NDT in Aerospace Manufacturing

What can fail, and why it matters

Aerospace parts are designed for long lives under repeated loads, and the dominant threat is fatigue: small cracks that start at a flaw and grow a little with every cycle. In manufacturing, the flaws that matter include inclusions and voids inside the metal, forging laps and seams, porosity in castings, cracks from heat treatment or grinding, and in composites, porosity, delaminations and foreign material trapped between plies.

The consequences of missing one can be severe. On July 19, 1989, United Airlines Flight 232, a DC-10, lost all hydraulic power after its tail-mounted engine's stage 1 fan disk broke apart in flight, and the aircraft crashed during an emergency landing at Sioux City, Iowa. The National Transportation Safety Board report (NTSB/AAR-90/06) traced the failure to a fatigue crack that grew from a metallurgical defect in the titanium disk, and found that the crack had not been detected during an earlier overhaul inspection. The board cited human factors limitations in the inspection and quality control procedures. The accident shaped aerospace thinking on melt quality, inspection of rotating engine parts and the reliability of inspection processes — not just the methods, but how consistently people apply them.

Beyond safety, escapes are expensive. A nonconforming part discovered at the engine or airframe maker can mean quarantines, record reviews across many lots and a customer corrective action that follows a supplier for years.

Where NDT is used

• Liquid penetrant testing (PT) is one of the most common methods in aerospace manufacturing, used on machined, forged and cast parts in non-magnetic alloys such as aluminum, titanium and nickel superalloys.
• Magnetic particle testing (MT) is used on steel parts such as landing gear components, shafts and fasteners to find surface and near-surface cracks.
• Ultrasonic testing (UT) examines billet, bar, plate and forgings for internal defects before and after forging, and is a primary method for composite laminates, often with automated immersion or squirter systems.
• Eddy current testing (ET) detects surface cracks, checks bolt holes and sorts materials by conductivity to confirm heat treatment.
• Radiography and CT (RT/CT) examine castings, welds and, increasingly, additively manufactured parts.
• Visual inspection (VT), thermography and bond testing support composite and bonded structure inspection.

Macro-etch and blue etch anodize inspections, which reveal grain flow and processing damage, are used alongside NDT on some parts, and the line between them is set by the customer specification.

How inspection fits into the supply chain

A single engine disk can pass through a melter, a billet converter, a forge, a heat-treater, a machine shop and a finishing house, and several of them may perform NDT. Each step is defined by the engineering drawing and the customer's process specifications, which say which method, which technique, which sensitivity and which acceptance criteria apply.

Because the part leaves each supplier looking the same whether it was inspected well or badly, the industry controls the process rather than trusting the result: approved procedures, qualified and certified personnel, calibrated and verified equipment, controlled materials such as penetrants and couplants, and records that tie every inspection to the part serial or lot.

Standards and programs

• NAS410 sets requirements for the qualification and certification of NDT personnel in the aerospace industry, with certification levels written as Level 1, Level 2 and Level 3. Its European counterpart is EN 4179.
• Nadcap, administered by the Performance Review Institute, provides industry-managed accreditation of special processes including NDT. Many primes require it of suppliers performing NDT on their parts.
• ASTM standard practices such as E1417 (liquid penetrant) and E1444 (magnetic particle), and SAE documents such as AMS 2644 for penetrant materials and AMS-STD-2154 for ultrasonic inspection of wrought metals, are frequently invoked by customer specifications.
• AS9100 quality management requirements and customer-specific supplier quality manuals sit over all of it.

Customer specifications frequently tighten or modify these general documents, so the governing requirement for a particular part is always the combination of drawing, purchase order and flowed-down specifications.

What this means for buyers and suppliers

If you buy aerospace parts or NDT services, verify the basics before the first order: is the supplier approved by your customer for this method, is its Nadcap accreditation current for the right scope, and are its inspectors certified under NAS410 to the level the work requires? Then look at the details auditors look at — procedure revision control, the Level 3's involvement, eye examinations, penetrant system checks, and how the supplier handles a rejected part.

If you are a supplier, the most common problems are not exotic. They are an expired eye exam, a certification that lapsed without anyone noticing, a procedure that no longer matches the way the work is done, or a daily system check that was skipped. Each is small; together they are what turns an audit into a finding.

How NDT LLC can help

NDT LLC helps aerospace suppliers build and maintain NDT programs, write NAS410 written practices and procedures, prepare for Nadcap audits and respond to findings, and provide training for personnel at every level. Contact us to discuss your program or an upcoming audit.