Gulfstream Stays Safe with UT
“advanced” UTPA has significantly reduced aircraft manufacturing and in-service inspection time and cost throughout the entire aviation industry.
Aviation Maintenance magazine asked Gulfstream Aerospace Corp. to describe in its own words how it optimizes ultrasonic testing.
Gulfstream uses ultrasonic NDT to inspect both metal and carbon-fiber composite structures for structural integrity. This testing enhances the confidence Gulfstream customers have in the safety and quality/reliability of the company’s aircraft. By using NDT, including ultrasonic NDT, in the manufacturing pro- cess, an OEM can detect possible discrepancies during manu- facturing that are non-conforming to type design to assure that proper corrective actions are taken prior to issuance of the standard certificate of airworthiness. Gulfstream also uses ultra- sonic NDT in its product support organization for aircraft repair and maintenance. We use different ultrasonic techniques, including immersion squirter and immersion reflector plate, C-scans and A-scans to examine flight control surfaces such as wings, flaps and stabilizers. We require our suppliers of flight control structures/parts to do in-house ultrasonic NDT testing before they send those structures/parts to us.
Gulfstream uses C-scan systems as well as portable hand- scan equipment. This type of equipment uses high-frequency sound waves that are transmitted into a material to detect possible imperfections or to locate possible changes in mate- rial properties. An example of a C-scan system that we use is the MAUS V system. This piece of equipment can scan deep into multilayered aluminum in search of corrosion and/ or cracks. An example of a portable type of ultrasonic NDT equipment that we use is an OmniScan.
Most non-structural components are not inspected using ultrasonic NDT. The additional expense may not be required. NDT methods are used when engineering-defined standards are available to ensure the NDT equipment used has the capability to detect the minimum flaw size for the specific component being tested.
— Mitch Birzer, senior manager of quality assurance, Gulfstream Aerospace Corp., Savannah, Ga.
Composites and UT Aircraft manufacturers’ like Boeing and Airbus usage of composite materials has been expanding from secondary to primary structures over the past years. An example of this is contrasting the materials used to construct the Boeing 777 to the newer Boeing 787. The 777 is comprised of 12 percent composite materials, and 50 percent aluminum. However, the 787 is comprised of 50 percent composites and 20 percent aluminum. The Airbus A380 and A350 incorporate significant composite structures, and even military aircraft like F22 fighter and A400 transports have seen composite usage growth. Composite usage has risen because it can be tailored to be lighter and stronger than its aluminum equivalent, more damage tolerant, and more repairable, allowing aircraft to fly faster, farther and carry heavier loads while costing less to maintain. This aircraft composite material surge has necessitated the development of NDT techniques that can identify internal composite structure cracking, material porosity, dis-bonds and delamination, all of which can compromise structural integrity. Typical carbon fiber composites transmit ultrasound efficiently, and the flaw detection and thickness gaging techniques that have long been applied to metal parts can be readily adapted for most composite material inspection. Metal airframe structures deform/dent upon impact, and this damage is usually identifiable with visual inspection. However, “many composite structures show little or no visual evidence of impact damage,” says Lasser. “In particular, low-velocity impacts can cause a significant amount of delamination, even though the only external indication of damage may be a very small surface indentation. This BVID can cause significant degradation of structural properties. If the damaged laminate is subjected to high- compressive loading, buckling failure may occur. Ultrasonic NDT is particularly well suited to finding BVID.”
While it excels at testing composites, as with all NDT methods, ultrasonics has limitations. Aerospace surfaces must be accessible to transmit ultrasound. It cannot successfully inspect any material where it cannot penetrate or provide resolution. Components that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect. Linear defects oriented parallel to the sound beam may go undetected. For components that are riveted and not bonded, ultrasonics cannot pass through the air gap to inspect below them. “Eddy current testing is typically used for many inspections like cracking around, between and inside of aircraft fastener-holes, because it is a much faster and/or more reliable NDT inspection method fastener-hole inspection,” says Nelligan. Also, materials that are highly attenuative absorb the ultrasonic signals and can make it very difficult to inspect using the NDT ultrasonic method. These types of materials usually require an alternative NDT method and instrumentation such as bond testing. “Solid foam components are not testable with UT, as UT does not penetrate solid foam,” says Maurer. “Components sensitive to humidity should not be used by liquid-coupled UT.”
Learning ultrasonics Skill and training is more extensive with ultrasonics than with other test methods. Inspectors need adequate training so they can definitely answer, “is the UT inspection I’m performing ensuring the quality or flightworthiness of this part?” To get to this point, UT operators are subject to certification. They follow training courses
34 Aviation Maintenance |
avm-mag.com | February / March 2012
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