01What an airworthiness directive is, legally
An airworthiness directive is not guidance, a recommendation, or a manufacturer's suggestion. It is a legal instrument that makes a specific corrective action mandatory for a defined population of aeronautical products.
Under the FAA system, 14 CFR 39.3 states plainly that airworthiness directives "are legally enforceable rules that apply to the following products: aircraft, aircraft engines, propellers, and appliances." The consequence is set out in 14 CFR 39.7: anyone who operates a product that does not meet the requirements of an applicable AD is in violation. And because 14 CFR 39.9 treats each operation as a separate violation, the exposure compounds with every flight rather than sitting as a single administrative failing.
In the European system, EASA issues ADs under the Basic Regulation (EU) 2018/1139, with issuance criteria in Part-21 21.A.3B of Regulation (EU) No 748/2012. The obligation to comply sits on the continuing-airworthiness side, in Part-M M.A.303 of Regulation (EU) No 1321/2014: any applicable AD must be carried out within the requirements of that AD, unless otherwise specified by the Agency.
Two structural points follow from this, and they matter more in practice than the citations themselves.
- An AD is issued against a type design, not against your aircraft.
14 CFR 39.5describes the trigger: the authority finds an unsafe condition in a product, and that condition is likely to exist or develop in other products of the same type design. Everything difficult about AD management flows from that sentence — the AD is written for a population, and someone has to decide whether your individual aircraft is in it. - Modification does not exempt you.
14 CFR 39.15is explicit: an AD applies to each product identified in it, even if that individual product has been changed. If the change genuinely affects your ability to comply,14 CFR 39.17requires an approved alternative method — unless the change eliminated the unsafe condition. "We already did something similar" is not a compliance position.
In audit findings, the failure is rarely a missed AD that nobody knew about. It is far more often an AD marked not applicable with no recorded reason. An unevidenced N/A is functionally the same as no assessment at all, and it is the easiest finding an auditor will ever write.
02Who issues ADs — State of Design and State of Registry
Two different states have roles, and confusing them is the root of most cross-border AD errors.
The State of Design is the state that certified the type — the authority responsible for the type certificate. It investigates the unsafe condition and issues the originating AD. For an Airbus airframe this is EASA; for a Boeing airframe, the FAA; for a Bombardier product, Transport Canada; for an Embraer product, ANAC.
The State of Registry is the state where your individual aircraft is registered. Under the ICAO framework — airworthiness directives being the common form of the mandatory continuing airworthiness information referred to in Annex 8 — the State of Registry is responsible for ensuring that mandatory continuing airworthiness information reaches owners and operators and is complied with.
In practice, the State of Registry normally adopts or mirrors the State of Design's ADs rather than re-deriving them. EASA, for instance, endorses State of Design ADs for non-EASA type-certificated products unless it issues its own directive instead. But adoption is not automatic in every direction and not instantaneous, which is precisely why an operator running a mixed fleet cannot rely on a single feed.
03EASA and the FAA compared
The two systems share the same safety logic and differ in almost every mechanical detail. Teams that work in one and assume the other behaves the same way generate a predictable class of error.
| Aspect | EASA | FAA |
|---|---|---|
| Legal basis | Reg. (EU) 2018/1139; issuance under Part-21 21.A.3B of Reg. (EU) No 748/2012 | 14 CFR Part 39 |
| Compliance obligation | Part-M M.A.303, Reg. (EU) No 1321/2014 | 14 CFR 39.7 — operating non-compliant is a violation |
| Legal form | Agency decision, published as an AD document | Rulemaking — ADs are part of the CFR, published in the Federal Register as amendments (14 CFR 39.13) |
| Reference format | Year-sequence, e.g. 2026-0123; emergency ADs carry an -E suffix | Year-biweek-sequence, e.g. 2026-14-07 |
| Consultation | Proposed AD (PAD) published for comment before the final AD | Notice of Proposed Rulemaking (NPRM) in the Federal Register |
| Urgent route | Emergency AD, issued without prior consultation | Emergency AD / immediately adopted rule; may be issued to affected operators directly |
| Deviation route | Agency-approved alternative, per the AD text and M.A.303 | AMOC under 14 CFR 39.19; the responsible office is named in the AD (39.21) |
| Ferry to repair | Per the AD text and national permit-to-fly provisions | 14 CFR 39.23 — special flight permit, unless the AD prohibits it |
| Publication | EASA AD repository / Safety Publications Tool | Federal Register; FAA Dynamic Regulatory System |
Most AD-management tooling on the market covers one authority. If your fleet, or your lessor's portfolio, spans both registers, the reconciliation between these two columns is currently manual work performed by an experienced engineer — and it is exactly the work that gets skipped under time pressure. See also our CAMO and Part-M guide.
04The AD lifecycle
An AD does not begin at publication and does not end at compliance. Understanding the full lifecycle is what lets a CAMO anticipate rather than react.
The consultation stage is an underused planning asset. A Proposed AD or NPRM is a public signal, typically months ahead of the final AD, that a specific population is about to acquire a mandatory task. Operators who read PADs schedule the work into an existing maintenance opportunity; operators who wait for the final AD often pay for an unscheduled input.
05How applicability is actually determined
This is the technically hard part of the whole discipline, and it is where most tooling stops short. The question "does this AD apply to this aircraft?" is not answered by the registration mark, the model name printed on the aircraft, or a simple text match on the manufacturer.
Applicability is a chain of assertions, and every link must hold.
Why registration is the wrong anchor
A registration mark is an administrative label. It changes when the aircraft is sold, re-registered or moved between states, and it carries no information about type design. The manufacturer's serial number persists for the life of the airframe; the type certificate and its data sheet define the population that an AD names. Any applicability process anchored on registration will drift the moment an aircraft changes register — which, for leased assets, is routine.
The three-dimensional problem
Applicability is rarely a single lookup. A typical transport-category determination requires three simultaneous conditions to be evaluated: the airframe against its TCDS model list, the engine against its own type certificate and its own AD stream, and the configuration — modifications, STCs, service bulletins already embodied — against the exclusions written into the AD's effectivity clause. An AD can apply to an airframe model but exclude aircraft that have embodied a particular modification; it can apply only to airframes fitted with a specific engine variant; it can apply to the engine irrespective of what airframe it happens to be hanging on this month.
Where the AD's effectivity text and the TCDS appear to disagree — typically after a modification or a re-designation — the AD text governs. The data sheet describes the certificated configuration; the AD defines its own applicable population, and it is entitled to draw that boundary differently. Engineers who resolve the conflict in favour of the TCDS produce confident, documented, wrong answers.
06Reading an AD: the parts that decide your outcome
Most of an AD is boilerplate. Four fields determine what you actually have to do, and by when.
14 CFR 39.21 each AD names the responsible office for exactly this reason.07Compliance times, and how to read them
A compliance time is not a due date. It is a rule for computing a due date, and the rule can reference several different clocks running at different rates.
Reading rules that prevent most errors
- Identify the reference point before the interval. "Within 24 months" is meaningless until you know 24 months from what — the effective date, the last inspection, or the date the part entered service.
- Treat "whichever occurs first" as a hard constraint. Compute every clock, take the earliest, and record which one governed. Auditors ask.
- Distinguish terminating action from repetitive action. A repetitive inspection is closed only by an approved terminating action; until then it belongs in the maintenance programme forever.
- Watch for grace provisions and their conditions. Some ADs permit deferral subject to a specific inspection or limitation. The deferral is only valid while the condition is met and recorded.
08Supersession, revision and cancellation
ADs are living documents. Three things can happen to one, and each has a different consequence for your fleet.
| Event | What it means | What you must do |
|---|---|---|
| Revision | The same AD is re-issued with changed text — often the effectivity, the referenced service bulletin revision, or the compliance time. | Re-run applicability across the whole affected fleet. A revision can pull aircraft into scope that were previously out. |
| Supersession | A new AD replaces an earlier one, normally broadening the population or adding a terminating action. | Comply with the new AD. Do not delete the earlier compliance record — it remains the evidence of what was done and when. |
| Cancellation | The AD is withdrawn, usually because the unsafe condition was addressed by other means. | Stop future recurring action, and record the cancellation with its reason. The historical compliance record still stands. |
The most expensive supersession error is quiet: a superseding AD widens the serial-number effectivity, nobody re-assesses the fleet because the original AD was already "closed", and three aircraft that were legitimately out of scope in 2023 have been in scope since 2025. This is discovered at lease return, where it is at its most expensive to fix and its most damaging to the relationship.
09Alternative Methods of Compliance
An AMOC is an authority-approved alternative to the actions or timing an AD prescribes, on the basis that it provides an acceptable level of safety. It is not a waiver and it is not self-declared.
In the FAA system the route is defined: 14 CFR 39.19 allows an operator to propose an alternative method of compliance or a different compliance time, submitted for approval to the principal inspector or the responsible office; 14 CFR 39.21 notes that each AD identifies the office that holds information on previously approved AMOCs. In the EASA system the equivalent deviation is agreed with the Agency, consistent with the "unless otherwise specified by the Agency" wording in M.A.303 and whatever the AD itself provides.
- An existing AMOC may already solve your problem. Because the AD names the holding office, checking for previously approved AMOCs before engineering your own is usually the fastest route.
- An AMOC is granted to a specific applicant and scope. Another operator's AMOC is a useful precedent, not a licence.
- The AMOC is part of your compliance evidence. It must be filed against the aircraft with the AD record, not in a separate correspondence folder — which is where it usually ends up, and where an auditor will not accept it.
10Records and evidence: proving compliance
Compliance that cannot be demonstrated is, for every practical purpose, non-compliance. At an audit, a lease return or a sale, the aircraft is not assessed — the records are.
A defensible AD record for a single directive answers six questions without the reader having to reconstruct anything:
- Which AD, at which revision. The reference alone is insufficient once revisions exist.
- Applicable or not — and on what basis. A not-applicable determination needs its reasoning recorded as rigorously as a compliance record. This is the single most common gap.
- What was done. The action, the referenced document at the mandated revision, and the work order or CRS reference.
- When, against which clock. Date, flight hours and cycles at compliance, and which clock governed.
- What comes next. For repetitive actions, the next due point on every applicable clock.
- Who determined it. The person or system, and the date of the determination — because a determination made against a superseded AD revision is not evidence of anything.
Records gaps are a valuation problem, not an administrative one. In commercial leasing, records deficiencies are the dominant cause of redelivery overspend, and incomplete back-to-birth traceability on life-limited parts directly reduces engine value. In general aviation, missing logbooks are commonly assessed at around a fifth of an aircraft's retail resale value. The paperwork is a material part of the asset. Our technical records guide covers retention, transfer and back-to-birth traceability in full.
11Dual-registry operations
Operators and lessors holding aircraft on more than one register carry a compounded obligation that most processes — and most software — handle badly.
The same underlying unsafe condition may be addressed by an EASA AD and an FAA AD which differ in effectivity wording, compliance time, referenced service bulletin revision, or all three. Neither authority is wrong; each is regulating its own register. The operator must satisfy the directive applicable through its State of Registry for each aircraft, and must be able to show which one it applied.
The practical control set
- Track by register, not by fleet. A single "AD list" across a mixed-register fleet will eventually apply the wrong authority's compliance time to an aircraft.
- Reconcile at the unsafe condition, not the AD number. Two AD references can address one condition. Mapping them to a common root is what prevents double work and missed divergence.
- Re-assess on register change. Moving an aircraft between registers changes which authority's directives govern. Treat it as a full re-determination event, not an administrative update.
- Watch post-Brexit divergence. UK CAA and EASA directives for the same type can now differ in timing and content. Assuming equivalence is no longer safe.
12Common failure modes
From audit findings and lease-return disputes, the same small set of errors recurs across operators of every size.
| Failure | How it happens | Control |
|---|---|---|
| Unevidenced N/A | An AD is assessed as not applicable and closed with no recorded reasoning. | Require the same evidence standard for a negative determination as for a positive one. |
| Stale determination | Applicability was assessed once, against a revision of the AD that no longer exists. | Bind every determination to an AD revision, and re-assess on revision or supersession. |
| Wrong clock | Calendar time applied where the AD specified flight hours or cycles, or the reference point misread. | Record which clock governed on every compliance entry. |
| Registration-anchored tracking | Fleet data keyed on registration; the aircraft is re-registered and its history fragments. | Key everything on manufacturer serial number. |
| Engine ADs missed on swap | The engine carries its own AD stream and moves between airframes. | Track engine and propeller ADs against the engine serial, independent of the airframe. |
| Superseded SB revision | Work performed to a later SB revision than the one the AD mandates. | Check the mandated revision at the point of work, not at the point of planning. |
| AMOC filed elsewhere | Approval sits in email or a project folder rather than the aircraft record. | File the AMOC against the aircraft, with the AD record. |
13Key terms
- AD
- Airworthiness Directive — a legally enforceable instrument requiring action on a defined population of products.
- PAD / NPRM
- Proposed AD (EASA) and Notice of Proposed Rulemaking (FAA) — the consultation drafts preceding a final AD.
- AMOC
- Alternative Method of Compliance — an authority-approved alternative to an AD’s actions or timing.
- Effectivity
- The clause defining the population an AD applies to. The exclusions decide more cases than the inclusions.
- TCDS
- Type Certificate Data Sheet — defines the certificated configuration and is the anchor for applicability.
- State of Design
- The authority holding the type certificate; issues the originating AD.
- State of Registry
- Where the individual aircraft is registered; the route by which the obligation reaches the operator.
- Supersession
- Replacement of an AD by a new one, often widening effectivity. Requires fleet-wide re-assessment.
- Terminating action
- An action that closes a repetitive requirement. Until one is embodied, the recurring task is permanent.
- Special flight permit
- Authorisation to ferry a non-compliant aircraft to a repair facility, unless the AD prohibits it.
14Frequently asked questions
Is an airworthiness directive mandatory?
Yes. Under 14 CFR 39.3 FAA airworthiness directives are legally enforceable rules, and 14 CFR 39.7 makes operating a product that does not meet an applicable AD a violation — with 14 CFR 39.9 treating each operation as a separate violation. In the EASA system, compliance is required by Part-M M.A.303 within the requirements of the AD, unless otherwise specified by the Agency.
Does an AD still apply if the aircraft has been modified?
Yes. 14 CFR 39.15 states that an AD applies to each product identified in it even if the individual product has been changed. Where the change affects your ability to comply, 14 CFR 39.17 requires an approved alternative method — unless the change eliminated the unsafe condition. Embodying a similar modification on your own initiative does not create an exemption.
Does an EASA AD apply to an aircraft on the FAA register?
Not directly. The obligation reaches the operator through the State of Registry. Where the State of Design differs, its ADs are normally adopted or mirrored by the State of Registry — but wording and timing can differ. An operator with aircraft on both registers must track both authorities for the same type and be able to demonstrate which directive was applied to which aircraft.
What is an AMOC and who can use one?
An Alternative Method of Compliance is an authority-approved alternative to the actions or compliance times in an AD, on the basis that it provides an acceptable level of safety. Under 14 CFR 39.19 a proposal goes to the principal inspector or responsible office; 14 CFR 39.21 notes each AD identifies the office holding previously approved AMOCs. An AMOC is granted to a specific applicant and scope — another operator's approval is a precedent, not a licence.
Can I fly the aircraft to a facility to comply with an AD?
Sometimes. 14 CFR 39.23 provides that some operators hold authority to fly the aircraft to a repair facility, and others may obtain a special flight permit from the local FAA office — unless the AD itself prohibits it. Always check the AD text: a directive addressing a condition that is unsafe in flight will often forbid ferry.
What is the difference between an AD and a service bulletin?
A service bulletin is issued by the design approval holder and is not mandatory in itself. An AD is issued by an airworthiness authority and is mandatory. An AD frequently makes a specific revision of a service bulletin mandatory — at which point the actions in that SB revision acquire legal force for the applicable population, and only that revision does.
How often should AD applicability be re-assessed?
Continuously for new issues, and as an event whenever anything in the applicability chain changes: a new or revised AD, a modification or STC embodiment, an engine change, a change of register, or a change to the recorded configuration. Periodic re-assessment on a fixed calendar alone will miss revisions that widen effectivity between reviews.