatigue cracks, but is being developed further to enable measurement of the thickness of hidden skin-corrosion.

Krautkramer Branson, a subsidiary of Emerson Electric, began delivering its Crack Finder eddy-current inspection system in February, less than six months after it signed a technology-transfer agreement with NASA and 12 months after it was selected as an industry partner to commercialise the technology.

The Crack Finder is a hand-held unit designed for rapid scanning of aircraft skins to detect surface-breaking cracks. The system uses self-nulling eddy-current technology developed by NASA which enables the device to recalibrate automatically to zero "...so that each imperfection produces a reading or reaction upon detection", the agency says.

In standard eddy-current testing, cracks are detected as variations in the electrical field induced in the skin by a magnetic probe. Training is required to enable inspectors to interpret the readings.

Dr Min Namkung, of the Non-Destructive Evaluation Sciences Branch, says the key features of the Crack Finder are: "...rapid scanning speed, simplicity of use, minimal operator training requirements and low cost". NASA says the "Simpson probe" technology, named after developer John Simpson of Lockheed Engineering and Sciences, is "...highly accurate, reliable, simple and economical".

Krautkramer is now developing other products based on the self-nulling eddy-current technology, including metal-thickness and coating-thickness gauges. The metal-thickness gauge, scheduled to be available in mid-1995, will be able to detect corrosion on aircraft skins, the company says.



Generazio says that the other NDE technologies are all in various stages on commercialisation, with thermal diffusivity closest to being ready for transfer to industry. "It takes about nine months to commercialise a technology," he says. NASA is working with several companies on each of the technologies and plans to sign several licence agreements covering different applications of each technique.

As the NASIP is focusing on the problem of assessing the condition of ageing aircraft, much of the effort so far has been on developing NDE technologies applicable to metallic structures. Most of the technologies developed can be used, with modification, to inspect composite structures, Generazio says. Future work will develop NDE systems specifically for inspecting composite structures for damage, he suggests.

While NASA has been developing new technologies, the US Federal Aviation Administration has been concerned with the determining the effectiveness of existing and emerging non-destructive inspection (NDI) techniques in airline service. To this end, it established the Ageing Aircraft NDI Validation Center (AANC) at Sandia National Laboratories in Albuquerque, New Mexico.

One of the centre's first tasks was to assess the reliability of eddy-current inspections, in use for detecting cracks around rivets, which can provide early signs of corrosion or fatigue in lap joints. To check the capability and reliability of eddy-current methods in use, Sandia built a simulated fuselage-section which could be shipped to maintenance centres to provide a common assessment exercise for all inspectors.

The 7m-long test section, featuring damage commonly found in ageing aircraft, was taken to seven maintenance centres to test the inspectors' ability to detect various sizes of crack. The experiment also collected data on the human factors involved in inspections, and their impact on the results obtained.

The tests showed that, while crack-detection requirements were being met, there was a wide variation between centres, and between inspectors, in how the eddy-current equipment was being used. The results are being used to improve maintenance practices and procedures, Sandia says, and the FAA plans to run similar experiments to assess the reliability of emerging NDI technologies. Generazio says that NASA is already working with the Sandia-based AANC to validate advanced inspection techniques.



KC Aviation mobilises NDT

KC Aviation, a Dallas, Texas-based company which specialises in business-aircraft maintenance and modification, is expanding its fleet of mobile non-destructive-test (NDT) vehicles to meet customer demands for on-site inspections to avoid aircraft downtime.

The company, a subsidiary of paper-products giant Kimberly-Clark, has four vehicles based at its Dallas, Appleton (Wisconsin), and Westfield (Pennsylvania) sites. A fifth vehicle will be based in Atlanta, Georgia, from late June, and a sixth will be added in Los Angeles in 1996. KC also has mobile engine-diagnostic and repair capabilities.

The company says that more of its customers are performing C-checks at their home bases, but lack the required inspection capabilities. KC's vehicles are equipped to perform a wide range of non-destructive inspections, including: die-penetrant, eddy-current, magnetic-particle, radiography and ultrasonic. All chemicals used are recovered for disposal by KC. The vehicle water-systems are closed-loop, to avoid site contamination.

KC uses a Phillips Comscan X-ray system to inspect the aft pressure-bulkheads of Gulfstream II and II business-jets for corrosion. The containerised Comscan unit allows the bulkheads to be inspected in situ, without the need to remove skin panels. The system has a video output, eliminating the need for film.

The Comscan unit uses Compton-effect backscattered radiation, allowing both the X-ray generator and image-intensifier detector to be mounted on the same side. (In conventional radiography, the X-ray source and the film have to be located on opposite sides of the structure to examined.) This simplifies inspection of the hard-to-access pressure-bulkheads.

KC says that the Comscan is one of only five such units available in the USA for Gulfstream inspections. The unit was acquired in a joint venture with Dallas Airmotive, which has a contract to provide radiographic inspection services to E-Systems' aircraft-modification division in Greenville, Texas.


Source: Flight International