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The Complete Guide to FAA Airworthiness Directive (AD) NDT Compliance

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The Complete Guide to FAA Airworthiness Directive (AD) NDT Compliance

Introduction

An Airworthiness Directive is the FAA telling you, in legally binding language, that a known unsafe condition exists on a type of aircraft and that you have to do something about it before the airplane keeps flying. A large share of those directives are closed out with nondestructive testing. The AD names a part, describes the crack or corrosion the FAA is worried about, and points you at an inspection. Most of the time that inspection is eddy current, phased array ultrasonic, radiography, or fluorescent penetrant on a specific structure with a specific threshold and a specific repeat interval. A concrete method example is the eddy current array special detailed inspection of Boeing longitudinal skin splices.

That is where a lot of operators and repair stations get tripped up. The AD is the law, but the AD usually does not contain the inspection procedure. It references a service bulletin or an NDT manual, and the actual technique lives there. Getting compliance right means reading the directive carefully, pulling the correct revision of the referenced document, running the inspection to that exact technique, and documenting it so an FAA inspector or a records auditor can trace every step. This guide walks through how that works, what standards govern the people and the methods, and how Baron NDT closes out AD inspections day in and day out.

What an Airworthiness Directive actually is

The FAA issues ADs under 14 CFR Part 39. When the agency finds an unsafe condition that is likely to exist or develop on other products of the same type design, it publishes a directive that makes corrective action mandatory. You cannot legally operate the aircraft once an AD applies to it unless you comply with the directive or hold an approved alternative method of compliance.

ADs fall into a few buckets. Some are one-time inspections. Some are repetitive, meaning you inspect, then re-inspect at a set interval (flight cycles, flight hours, or calendar time) for the life of the airframe or until a terminating action is done. Some require a modification or part replacement that ends the repetitive requirement. NDT shows up most often in the repetitive inspection category, because the FAA wants ongoing surveillance of a fatigue or corrosion location without scrapping the part.

Emergency vs final rule ADs

An emergency AD takes effect immediately, often before it is formally published, and usually targets a condition that could cause an accident in the near term. A standard AD goes through a Notice of Proposed Rulemaking with a comment period before it becomes a final rule. For the inspector on the floor the difference is mostly timing. The compliance work is the same: read it, find the referenced procedure, do the inspection correctly, and document it.

How to read an AD for NDT compliance

Every AD has a structure, and reading it in order keeps you out of trouble.

Applicability

First confirm the AD actually applies to your aircraft. Applicability lists the make, model, and often the serial number or line number ranges and sometimes the specific configurations affected. An AD written for certain 737-600 through -900 line numbers does not apply to an airframe outside that range. Get this wrong in either direction and you have either done unnecessary work or missed a mandatory inspection.

Compliance times

This is the threshold and the interval. The threshold is when the first inspection is due, usually expressed in total flight cycles or total flight hours since the aircraft was new, or within so many cycles after the AD effective date. The interval is how often you repeat it. Miss the threshold and the aircraft is not airworthy. These numbers drive the maintenance planning, so they get captured in the tracking system before anything else.

Required actions and referenced documents

The heart of the AD is the action paragraph. It will say something like inspect the area for cracking in accordance with a named service bulletin or a section of the manufacturer NDT manual, at a specific revision. The directive almost never spells out the probe, the frequency, the scan plan, or the acceptance criteria. Those live in the referenced document. You have to pull the exact revision the AD calls out, because revisions change techniques, reference standards, and accept/reject limits. Running the wrong revision is a finding waiting to happen.

Alternative methods of compliance

An AMOC is an FAA-approved alternative to the literal AD requirement. If an operator has a better inspection, a different interval supported by analysis, or a terminating repair, they can request approval. You can only use an AMOC that has been approved for your specific situation, and the approval letter becomes part of the compliance record.

NDT methods used to close out ADs

The method is dictated by the defect the AD is chasing and the structure it lives in. A Level III selects and verifies the technique against the referenced procedure, but the common pairings look like this.

Eddy current and eddy current array

Eddy current is the workhorse for surface and near-surface fatigue cracking in aluminum airframe structure, fastener holes, and skin. It needs no couplant, works through thin paint and primer, and is fast on long runs. Recent directives lean on it heavily, from the A330 windshield frame HFEC inspection to the 737 Classic aft drain mast inspection, and it pairs with ultrasonic on the 767 scribe-line cracking directive. Eddy current array adds a row of coils so a single pass covers a wide footprint with a C-scan image, which is exactly what you want over chem-mill steps and skin maps. The 737 crown skin chem-mill AD is a textbook case, and we cover the technique in detail in our writeup on eddy current array inspection of 737 crown skin chem-mill steps. For the method end to end, see our guide to eddy current testing.

Phased array ultrasonic and HFEC

When an AD targets cracking that starts at a faying surface or a fastener row in a lap splice, you often need a combination of phased array ultrasonic for the subsurface crack and high frequency eddy current for the surface side. The Boeing 737NG lap splice directive is a good example of a combined PAUT and HFEC closeout, which we break down in our post on 737NG lap splice NDT for AD 2023-13-05.

Radiography

Radiography earns its place when the suspect area is hidden under structure, when you need to see a crack under a doubler, or when you are looking at internal condition you cannot reach from the surface. It is slower and carries radiation safety overhead, but it sees what surface methods cannot. The fundamentals are in our guide to radiographic testing.

Fluorescent penetrant and magnetic particle

FPI catches surface-breaking cracks on disassembled or accessible parts, and it is heavily used on engine hardware and fittings called out in engine ADs. The fan blade leading edge work in our fan blade FPI post shows the workflow. Magnetic particle covers ferromagnetic parts like landing gear and steel fittings.

Real AD examples

737 crown skin chem-mill, AD 2013-08-15

This directive addresses fatigue cracking in the fuselage crown skin at the chem-mill steps on certain 737 Classic and NG airframes. The referenced service bulletin sets up a repetitive eddy current inspection along the chem-mill pockets. Array probes make this practical, because a single C-scan pass images the step transition where the cracks initiate instead of dragging a single coil down every line.

737NG lap splice, AD 2023-13-05

This one targets the lap splices and references service bulletin 737-53-1399. Compliance runs a combination of phased array ultrasonic and high frequency eddy current to catch cracking at the upper rivet row and the faying surface. It is repetitive, so the operator inspects at the threshold and then on interval, and each event has to be recorded against the AD.

Engine ADs

Engine directives frequently call for fluorescent penetrant or eddy current on fan blades, disks, and dovetail slots, often tied to a manufacturer service bulletin and performed at a shop visit. Because the parts come off the engine, the inspection is done on the bench under controlled conditions, which is its own discipline covered in our guide to NDT in aircraft engine and MRO.

Standards and certifications behind AD compliance

An AD inspection is only valid if the people, the procedures, and the equipment all meet the governing standards.

  • 14 CFR Part 39 is the regulation that makes ADs mandatory and defines AMOCs.
  • 14 CFR Part 145 governs repair stations. Baron NDT operates under FAA Repair Station Certificate 5NDR545D, which means our quality system, procedures, and personnel are FAA accepted for this work.
  • 14 CFR Part 43 covers maintenance, the return to service, and the records you have to make.
  • NAS 410 sets the qualification and certification requirements for NDT personnel in aerospace, including training hours, experience, eye exams, and the Level I, II, III structure. SNT-TC-1A serves the same role on the general industry side.
  • OEM NDT manuals and service bulletins from Boeing, Airbus, and Bombardier contain the actual inspection techniques the ADs reference. The correct revision is mandatory.
  • ASTM E1417 (liquid penetrant), ASTM E1444 (magnetic particle), and ASTM E1742 (radiographic examination) back up the method procedures where the OEM document points to industry standards.

The thread that ties this together is traceability. A Level III certified to NAS 410 writes or approves the technique, a qualified Level II runs it, and the Part 145 quality system proves the equipment was calibrated and the standards were current.

Documentation and records

The inspection is only half the job. AD compliance lives or dies on the paperwork, because months or years later an auditor has to confirm the work was done correctly and on time. A complete record ties the AD number and revision to the aircraft, states the method and the referenced procedure with its revision, identifies the technician and their certification level, lists the equipment and calibration standards used, records the findings, and shows the next due point for repetitive items. The maintenance tracking system carries the threshold and interval forward so the next inspection does not slip. For repair station work, the NDT report and the return to service entry have to agree. Gaps here turn a good inspection into a compliance finding. For related guidance, see our coverage of How to Stay Compliant with the Latest FAA Heavy Aircraft ADs Without Grounding Your Fleet and Airbus A320 Family AD 2026-10-15: Rototest and HFEC Inspection of Cold-Worked Fastener Holes.

Advantages and limitations of NDT for AD compliance

The advantage is obvious. NDT lets you satisfy a directive without tearing the structure apart or replacing serviceable parts. You inspect, you document, you keep flying, and you catch the fatigue or corrosion before it becomes a failure. Repetitive inspections give the FAA ongoing surveillance of a known weak point at a fraction of the cost and downtime of a redesign.

The limitations are real and worth respecting. Every method has a smallest detectable flaw, and the AD is written around that capability, which is why you cannot substitute a method on your own without an approved AMOC. Access drives everything. If the area is buried, you may need radiography or disassembly, and that changes the cost and the schedule. Technique sensitivity depends on surface condition, paint thickness, probe selection, and calibration, so a sloppy setup can miss a crack the method is fully capable of finding. None of these are reasons to avoid NDT. They are reasons to run it under a controlled procedure with qualified people.

Best practices

  • Confirm applicability against the actual serial or line number before you touch the aircraft.
  • Pull the exact revision of the service bulletin or NDT manual the AD references, not the latest revision you happen to have.
  • Capture the threshold and interval in the tracking system on day one so repetitive inspections never slip.
  • Calibrate against the correct reference standard for the technique and record the calibration in the report.
  • Use the method the AD calls out, and if you want a better one, get an AMOC approved before you rely on it.
  • Write the report so a third party can trace the AD, the procedure revision, the technician certification, and the next due point without calling you.
  • Keep your NDT personnel current to NAS 410, including the annual eye exam, because an expired certification invalidates the inspection.

Frequently asked questions

Does the AD itself contain the NDT procedure?

Usually no. The AD names the unsafe condition and the required action, then references a service bulletin or a section of the OEM NDT manual at a specific revision. The actual technique, probe, calibration, and acceptance criteria live in that referenced document.

Can I use a different NDT method than the one the AD specifies?

Not on your own. You have to comply with the method the AD calls out unless you obtain an FAA-approved Alternative Method of Compliance for your specific situation. The AMOC approval becomes part of your compliance record.

What happens if a repetitive AD inspection is missed?

The aircraft is not airworthy past the due point. Repetitive ADs require inspection at the threshold and then at each interval, so the tracking system has to carry the next due point forward. A missed inspection is a compliance finding and grounds the aircraft until it is resolved.

Who is qualified to perform AD inspections?

NDT personnel certified under NAS 410 at the appropriate level, working under an FAA Part 145 repair station or equivalent approval. A Level III approves the technique and a qualified Level II typically performs it, with all certifications current.

How long do I have to keep AD compliance records?

AD compliance records are part of the aircraft maintenance records and are kept per Part 43 and Part 91 record requirements. For repetitive ADs, you keep the record at least until the work is superseded by the next required inspection, and the running history is part of the aircraft logbook.

Conclusion

AD compliance is where regulation, inspection skill, and recordkeeping all have to line up. The directive is the law, the referenced procedure is the technique, and the documentation is the proof. Get the applicability right, run the correct method to the correct revision, and write a report that stands on its own.

Baron NDT closes out Airworthiness Directive inspections under FAA Repair Station Certificate 5NDR545D with NAS 410 certified personnel and a Part 145 quality system built for traceability. From the 737 crown skin and lap splice ADs to engine and fitting directives, we run the inspection to the referenced procedure and hand back records an auditor can follow. For the full picture of how these methods fit across the airframe, start with our ultimate guide to aircraft NDT inspection. To put an AD inspection on the schedule, contact Baron NDT in Jacksonville, Florida at 904-304-2907.