Four documents cover most of the world's nondestructive testing (NDT) personnel certification: NAS410 in aerospace, SNT-TC-1A and CP-189 across general industry, power and pressure equipment, and ISO 9712 wherever certification is run centrally. They share a vocabulary — three levels, general, specific and practical exams, a vision check — which makes them look interchangeable. They are not. The biggest difference is not in any number. It is who certifies you and whether the document is a requirement or a recommendation.
This guide compares them side by side, then works through the questions that decide which one applies to you.
Employer-based or central certification
Three of the four are employer-based. Under NAS410, SNT-TC-1A and CP-189, your employer certifies you, against its own written practice. A certification is the employer's written statement that you met its requirements, so it belongs to that employer. Leave, and it ends: NAS410 revokes it when employment ends, and SNT-TC-1A treats it as revoked. A new employer can recognise your history and your earlier exams, but it issues its own certification.
ISO 9712 works the other way. An independent certification body examines and certifies you, the certificate is yours, and it travels with you between employers. The employer still decides what you may do on its work — ISO calls that operating authorisation — but the qualification itself sits outside the company.
That is why the schemes lean on each other. NAS410 and SNT-TC-1A both let an employer accept a current central certificate (ASNT, ISO 9712, or an aerospace national NDT board under NAS410) in place of part of its own exams, once the responsible Level 3 or Level III has checked that it covers what the written practice needs. The specific and practical exams, which test your employer's own procedures, products and equipment, generally stay with the employer.
The four schemes at a glance
| NAS410 (Rev 6) | SNT-TC-1A (2024) | CP-189 | ISO 9712 | |
|---|---|---|---|---|
| Published by | Aerospace Industries Association; equivalent to EN 4179 | ASNT | ASNT, approved as an ANSI standard | ISO |
| What it is | Standard: minimum requirements | Recommended practice: guidelines the employer adapts | Standard: requirements as written | Standard for central certification |
| Who certifies | The employer | The employer | The employer | An independent certification body |
| Levels | Level 1-Limited, 1, 2, 3 | I, II, III (plus limited Level II) | I, II, III | 1, 2, 3 |
| Who runs the program | The Responsible Level 3 | The employer's Level III; the 2025 addendum defines a Responsible Level III | The employer's Level III | The certification body |
| Passing scores | 70% each exam, 80% average | 70% each written, 80% practical, 80% composite | 70% each written, 80% practical, 80% composite | Set by the standard; check your edition |
| Near vision | Yearly: Jaeger 1, Snellen 20/25 or ISO 18490 Tumbling E | Yearly: Jaeger 2 (or equivalent) | Required; check your edition | Required; check your edition |
| Certification lasts | Up to 5 years (Level 1-Limited: 2) | Recommended maximum 5 years | Set by the standard; check your edition | Set by the standard; check your edition |
Our own summary of the editions listed at the end of this guide. It is a map, not a substitute: check every figure against your own copy.
NAS410: aerospace
NAS410 is a National Aerospace Standard, published by the Aerospace Industries Association and kept technically equivalent to Europe's EN 4179. Revision 6 carries a revision date of 1 December 2025 and says it takes effect no later than six months after that date. Personnel certified to an earlier revision keep their certification until it expires.
It sets minimums. Each employer writes a written practice, approved by its Responsible Level 3, which may add to the requirements but not reduce them. Prime contractors are responsible for their sub-contractors' compliance, which is why an aerospace supplier often works to a prime's supplement as well. Levels are written in Arabic numerals, and Revision 6 has four: Level 1-Limited, Level 1, Level 2 and Level 3.
Read the full breakdown in the NAS410 guide.
SNT-TC-1A: general industry
SNT-TC-1A is ASNT's recommended practice, and it says so about itself: it is a framework, not a strict specification. Its hours, question counts and intervals are recommendations the employer adopts, changes with written justification, or tightens, in a written practice approved by its NDT Level III. Once written, that practice is the rulebook — the verb becomes “shall” — and it is what an auditor holds you to.
That flexibility is why two companies can both “follow SNT-TC-1A” and still certify differently. It is also why a code that wants certainty tightens it: ASME Section V (2025 edition) makes SNT-TC-1A (2024) mandatory rather than recommended for work under the code, and adds requirements of its own. More in the SNT-TC-1A guide.
CP-189: the ASNT standard
CP-189 comes from the same society as SNT-TC-1A and uses the same three levels, but it is an ANSI-approved standard. Its requirements apply as written rather than as starting points for the employer to adapt. Certification is still issued by the employer.
Codes and contracts that want every supplier on the same rulebook invoke it, and nuclear work is the classic example. ASME Section V (2025) accepts a written practice built on either SNT-TC-1A (2024) or CP-189 (2024). See the CP-189 guide.
ISO 9712: central certification
ISO 9712 sets out how an independent certification body qualifies and certifies NDT personnel in levels 1, 2 and 3. Because the certificate belongs to the individual, it suits people who move between employers and countries, and programs that want one qualification recognised across a whole sector. In the US, ASNT runs an ISO 9712-based program of its own, which the 2024 edition of SNT-TC-1A names alongside ASNT's older certificates as evidence an employer may accept.
Which one applies to you?
You rarely choose. The work chooses for you:
- Aerospace parts, for a prime or its supply chain: NAS410 (or EN 4179 in Europe), usually with the prime's own supplement. If you hold Nadcap accreditation for NDT, the audit checks your program against it.
- Pressure equipment built or inspected to the ASME code: whatever the construction code points to. For work that references Section V Article 1, that means a written practice built on SNT-TC-1A (2024) or CP-189 (2024), with Section V's own additions, or an ISO 9712-based central program for the parts it allows.
- Nuclear in-service inspection: Section XI and the contract, which typically means CP-189 plus Section XI's own rules.
- General industry, fabrication, structural and field work: usually SNT-TC-1A, through your written practice, unless a customer asks for something else.
- Naval ships and submarines: NAVSEA's own requirements (T9074-AS-GIB-010/271), which a contract invokes on top of a personnel scheme.
- Work across borders, or for owners who want portable certificates: ISO 9712 through a certification body.
Many shops live under two or three at once — an aerospace line and a code-work line in the same building. That is legal and common, but it means one technician can hold certifications under two schemes, with different numerals, different vision charts and different clocks.
Levels 1, 2 and 3, or I, II and III
The numerals are not decoration. NAS410 writes its levels in Arabic numerals and SNT-TC-1A and CP-189 in Roman, and practitioners use them to tell at a glance which scheme a certification was issued under. The roles line up broadly — Level 1 or I follows written instructions, Level 2 or II sets up, interprets and evaluates against the acceptance criteria, Level 3 or III owns procedures, techniques and the training and examination of others — but the details differ. NAS410, for one, lets a Level 1 accept or reject parts only when the written practice says so and the Responsible Level 3 approves it.
The vision check is where schemes differ most visibly
Every scheme wants an annual near-vision check and a color check, but the chart differs. SNT-TC-1A (2024) asks for Jaeger No. 2 or equivalent at not less than 12 inches, in at least one eye, natural or corrected — and adds that eye drops that improve acuity may not be used. NAS410 Revision 6 asks for one of three options: Jaeger No. 1 at not less than 12 inches, Snellen 20/25 at 16 inches, or the Tumbling E test of ISO 18490:2015. A technician certified under both needs to pass the stricter chart. The vision exam guide goes further.
How TRACE handles it
TRACE records each certification with its method and level, and the numerals follow the scheme: NAS410 banks and exams use 1, 2 and 3, SNT-TC-1A uses I, II and III. Vision exams, recertification dates and the documents behind them sit on the same technician record, with reminders before each one falls due.
If you examine your own people, the Certification Testing module runs general, specific and practical exams and credits the score to the certification, and the NDT Question Bank supplies original questions by method and level, with sets written to NAS410, to SNT-TC-1A and to individual standards.
Frequently asked questions
What is the difference between NAS410 and SNT-TC-1A?
NAS410 is a mandatory standard for aerospace NDT personnel: its minimum training hours, experience, exam scores and five-year recertification are requirements, and the Responsible Level 3 answers for the program. SNT-TC-1A is a recommended practice for general industry: it offers recommended hours and rules that each employer adapts in its own written practice, which then becomes the rulebook. Both are employer-based, and both use three main levels, but NAS410 writes them 1, 2 and 3 and SNT-TC-1A writes them I, II and III.
Is ISO 9712 accepted in place of SNT-TC-1A?
Sometimes, and only where the contract or code says so. ISO 9712 certification is issued by an independent certification body rather than by your employer. ASME Section V (2025), for example, lets an ISO 9712-based central program cover training, experience and the general and method exams, but the employer's written practice and its own specific and practical exams still apply. NAS410 and SNT-TC-1A both let an employer credit a current ISO 9712 or other central certificate toward parts of the exam process when the Level 3 or Level III has checked it.
What is the difference between SNT-TC-1A and CP-189?
Both come from ASNT and both are employer-based. SNT-TC-1A is a recommended practice written with "should", which the employer adapts; CP-189 is an ANSI-approved standard written with "shall", so its requirements apply as written. Contracts and codes that want a fixed rulebook, rather than each employer's own version, invoke CP-189.
Is NAS410 the same as EN 4179?
They are maintained to match. NAS410 Revision 6 states that it is technically equivalent to EN 4179, Edition 7, and it lets personnel certified to earlier revisions of either document keep their certification until it expires.
How long does an NDT certification last?
Under NAS410 and SNT-TC-1A, five years at most, with the expiry falling on the last day of the month. NAS410's Level 1-Limited certifications last two years at most. In between, the vision exam (yearly for near vision) and, under NAS410, the annual proficiency review can suspend a certification that has not yet expired.
Checked against
- NAS410 Revision 6, Aerospace Industries Association, revision date 1 December 2025
- ASNT SNT-TC-1A, 2024 edition, with the 2025 addendum
- ANSI/ASNT CP-189, 2016 edition with the 2018 addendum (ASME Section V, 2025 edition, cites a 2024 edition of CP-189, which we have not reviewed)
- ASME Boiler and Pressure Vessel Code Section V, 2025 edition, Article 1 (T-120)
This guide summarises the documents above in our own words to help you find your way around them. It is not a substitute for them: requirements change between editions and contracts add their own, so check every figure against your own licensed copy and your written practice before you rely on it.
Named for reference only. TRACE is an independent product: it is not affiliated with, endorsed by, sponsored by or accredited by ASNT (publisher of SNT-TC-1A and CP-189), the Aerospace Industries Association (publisher of NAS410), ASD-STAN (publisher of EN 4179), ISO or ASME, or by any other organization whose standards or programs it describes. Their names and marks are their own.