NDT is now compulsory for many aerospace companies, as it plays a safety-critical role in the production process. As an NDT’s reliable partner supporting clients in the Aerospace industry, PROMAG’s job is to make sure planes stay in the sky, take off and land safely, and millions of people travel safely every day. PROMAG has been working with aircraft parts and components manufacturers, MRO companies and other aviation professionals to continually assist our customers in improving quality, increasing production capabilities and lowering costs.
For part and component non-destructive testing, we have experience and references in providing Magnetic Particle Testing(MPI) and Fluorescent Penetrant Inspection(FPI) system solutions for structures, bridges, supports, ribs, fuselage parts, fuselage skin, turbine blades, valves, dampers, vanes, valve rings, nozzles, gearboxes, blisks, frames, compressor case details, flanges, torque tube assembly and many more.
NDT Techniques in Aerospace
Modern NDT techniques can be broadly divided into two sub-categories: surface techniques, and sub-surface techniques. Surface techniques is a term that describes those used to detect surface-level defects, such as fluorescent penetrant inspection, which can be effective at locating cracks or other flaws on the surface of the part. Sub-surface techniques are those used to recognize flaws below the surface of the part. Methods include ultrasonic testing and radiographic testing.
Fluorescent penetrant inspection, known as FPI, is also referred to as penetrating testing (PT) and is a form of liquid penetrant inspection (LPI). This is a common NDT surface application in the aerospace manufacturing industry. Components are coated with a dye that shows up under ultraviolet light. The surface coating is then washed off so that any excess dye on the surface of the part is removed, and only dye that has penetrated into surface-level discontinuity remains. The part is then examined under ultraviolet light to determine whether any dye is visible; if so, the indication is evaluated to determine the integrity of the part.
A common sub-surface technique in aerospace is ultrasonic testing (UT). Also widely used in medical applications, this works by directing high-frequency sound waves at the part to detect sub-surface defects. The sound waves travel through the part but are reflected back where flaws or material inconsistencies are present. An analysis of the reflected beam is conducted to gain insight into the meaning of the indication.
Another NDT technique that is also ubiquitous in the medical industry is radiographic testing (RT), more typically known as X-ray. This is another common sub-surface technique, particularly used to validate weld integrity. More recently, digital radiography has become available, as well as the more traditional film method.
These methods, along with magnetic particle inspection (MPI) and eddy current survey, are part of the Nadcap scope of accreditation for nondestructive testing. MPI is used specifically for defect recognition in ferromagnetic materials and works by analyzing the behavior of magnetic particles when a magnetic field is applied to the part. Similarly, eddy current testing employs an alternating current in a test coil to detect defects by inducing an alternating magnetic field in the part.
The NDT Level III must be involved in the process of verifying all NDT requirements and capabilities. According to NAS410, the NDT Level III should have the skills and knowledge to interpret codes, standards and other contractual documents that control the NDT method(s), such as customer-specific requirements. This individual must also be capable of selecting the appropriate method and technique for a specific inspection, preparing and verifying the adequacy of procedures and work instructions, and must have a good general knowledge of other NDT methods and product manufacturing and inspection. The NDT Level III also approves the NDT procedures and is technically responsible for the NDT facility and staff.
This last responsibility, for NDT personnel, is critical. Nadcap audit data shows that out of 1,085 audits conducted in 2011, five of the top ten non-conformances identified were related to AC7114 5.0, which relates to personnel certification.
More recently, the following have been identified during Nadcap audits as causing issues with compliance among the global aerospace supply chain. There is value in reviewing these to determine whether any are “easy targets” for improvement, whether or not Nadcap is applicable to your company.
- Sub-Tier Flow Down (AC7114 –2.2 and 4.2.1) The audit checklist asks whether there is “a procurement procedure to ensure that Prime contractor requirements are adequately imposed on the sub-tier supplier” for services such as calibration, cleaning/etching facilities, process control tests, and others. This typically generates a non-conformance where aerospace suppliers are not adhering to their written procedure.
- Training Plan (AC7114 –1.7) It is a requirement for aerospace companies performing NDT services to have a formal plan showing objectives for obtaining certification for trainees. Individuals shall be documented as trainees and be actively participating in a training program for a stated NDT method for a limited and specified period of time. This shall be addressed in the company’s written practice. Non-conformances arise where the “limited and specified period of time” is not addressed in the procedure, and where companies simply state that they have no trainees; however, what is important is that the procedure exists for whenever they have trainees, not that it’s not applicable right now.
- Calibration (AC7114 –1.3) The Nadcap audit checklist question states: Do purchase orders or procedures for calibration services recognize or provide traceability to requirements for the standards to be used, accuracy, range of use and number of points to be checked (three minimum unless otherwise stated in the applicable method standard) encompassing that range? Non-conforming companies do not provide the required information to the calibration agency or cannot show how they provided that information originally. Self-evidently, this has the potential to impact the integrity of the component.