Airworthiness Directives: How to Read an AD, Part 39 Compliance, AMOCs, and the SB/SAIB Distinction
Updated: Aug 27
Airworthiness Directives are legally enforceable regulations the FAA issues to correct unsafe conditions in aircraft, engines, propellers, and appliances — and for owners and operators, understanding them is essential not just for compliance but for safety. But ADs trip up a lot of pilots and owners in specific ways: they're actual federal law (not guidance), they apply by serial number and installed equipment (not just aircraft type), they come in flavors of compliance timing that matter enormously, and they're routinely confused with Service Bulletins and Special Airworthiness Information Bulletins, which carry completely different legal weight. Understanding how ADs actually work turns a vague obligation into a manageable, well-defined responsibility.
This post covers Airworthiness Directives in practical depth: what an AD legally is (and the Part 39 structure behind it), how ADs are developed and how emergency ADs differ, how to read an AD number, the types of compliance timing, AMOCs, the critical distinction between ADs, Service Bulletins, and SAIBs, how ADs apply to engines and appliances independently, and the owner's continuing responsibility.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What an AD Legally Is
An Airworthiness Directive is a legally enforceable rule — actual federal regulation — issued by the FAA when an unsafe condition exists in a product and is likely to exist or develop in other products of the same type design.
The legal status:
ADs are codified in 14 CFR Part 39
They ARE federal regulations (not advisory, not guidance)
Compliance is mandatory (14 CFR 39.7: no person may operate a product to which an AD applies except in accordance with that AD)
Violating an AD is violating the federal aviation regulations
What ADs apply to:
Aircraft
Aircraft engines
Propellers
Appliances (avionics, components, parts)
Any "product" with an unsafe condition
The "unsafe condition" basis:
An AD is issued when an unsafe condition is found
AND that condition is likely to exist or develop in other products of the same type design
The fleet-wide risk is what triggers an AD
Individual problems (not fleet-wide) are handled differently
Why Part 39 matters:
Part 39 is the regulatory home of ADs
39.7 makes compliance mandatory
39.9 addresses the consequences of non-compliance (the product isn't airworthy)
The AD itself becomes part of the regulations when issued
The key point:
An AD isn't a recommendation or a manufacturer's suggestion
It's federal law with the force of regulation
Non-compliance is both unsafe AND illegal
This is what distinguishes ADs from Service Bulletins (below)
How ADs Are Developed
Understanding the process clarifies why ADs carry the weight they do.
The standard rulemaking process:
An unsafe condition is identified (see sources below)
The FAA issues a Notice of Proposed Rulemaking (NPRM) for the proposed AD
Published in the Federal Register with a public comment period
The public (owners, manufacturers, operators) can comment
The FAA considers comments and issues the Final Rule AD
The AD becomes effective on a specified date
The sources of unsafe conditions:
Accident or incident investigations
Manufacturer reports
Service Difficulty Reports (SDRs)
Testing or engineering analysis
Findings by international aviation authorities (foreign ADs on foreign-made aircraft)
The foreign AD connection:
For aircraft/products made in other countries, the foreign authority (EASA, etc.) may issue an AD
The FAA typically issues a corresponding AD (adopting or paralleling it)
This is why a European-made aircraft gets FAA ADs based on EASA findings
International coordination
The comment period value:
The NPRM process lets stakeholders provide input
Sometimes the AD is modified based on comments
Balances safety with practicality
Democratic rulemaking
Emergency ADs: The Exception to the Process
Emergency ADs skip the normal process because of urgency.
What an emergency AD is:
Issued when an immediate safety risk exists
Effective IMMEDIATELY (upon receipt, no comment period)
Bypasses the NPRM process (there's no time)
Often requires grounding until corrective action
Why the process is skipped:
The normal rulemaking (NPRM, comment period) takes time
An immediate hazard can't wait
The emergency AD takes effect at once
Safety demands immediate action
How emergency ADs are distributed:
Sent directly to known owners/operators of the affected product
By priority means (to ensure receipt)
Effective on receipt
Later often followed by a standard AD (with the normal process) to make it permanent
The grounding aspect:
Emergency ADs frequently ground the aircraft
"Before further flight" compliance
The aircraft can't fly until the AD is addressed
The most urgent category
The practical impact:
If you receive an emergency AD, it's effective immediately
You may not be able to fly until you comply
Take it seriously (it's an immediate hazard)
Address it before further flight if required
How to Read an AD Number
A practical skill the basic treatment doesn't cover: decoding the AD number.
The format:
ADs are numbered like 2024-15-08
The format is: YEAR–BIWEEK–SEQUENCE
2024: the year issued
15: the biweekly period number (the FAA publishes ADs in biweekly batches)
08: the sequence number within that period
Decoding 2024-15-08:
Issued in 2024
In the 15th biweekly period of the year
The 8th AD in that period
A unique identifier
Why the numbering matters:
Each AD has a unique number
You reference the number for compliance records
You search the number in the FAA database
The number identifies the specific AD
The revision aspect:
ADs can be revised (superseded or amended)
A revised AD may supersede an earlier one
The records should note the current applicable AD
Superseding ADs replace the ones they supersede
The superseding concept:
A new AD can SUPERSEDE an old one (replace it entirely)
The superseded AD is no longer applicable (the new one takes over)
Compliance records track which is current
Superseding is common as understanding evolves
Types of Compliance Timing
ADs specify WHEN compliance is required, and the timing type matters enormously.
Before further flight:
The most urgent
The aircraft cannot fly until the AD is complied with
Common for emergency ADs
Grounds the aircraft immediately
Within a calendar period:
Comply within a specified number of days/months
Calendar-based deadline
Allows continued operation until the deadline
Then compliance is required
Within a number of flight hours:
Comply within a specified number of hours of time in service
Hours-based deadline
Continue operating until the hours are reached
Common for wear-related conditions
Within a number of cycles:
Comply within a specified number of cycles (takeoff/landing cycles)
Cycle-based (for fatigue-related conditions)
Cycles matter for pressurization/structural fatigue
Some ADs use cycles
At the next inspection:
Comply at the next annual, 100-hour, or specified inspection
Ties the AD to a scheduled event
Convenient timing
Common for less urgent conditions
Recurring intervals:
For recurring ADs, comply repeatedly at specified intervals
Every X hours, months, or cycles
Ongoing compliance
The recurring requirement
Why the timing type matters:
"Before further flight" grounds you now
A calendar/hours deadline lets you plan
Recurring means ongoing tracking
Know the compliance timing for each applicable AD
Recurring vs. One-Time ADs
The distinction that causes the most compliance problems.
One-time ADs:
Require a single corrective action
Once complied with and documented, no further action
The condition is permanently addressed (part replaced, modification made)
Done and done
Recurring ADs:
Require repeated inspections or actions at specified intervals
Remain in effect for the life of the aircraft/component (unless terminated)
Ongoing compliance required
The most common cause of grounding at inspection
Why recurring ADs cause problems:
They must be tracked and complied with repeatedly
Missing an interval means non-compliance
Easy to overlook over years of ownership
A lapsed recurring AD grounds the aircraft
The termination possibility:
Some recurring ADs can be TERMINATED by a permanent fix
A modification that eliminates the condition ends the recurring requirement
"Terminating action"
Ends the ongoing inspections
The tracking imperative:
Recurring ADs demand diligent tracking
Maintenance tracking software or careful logbook review
Know the next due time for each recurring AD
The owner's responsibility
Methods of Compliance and AMOCs
How an AD is complied with, and the alternative-method provision.
The specified method:
Each AD specifies HOW to comply
Inspections, part replacement, modifications, operational limitations, software/firmware updates
The AD's instructions must be followed
The prescribed method
The compliance methods:
Inspection: check for the condition
Part replacement: replace a defective/suspect part
Modification: alter the product to fix the condition
Operational limitation: restrict operation (speed, etc.)
Software/firmware update: for avionics ADs
Alternative Method of Compliance (AMOC):
An AD may allow an AMOC
An alternative way to address the unsafe condition
Must be APPROVED by the FAA (not self-determined)
Provides flexibility when the standard method isn't feasible
When AMOCs are used:
The standard compliance method isn't practical for a specific situation
An equally effective alternative exists
The FAA approves the alternative
Documented like any compliance
The AMOC approval:
Request submitted to the FAA
The FAA evaluates whether the alternative addresses the unsafe condition
If approved, the AMOC satisfies the AD
Not a way around the AD — an approved alternative
ADs vs. Service Bulletins vs. SAIBs: The Three-Way Distinction
A critical distinction the basic treatment mentions but conflates. These three have very different legal weight.
Airworthiness Directive (AD):
Issued by the FAA
MANDATORY (federal regulation, Part 39)
Compliance required by law
Non-compliance = unairworthy and illegal
Service Bulletin (SB):
Issued by the MANUFACTURER
Generally NOT mandatory for Part 91 operators
A manufacturer's recommendation (a fix, improvement, or inspection)
Becomes mandatory ONLY if an AD references/adopts it
Special Airworthiness Information Bulletin (SAIB):
Issued by the FAA
NOT mandatory (advisory/informational)
Alerts owners to a condition that doesn't (yet) warrant an AD
Recommends action but doesn't require it
The key distinctions:
Issued By | Mandatory? | |
AD | FAA | YES (Part 39) |
Service Bulletin | Manufacturer | No (unless an AD adopts it) |
SAIB | FAA | No (advisory) |
Why the distinction matters:
Only ADs are legally required (for Part 91)
SBs are recommendations (though often wise to follow)
SAIBs are informational alerts
Confusing them leads to errors (thinking an SB is mandatory, or ignoring an AD)
The SB-to-AD pipeline:
A manufacturer issues an SB for a condition
If the FAA determines it's an unsafe condition (fleet-wide), it issues an AD
The AD often references the SB (making the SB's procedure mandatory)
SBs sometimes become ADs
The Part 91 vs. commercial distinction:
For Part 91 (private) operators, SBs are generally not mandatory
For commercial operators (Part 121/135), some SBs may be required by their maintenance program
ADs are mandatory for everyone
Know your operating rules
How ADs Apply: Engines and Appliances Independently
A trap the basic treatment touches on but deserves emphasis: ADs follow the component, not just the airframe.
The serial-number and equipment basis:
ADs apply based on make, model, serial number, AND installed equipment
Not just the aircraft type
A specific serial-number range may be affected
Installed appliances/engines have their own ADs
Airframe ADs:
Apply to the aircraft (by make/model/serial)
The airframe itself
Structural, systems, etc.
Engine ADs:
Apply to the ENGINE (independent of the airframe)
The same engine in different aircraft gets the engine ADs
Track engine ADs separately
The engine has its own AD history
Propeller ADs:
Apply to the propeller
Independent of the airframe and engine
Track propeller ADs
Own AD history
Appliance ADs:
Apply to appliances (avionics, components)
A specific radio, autopilot, or component
If you install it, its ADs apply to your aircraft
Often overlooked
The installed-equipment trap:
When you install a component (radio, autopilot), its ADs come with it
Owners forget to check appliance/avionics ADs
A newly installed (or existing) appliance may have applicable ADs
Check all installed equipment
Why this matters:
AD research isn't just "what ADs apply to my aircraft model"
It's the airframe AND engine AND propeller AND every appliance
By serial number and installed equipment
Comprehensive research is required
Who Is Responsible and How Compliance Is Documented
The responsibility and documentation, which the basic treatment covers but can be sharpened.
The owner/operator's responsibility (91.403):
The owner/operator is PRIMARILY responsible for maintaining the aircraft in airworthy condition
This INCLUDES ensuring AD compliance
91.403(a): the owner/operator is primarily responsible for maintaining the aircraft in an airworthy condition, including compliance with ADs
The buck stops with the owner
Maintenance personnel:
Perform the required inspections/actions
Document compliance in the maintenance records
The mechanic does the work and signs it off
But the owner ensures it happens
The pilot's responsibility:
The PIC must determine the aircraft is airworthy before flight (91.7)
This includes AD compliance
Especially important when renting or flying unfamiliar aircraft
Verify before flying
How compliance is documented:
Recorded in the aircraft maintenance logbooks
Must include: AD number and revision, description of the action taken, date and aircraft total time, next due time (for recurring ADs), signature and certificate number of the mechanic/inspector
Complete documentation is required
Incomplete records can render the aircraft unairworthy
The documentation detail:
AD number and revision (which AD, which version)
Method of compliance (what was done)
Date and aircraft total time in service
For recurring ADs: the next due time
Signature and certificate number
Why documentation matters:
Missing/incomplete AD documentation makes the aircraft unairworthy (can't prove compliance)
The records ARE the proof
An IA checks AD compliance at the annual (via the records)
Documentation is as important as the compliance itself
Finding Applicable ADs
How to actually research ADs, expanded from the basic list.
The FAA AD database:
The FAA's Dynamic Regulatory System (DRS) contains ADs
Searchable by make, model, and more
The authoritative source
Free online
How to research comprehensively:
Search by aircraft make/model/serial number
Search for the ENGINE make/model
Search for the PROPELLER make/model
Search for each installed APPLIANCE (avionics, components)
Cross-reference serial numbers and applicability
Maintenance tracking services:
Commercial services track ADs for your specific aircraft
Automatically notify of new/recurring ADs
Manage the compliance schedule
Valuable for owners
The pre-purchase AD research:
Before buying an aircraft, research its AD compliance
A pre-purchase inspection includes AD status
Uncomplied ADs are a liability (and cost)
Critical for buyers
The annual/100-hour check:
The IA verifies AD compliance at the annual
Reviews the records for applicable ADs
Confirms recurring ADs are current
A compliance checkpoint
Why comprehensive research matters:
ADs apply by serial number and installed equipment
Missing an applicable AD (especially appliance ADs) is common
Comprehensive research catches them all
Diligence required
Common AD Pitfalls
The errors that ground aircraft, expanded.
Assuming an AD no longer applies after resale:
ADs follow the AIRCRAFT (and its components), not the owner
A new owner inherits all the AD compliance requirements
Recurring ADs continue
Resale doesn't reset ADs
Missing recurring inspection intervals:
The most common problem
Recurring ADs must be complied with repeatedly
Missing an interval = non-compliant = grounded
Track diligently
Failing to check appliance and avionics ADs:
Owners check airframe ADs but forget appliances
Installed avionics/components have their own ADs
A common oversight
Check ALL installed equipment
Relying on incomplete logbook entries:
Incomplete AD documentation makes the aircraft unairworthy
The records must be complete (AD number, action, date, time, signature, next due)
Verify the documentation
Incomplete = a problem
Overlooking engine/propeller ADs:
The engine and propeller have their own AD histories
Independent of the airframe
Track them separately
Don't forget them
The consequences:
Even ONE missed AD can ground the aircraft
Enforcement action for operating non-compliant
Unairworthy status
Avoidable with diligence
ADs and Airworthiness
The connection to the fundamental airworthiness concept.
The airworthiness definition:
An aircraft is airworthy only if:
It conforms to its type design (INCLUDING AD compliance), AND
It is in a condition for safe operation
Both required
Why AD non-compliance violates both:
Uncomplied ADs mean the aircraft doesn't conform to the (AD-modified) type design
AND the uncorrected unsafe condition means it's not in a condition for safe operation
So a missed AD violates BOTH airworthiness conditions
Doubly unairworthy
The practical effect:
An aircraft with an overdue AD is NOT airworthy
It cannot be legally flown
Regardless of how it performs
AD compliance is part of airworthiness
The PIC's determination:
The pilot determines airworthiness before flight (91.7)
This includes AD compliance
Flying with a known overdue AD is a violation
The pilot's responsibility too
Final Thoughts
Airworthiness Directives are a cornerstone of aviation safety — the mechanism by which the FAA corrects known hazards across entire fleets, with the full force of federal regulation behind them under 14 CFR Part 39. Understanding how they work makes compliance manageable: know that ADs are mandatory (unlike Service Bulletins and SAIBs), that they apply by serial number and installed equipment (airframe, engine, propeller, and every appliance), that the compliance timing ranges from "before further flight" to recurring intervals, and that complete documentation is as essential as the compliance itself.
For owners, staying proactive with comprehensive AD research and meticulous documentation is one of the most important responsibilities of aircraft ownership. While tracking ADs requires diligence, compliance protects not only regulatory standing but lives — which is, after all, why ADs exist.
On the Written Test and Checkride
Airworthiness Directives appear on tests and checkride orals. The most commonly tested topics:
What an AD is (mandatory, Part 39)
One-time vs. recurring ADs
ADs vs. Service Bulletins (mandatory vs. recommended)
Who is responsible for compliance (owner/operator)
How compliance is documented
That ADs apply by serial number and installed equipment
Quick Reference
What an AD Is:
Legally enforceable FAA regulation (14 CFR Part 39)
Corrects an unsafe condition likely fleet-wide
MANDATORY (39.7)
Applies to aircraft, engines, propellers, appliances
How ADs Are Developed:
Standard: NPRM → comment period → Final Rule AD
Sources: accidents, manufacturer reports, SDRs, analysis, foreign authorities
Emergency AD: effective immediately (no comment period)
Reading an AD Number (e.g., 2024-15-08):
2024: year
15: biweekly period
08: sequence number
ADs can supersede earlier ADs
Compliance Timing:
Before further flight (grounds the aircraft)
Within calendar period (days/months)
Within flight hours
Within cycles
At the next inspection
Recurring intervals
One-Time vs. Recurring:
One-time: single action, then done
Recurring: repeated at intervals (most common cause of grounding)
Recurring can be terminated by a permanent fix
Methods of Compliance:
Inspection, part replacement, modification, operational limitation, software update
AMOC: FAA-approved Alternative Method of Compliance
AD vs. SB vs. SAIB:
Issued By | Mandatory? | |
AD | FAA | YES |
Service Bulletin | Manufacturer | No (unless an AD adopts it) |
SAIB | FAA | No (advisory) |
ADs Apply By:
Make, model, SERIAL NUMBER, and installed equipment
Airframe ADs, engine ADs, propeller ADs, appliance ADs (all independent)
Check ALL installed equipment (appliances often missed)
Responsibility:
Owner/operator: primarily responsible (91.403)
Maintenance: performs and documents
Pilot: verifies before flight (91.7)
Documentation:
AD number and revision
Action taken
Date and aircraft total time
Next due time (recurring)
Signature and certificate number
Incomplete docs = unairworthy
Finding ADs:
FAA database (DRS)
Search airframe, engine, propeller, each appliance
Maintenance tracking services
Pre-purchase research
Key Principle:
ADs are mandatory federal regulations (Part 39) that correct fleet-wide unsafe conditions — unlike Service Bulletins (manufacturer, recommended) and SAIBs (FAA, advisory). They apply by serial number and installed equipment, so research must cover the airframe, engine, propeller, and every appliance. Compliance timing ranges from "before further flight" to recurring intervals, and complete documentation is required. A single overdue AD makes the aircraft unairworthy.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.