Explainer
Airworthiness Directive vs Service Bulletin
The Short Answer
Law Versus Recommendation
- Airworthiness directive (AD)
- Issued by the regulator. Under 14 CFR 39.3, FAA airworthiness directives are legally enforceable rules for aircraft, engines, propellers, and appliances. Operating a product that does not meet an applicable AD is a violation (39.7), and a new violation each time you operate it (39.9). It says what must be done and by when.
- Service bulletin (SB)
- Issued by the manufacturer, or the design approval holder. It describes an inspection, modification, or change the manufacturer recommends or offers to every operator of the type. On its own it is guidance with the manufacturer's authority behind it, not law. It becomes a legal requirement for an operator when an AD adopts it.
For how a bulletin travels from the manufacturer to the people doing the work, and where that pipeline breaks, read how aviation documentation stays current with service bulletins. This piece covers the other half: the legal difference, and what each one does to the manuals.
14 CFR Part 39
How an AD Works
Part 39 is the legal framework for the FAA's airworthiness directive system. The sections that matter most to a documentation team:
- When the FAA issues one (39.5)
- When an unsafe condition exists in a product, and the condition is likely to exist or develop in other products of the same type design. That is why an AD names a type, not an individual aircraft.
- What it requires (39.11)
- Inspections you must carry out, conditions and limitations you must comply with, and actions you must take to resolve the unsafe condition.
- Alternative methods of compliance (39.19)
- Anyone may propose an alternative method of compliance, or a change in the compliance time, if the proposal provides an acceptable level of safety. The proposal usually goes through the principal inspector to the office named in the AD, and the alternative may be used only if that office's manager approves it.
- When the AD and the service document disagree (39.27)
- Where an AD incorporates a manufacturer's service document by reference, the document becomes part of the AD, and the AD may modify its directions. If the two conflict, you follow the AD.
The last one is the link between the two instruments. The FAA notes that a manufacturer's service bulletin can be referenced in an AD, and when it is incorporated by reference it stops being optional reading and becomes part of the directive. The AD can also narrow or change the bulletin, so the AD text, not the bulletin alone, is the instruction to follow.
In Practice
How an AD Often Mandates an SB
A typical sequence runs like this. The manufacturer finds a problem and issues a service bulletin with the inspection or fix. If the regulator concludes that an unsafe condition exists and is likely to exist in other aircraft of the type, it issues an AD, and the AD names the bulletin as an acceptable way to comply, with a deadline. The AD, not the bulletin, creates the obligation.
That is why the answer to “is a service bulletin mandatory?” is “it depends whether an AD cites it.” The manufacturer's label on the bulletin (recommended, alert, mandatory) tells you how urgent the manufacturer thinks it is, not whether the regulator requires it. EASA draws the same line: it issues Safety Information Bulletins as non-mandatory guidance and reserves airworthiness directives for what is mandatory, and it has published non-binding guidance on how manufacturers should write service bulletins that relate to ADs.
Alternatives exist, but only through the front door. An approved alternative method of compliance under 39.19 can replace the bulletin's method or change the compliance time. An operator's own decision that the bulletin is not worth doing cannot.
Records
Recording Compliance
An AD is a continuing obligation, and the record has to show it. Under 14 CFR 91.417, a Part 91 owner or operator must keep the current status of applicable airworthiness directives, including for each the method of compliance, the AD number and revision date, and, if the AD recurs, when the next action is due. Those fields reveal what the documentation must support: which AD, which revision, which method, and when it comes due again.
One neighboring rule shows how far an AD reaches. Under 14 CFR 91.213 and 121.628, equipment that an AD requires to be operable cannot be included in a minimum equipment list unless the AD provides otherwise.
The Library
How Manuals Absorb SBs and ADs
The legal difference is clean. The documentation consequence is the same for both: when a bulletin or directive is embodied, the content that describes the aircraft changes, and which aircraft it applies to changes with it. In practice, expect the effect to appear in three places:
- The maintenance manual. Procedures change for modified aircraft only, so the AMM must carry both the old and the new task with clear effectivity.
- The parts catalog. A modification can supersede a part number, so the illustrated parts catalog needs the new part, the old part, and which tails take which.
- The component maintenance manual. If the fix concerns a component, the shop manual changes on the vendor's schedule, which may not match the airframer's.
What differs is the clock. A bulletin can wait for the operator's planning cycle. A directive carries a deadline, so the revision that absorbs it needs to reach the floor before the deadline, and the library needs to show which directive each change answers. These changes belong to the same continuing airworthiness set described in instructions for continued airworthiness, and the full set of aircraft manual types shows where else a change can land. Flight-side documents change too; see the flight operations library.
Europe
EASA Airworthiness Directives
EASA works the same way in principle. Its airworthiness directives mandate actions to restore an acceptable level of safety, apply directly in EASA member states, and an aircraft to which an AD applies may be operated only in accordance with it. EASA also adopts the directives of other states of design for products on its register. The legal basis is 21.A.3B of Part 21 to Regulation (EU) 748/2012, which also requires the design approval holder to make the descriptive data and accomplishment instructions available to operators. An operator with aircraft on both registers tracks both.
FAQ
Questions We Hear
What is an airworthiness directive?
A regulator's legally enforceable requirement to inspect, repair, modify, or restrict a type of aircraft, engine, propeller, or appliance because of an unsafe condition. Under 14 CFR 39.3, FAA airworthiness directives are legally enforceable rules. EASA describes its directives as mandating actions to restore an acceptable level of safety.
What is the difference between an airworthiness directive and a service bulletin?
Authority. An AD comes from the regulator and is mandatory; a service bulletin comes from the manufacturer and is a recommendation unless an AD makes it mandatory. The two are often linked: an AD frequently adopts a manufacturer's service bulletin as the way to comply, and then the bulletin's instructions become part of the AD.
Is a service bulletin mandatory?
Not by itself in the regulatory sense. A service bulletin becomes mandatory when an airworthiness directive incorporates it by reference, or when an operator's own maintenance program or a contract makes it so. A bulletin marked alert or mandatory by the manufacturer describes urgency, not legal status. Check whether an AD cites it.
Can an operator comply with an AD in a different way than the service bulletin says?
Possibly, but only through an approved alternative method of compliance. Under 14 CFR 39.19, an operator may propose one if it provides an acceptable level of safety, and may use it only if the manager of the office identified in the AD approves it. An operator cannot simply decide the service bulletin is optional.
What does an operator have to record about ADs?
Under 14 CFR 91.417(a)(2)(v), a Part 91 owner or operator keeps the current status of applicable ADs, including for each the method of compliance, the AD number and revision date, and the time and date the next action is due if the AD recurs. Other parts of the regulations have their own record requirements, so check the one that applies to the operation.
Does EASA issue airworthiness directives too?
Yes. EASA issues or adopts airworthiness directives, which it says mandate actions to restore an acceptable level of safety and which operators must follow to fly an affected aircraft. Its Safety Information Bulletins, by contrast, carry non-mandatory information and guidance. The basis is 21.A.3B in Part 21 of Regulation (EU) 748/2012.
Get In Touch
Need Manuals That Track Every Change?
We build and maintain structured aviation publications with the effectivity and change tracking that bulletins and directives demand. Tell us what your library has to keep up with.