Aerospace is an industry that has experienced great growth in recent decades. NDT in aerospace has a special driver of its own due to the high levels of human traffic involved; the crash of a civil or military airliner has the ability to cause loss of life reaching catastrophic proportions. Therefore, strict NDT specifications have been set to detect very small cracks and defects in engine turbo discs, blades and airframe structures, in both production and ongoing maintenance.
Digital Radiography (DR) has existed in various forms (for example, CCD and amorphous Silicon imagers) in the security X-ray inspection field for over 20 years and is rapidly replacing the use of film for inspection X-rays in the Security and NDT fields. DR has opened a window of opportunity for the aerospace NDT industry due to several key advantages including excellent image quality, high POD, portability, environmental friendliness and immediate imaging.
DR in Ongoing Aerospace Maintenance
Israel Aircraft Industries’ (IAI) NDT lab provides NDT services for both production and maintenance of aircraft. It uses a digital flat imager in a wide range of maintenance applications: In one case, a nitrogen pressure tank was malfunctioning and exploding during pressure testing. Eight different shots of each tank using an Image Intensifier with a flat-panel, portable DR system revealed tiny cracks 0.05 mm wide and 0.4 mm long in the weld. An attempt was made to use film X-ray but without any success until the DR system was introduced.
A table put together by Mr. David Belo, IAI Radiography Level III Expert at the recent ISR ASNT Section, compares the advantages of DR X-ray as compared to film X-ray. The numbers speak for themselves; significant savings of time and money (annual savings of over $64,000 for a scope of 7,250 images) have been achieved.
DR in the Service of the US Air Force
The US Air Force (USAF) primarily requires NDT inspection for ongoing maintenance purposes, in order to inspect different parts of its aircraft for fatigue, corrosion and ageing (including wing sections, rotors and more). In 2000, the USAF carried out a study comparing DR to film X-ray methods. Due to the fact that deployment for NDT inspection purposes involves moving a vast amount of equipment and aircraft, the USAF sought a cost effective and compact mechanism to perform its NDT maintenance inspections.
It discovered that flat panel, battery-operated digital X-ray systems were in fact cheaper and faster, with a much smaller footprint (space required for shipping) than traditional film equipment (a 65% to 97% reduction).
DR flat-panel based X-ray systems required only 84 minutes for deployment, while film based X-ray equipment required several hours. Thus, DR featured 43% savings in the amount of time required to perform the inspection as compared to the film-based technology. Even more important was the fact that DR based NDT inspections offered a unique battery-operated, portable solution at unparalleled lower prices. According to the USAF, cost assessment showed that economically the Air Force could realize a return on its investment in only 3 months. The conclusion was quite clear – DR meets the needs of Expeditionary Air Force (EAF) technical orders.
These advantages can be observed in practical terms when considering another maintenance case: The rear rotor of an Apache broke off as the aircraft was taxiing on the runway before takeoff. According to procedure, the entire Apache fleet was grounded until each aircraft underwent NDT inspection. Due to the use of a portable DR X-ray system, the entire inspection was conducted in 2 days. If a film-based system had been used for this purpose, the process would have taken at least a week or more. This grounding procedure also applies when a civil aircraft malfunctions. Here, too, a portable DR system can save many man hours of inspection work, thus reducing economic consequences for the airline while ensuring top-rate inspection and preserving ultimate human safety without any compromise.