The Airworthiness Limitations Section is the legally binding part of an aircraft’s maintenance documentation, and it sets the mandatory replacement times, inspection intervals, and recurring tasks that no operator or mechanic can skip or extend. Everything else in the manufacturer’s manual is guidance. The ALS is regulation. Flying past any of its limits makes the aircraft unairworthy, and federal enforcement follows — grounding, certificate suspension or revocation, and civil penalties reaching $75,000 per violation for commercial operators.
What the ALS Contains
The ALS lives inside a larger document called the Instructions for Continued Airworthiness (ICA), which the manufacturer must provide for every certificated aircraft, engine, and propeller. The ICA covers cleaning, lubrication, troubleshooting, and the rest of the routine maintenance picture. Only the ALS is binding on operators.1eCFR. 14 CFR 21.50 – Instructions for Continued Airworthiness and Manufacturers Maintenance Manuals Having Airworthiness Limitations Sections
For large transport-category airplanes, Appendix H to Part 25 defines four categories of content that belong in the ALS:
- Structural inspection intervals and replacement times driven by the damage tolerance and fatigue evaluations under 14 CFR 25.571, including fatigue inspections of fuselage joints, wing spars, and other primary structure, plus mandatory retirement times for safe-life parts.
- Fuel tank system limitations, including critical design configuration control limitations (CDCCLs), inspections, and procedures required under 14 CFR 25.981 to control fuel tank flammability.
- Electrical wiring replacement times for electrical wiring interconnection system (EWIS) components under 14 CFR 25.1701.
- A limit of validity (LOV) — a total accumulated flight-cycle or flight-hour number beyond which the airplane cannot operate, because the engineering data supporting the structural maintenance program has not been validated past that point.
Those four categories are the Part 25 scope.2GovInfo. 14 CFR Part 25, Appendix H – Instructions for Continued Airworthiness Smaller aircraft certificated under Parts 23, 27, and 29 have parallel appendices scaled to their complexity, but the principle is the same: anything that must happen on a fixed schedule to keep the aircraft safe goes in the ALS.
Life-Limited Parts
Some components have a finite lifespan measured in flight hours, flight cycles, or calendar time. A turbine disk endures huge centrifugal stress every time the engine runs, and after a set number of cycles it must come out of service and be scrapped. Not overhauled, not inspected more closely — physically discarded. The same logic applies to certain landing gear components, rotor hubs, and high-pressure compressor stages. The retirement life is the mandatory limit, and the part’s visible condition at removal is irrelevant.3Federal Aviation Administration. AC 33.70-1 – Guidance Material for Aircraft Engine Life-Limited Parts
The reason is that fatigue cracking in highly stressed metals often cannot be detected by any inspection method until the crack is already large enough to cause failure. The safe-life philosophy removes the part before that point.
Mandatory Structural Inspections
Structural inspections work on a different principle. They are built around damage tolerance: a structure can safely carry a small crack as long as inspections find it before it grows to a dangerous size. The ALS prescribes when each inspection must happen and what technique to use, often specifying non-destructive testing methods like eddy current, ultrasonic, or fluorescent penetrant inspection. Inspection thresholds for single-load-path and fail-safe structures are established from crack growth analysis, assuming an initial flaw of the maximum probable size from manufacturing or service damage.4eCFR. 14 CFR 25.571 – Damage-Tolerance and Fatigue Evaluation of Structure
Certification Maintenance Requirements
Certification Maintenance Requirements (CMRs) address a different risk. A CMR is a periodic task established during original design certification to detect hidden failures in systems — not structures — that could combine with another failure to produce a catastrophic result. Consider a backup fire-detection loop that has silently failed. The airplane flies fine until the day a real fire starts and the crew gets no warning. CMRs exist to catch that silent failure before it matters.5Federal Aviation Administration. AC 25-19A – Certification Maintenance Requirements
CMRs come out of the system safety assessment process during type certification. They are operating limitations of the type certificate itself, and skipping one carries the same legal consequence as running past a life-limited part.
What the Regulations Actually Require
Three federal regulations create the compliance framework, and each targets a different person in the chain.
14 CFR 91.403(c) prohibits anyone from operating an aircraft that has an ALS unless every mandatory replacement time, inspection interval, and related procedure has been completed. The narrow exceptions cover alternative intervals approved under Part 121, Part 135 operations specifications, or an inspection program approved under 14 CFR 91.409(e).6eCFR. 14 CFR 91.403 – General
14 CFR 43.16 puts the same rule on the mechanic. Anyone performing maintenance or inspections specified in an ALS must follow that section exactly. A mechanic cannot substitute a different technique or extend an interval on personal judgment.7eCFR. 14 CFR 43.16 – Airworthiness Limitations
14 CFR 21.50 puts a duty on the design approval holder. The type certificate holder, or the holder of a supplemental type certificate, must furnish at least one complete set of ICA (including the ALS) to the owner of each aircraft on delivery, or on issuance of the first standard airworthiness certificate, whichever comes later.1eCFR. 14 CFR 21.50 – Instructions for Continued Airworthiness and Manufacturers Maintenance Manuals Having Airworthiness Limitations Sections
Penalties
Operating past an ALS limit is not a gray area. The aircraft is unairworthy, and the penalty depends on who committed the violation. For an entity other than an individual or small business — essentially, airlines and large operators — the maximum civil penalty is $75,000 per violation. For individuals and small businesses the cap is $17,062 per violation in most categories. For an airman acting in that capacity, the baseline maximum is $1,875 per violation.8eCFR. 14 CFR 13.301 – Inflation Adjustments of Civil Monetary Penalties
Beyond fines, the FAA can suspend or revoke pilot certificates, mechanic certificates, and air carrier operating certificates. Fixed-duration suspensions serve as discipline; indefinite suspensions ground a certificate holder until they demonstrate they meet standards again. Revocation ends the certificate, and the holder must reapply from scratch.9Federal Aviation Administration. Legal Enforcement Actions
Where To Find the ALS
Most manufacturers organize their maintenance manuals using the ATA numbering system (iSpec 2200 or S1000D). Under those standards, the ALS appears in Chapter 4 of the Aircraft Maintenance Manual, and Appendix H to Part 25 reinforces the point by requiring that the ALS be segregated and clearly distinguishable from the rest of the document.10Legal Information Institute. 14 CFR Appendix H to Part 25 – Instructions for Continued Airworthiness The separation exists so nobody treats a mandatory limit as an optional recommendation. Before any heavy maintenance visit, confirm your maintenance provider has the latest revision of Chapter 4. An outdated ALS produces work performed to superseded intervals that no longer satisfy current requirements.
Revisions and Airworthiness Directives
An ALS is not static. As a type accumulates fleet experience, the manufacturer and the FAA learn things that were unknown at certification: a fatigue crack showing up earlier than predicted, a failure mode nobody modeled, corrosion in an area assumed benign. When that happens, the type certificate holder revises the ALS, and those revisions become part of the mandatory requirements.
The FAA often accelerates the process with an Airworthiness Directive (AD). ADs are legally enforceable rules that can mandate inspections, impose operating limitations, or require modifications, and they apply to specific products by type, model, and serial number.11eCFR. 14 CFR Part 39 – Airworthiness Directives An AD may direct operators to incorporate a specific ALS revision, add a new inspection, or reduce a part’s life limit. Once an AD is effective, compliance is not optional. The airplane does not fly until the required action is done or an alternative method of compliance is approved.
Tracking revisions is the operator’s job. The manual is only valid if kept current, and documentation must be updated each time the manufacturer issues a revision.12Federal Aviation Administration. Airworthiness Limitations Section – MBB-BK117 D-3 Fleet operators need a revision management process that tracks every tail number against the current ALS version.
Alternative Methods of Compliance
The ALS sets the baseline, but the regulations recognize that a specific procedure is not always the only safe option. An operator may propose an alternative method of compliance (AMOC) that provides an equivalent level of safety. The approval bar is high, and the operator cannot use the alternative until the FAA formally approves it.13eCFR. 14 CFR 39.19 – May I Address the Unsafe Condition in a Way Other Than That Set Out in the Airworthiness Directive
The process starts with a written proposal to the operator’s principal inspector at the Flight Standards District Office. The proposal must identify the specific AD paragraph or ALS requirement, describe the alternative, and include substantiating data proving the alternative corrects the unsafe condition to an acceptable safety level. The principal inspector forwards the proposal, with comments, to the manager of the office identified in the AD.14Federal Aviation Administration. Alternative Methods of Compliance (AMOC)
After approval, the operator must notify the principal inspector before actually using the AMOC. A copy of the approval document stays in the aircraft’s maintenance records until ownership transfers, and the records must be available to FAA inspectors or NTSB investigators on request.
Records That Prove Compliance
Compliance means nothing if you cannot prove it. Under 14 CFR 91.417, each maintenance record entry must describe the work performed, the date it was completed, and the signature and certificate number of the person approving the aircraft for return to service. For life-limited parts, a running record of current status must be maintained for every airframe, engine, propeller, rotor, and appliance. Unlike routine maintenance entries that can eventually be discarded, life-limited part records transfer with the aircraft when it is sold.15eCFR. 14 CFR 91.417 – Maintenance Records
Certificated repair stations have their own retention rule under 14 CFR 145.219, which requires records to be kept for at least two years from the date an article was approved for return to service.16eCFR. 14 CFR 145.219 – Recordkeeping
Operators using electronic record-keeping systems must obtain authorization through Operations Specification A025 and meet requirements for controlled access, data integrity, backups, and auditability, including a demonstration of the system before approval.17Federal Aviation Administration. AC 120-78B – Electronic Signatures, Electronic Recordkeeping, and Electronic Manuals
Reporting Deficiencies Found During ALS Inspections
When a mandatory ALS inspection reveals a crack, corrosion, or permanent deformation beyond acceptable limits, the finding triggers more than a repair. Under 14 CFR 121.703, Part 121 certificate holders must file a Service Difficulty Report for any crack, permanent deformation, or corrosion of aircraft structures that exceeds acceptable thresholds, and for any failure or defect that has endangered or may endanger safe operation.18eCFR. 14 CFR 121.703 – Service Difficulty Reports
Reports go to FAA offices in Oklahoma City within 96 hours and must include the aircraft type and identification number, the nature of the failure, the part and system involved, the apparent cause, and the corrective action taken. If more information surfaces later, the operator files a supplement referencing the original report. Withholding a report because some details are still unknown is not permitted; submit what you have and supplement later.
These reports feed a fleet-wide safety database. When the FAA sees the same finding across multiple operators or tail numbers, that pattern often drives the next round of ALS revisions and Airworthiness Directives, closing the loop between inspection, reporting, and the limits that govern the fleet.