The core difference between airworthiness directives vs service bulletins is legal authority. Airworthiness Directives (ADs) are legally enforceable regulations issued by civil aviation authorities to correct unsafe conditions, whereas Service Bulletins (SBs) are technical recommendations published by manufacturers that remain optional for private operators unless incorporated into an AD or maintenance program.
During aircraft, engine, and rotable part transactions, blurring these two documents triggers expensive surprises at closing. Overlooking one open AD grounds an aircraft immediately, while skipping key manufacturer SBs can lower market value, reduce OEM warranty support, or breach commercial airline lease return terms.
Key Takeaways
- Airworthiness Directives are mandatory safety rules issued by regulators such as the FAA and EASA; flying past an AD deadline is illegal.
- Service Bulletins come from manufacturers and remain legally optional for private operators unless mandated by an AD or approved maintenance program.
- Buyers must audit AD applicability across airframe, engine, propeller, and installed appliance serial numbers rather than airframe model alone.
- Compliant AD records state the AD number, revision date, compliance method, completion date, next recurring due point, and mechanic sign-off.
Airworthiness Directives vs Service Bulletins: Legal Status Compared
Civil aviation authorities—such as the FAA under 14 CFR Part 39 and EASA under Part 21—issue Airworthiness Directives when service difficulty reports or investigations reveal an unsafe condition in a type-certificated aircraft, engine, propeller, or appliance. Operating past an AD deadline violates aviation law and invalidates standard or special airworthiness certificates. When severe hazards demand immediate grounding or pre-flight checks, regulators publish Emergency ADs bypassing standard public comment periods.
Service Bulletins originate from Original Equipment Manufacturers to distribute product improvements, reliability modifications, or inspection procedures, tiered from routine bulletins up to Alert Service Bulletins. Under US Part 91 general aviation rules, even a bulletin stamped "Mandatory" by the manufacturer is not legally compulsory by itself , though commercial carriers and EASA Part-CAMO operators may be bound through their approved maintenance programs .
| Document Type | Issuing Body | Legal Status | Primary Trigger | Cost Bearer |
|---|---|---|---|---|
| Airworthiness Directive | FAA, EASA, CAA | Legally mandatory | Unsafe flight condition | Owner / operator |
| Emergency AD | FAA, EASA, CAA | Mandatory immediately | Urgent safety hazard | Owner / operator |
| Alert / Mandatory SB | Aircraft or part OEM | Mandatory if adopted | High-priority safety / fix | Operator or warranty |
| Optional Service Bulletin | Aircraft or part OEM | Recommended / optional | Reliability or upgrade | Owner / operator |
| AMOC Letter | Civil aviation authority | Approved AD alternative | Equivalent safety method | Applicant / operator |
When Does a Service Bulletin Become Legally Mandatory?
Regulators and manufacturers frequently link the two documents through incorporation by reference. Instead of drafting lengthy teardown diagrams inside regulatory text, an FAA or EASA Airworthiness Directive will mandate compliance in accordance with the accomplishment instructions of a named manufacturer Service Bulletin and revision number. Here, the AD creates the legal obligation while the referenced SB supplies the required wrench-turning steps.
Service Bulletins also gain binding force outside AD rulemaking. Manufacturers sometimes embed specific inspection or life-limit bulletins inside the government-approved Airworthiness Limitations Section of a maintenance manual. Commercial agreements add another layer: anyone reviewing how airlines buy and lease aircraft sees that operating leases and engine pool contracts routinely require accomplishing specified reliability SBs before redelivery.
Checking AD and SB Status on Parts, Engines, and Aircraft
Parts traders cannot stop at airframe-level AD lists. Standalone ADs target appliances like fuel controls, starter-generators, and actuators by part number, dash number, and serial range—including affected PMA parts or suspect batches flagged alongside counterfeit parts warnings. When assembling required documents to sell used aircraft parts, check stamped modification plates on the housing and verify exact AD and SB status remarks inside FAA 8130-3 and EASA Form 1 certification signed by a certificated Part 145 repair station.
Engine and whole-aircraft buyers must audit historical logs line by line. When evaluating how to buy a used aircraft engine, verify repetitive borescope ADs and rotor disk SBs against shop teardown reports. Similarly, buyers following a structured process on how to buy a used aircraft should generate a fresh AD list directly from the State of Registry database rather than trusting a seller's summary sheet, before researching where to buy parts online for open squawks.
What Must Appear in Compliant Logbook Records?
Under FAA 14 CFR 91.417(a)(2)(v) and parallel international continuing airworthiness rules, owners must maintain permanent status records of applicable Airworthiness Directives and transfer them when selling the aircraft. Every entry needs the AD number, amendment or revision date, accomplishment date, airframe or engine hours and cycles at compliance, and the exact compliance method performed.
For recurring ADs—such as a repetitive eddy-current spar check—the status list must state the exact hours, cycles, or calendar date when the next inspection falls due. Whenever compliance relies on an Alternative Method of Compliance (AMOC), verify that the authority's approval letter sits in the permanent binder and applies to that specific serial number rather than solely to a prior operator.
Common Audit Mistakes That Derail Pre-Purchase Reviews
Three record-keeping errors repeatedly stall closings. First, sellers hand over a clean computerized AD summary printout but cannot produce the underlying signed maintenance logbook entries showing a certificated mechanic or repair station actually performed the work. Without a valid return-to-service signature, a tracking spreadsheet carries no legal authority.
Second, mechanics occasionally mark an airframe AD "Not Applicable by serial number" while overlooking a replacement magneto, valve, or turbocharger installed years later that falls inside an appliance AD's serial block. Third, incorporating a newer Service Bulletin revision when the AD strictly specifies an earlier revision—without "or later approved revision" language—leaves the product technically non-compliant absent an approved AMOC.
Frequently Asked Questions
Is a manufacturer "Mandatory Service Bulletin" legally required?
For private aircraft operating under FAA Part 91, a Service Bulletin labeled "Mandatory" by the manufacturer is generally not legally compulsory unless enforced by an Airworthiness Directive or Airworthiness Limitations Section . However, commercial air carriers, EASA Part-CAMO fleets, and aircraft leases frequently make manufacturer SBs contractually or operationally mandatory.
What happens if you buy an aircraft or part with an open AD?
Buying a part or aircraft with an unaccomplished AD is legal, but the product cannot legally fly or return to service once past the AD compliance deadline. Smart buyers deduct the estimated shop labor and kit cost of open ADs from the purchase price or require seller compliance before closing.
What is an AMOC for an Airworthiness Directive?
An Alternative Method of Compliance (AMOC) is formal written approval from a civil aviation authority allowing an operator to satisfy an Airworthiness Directive using a different inspection, repair, or modification procedure than specified in the AD, provided it achieves an acceptable level of safety.