Explainer
The Airworthiness Limitations Section (ALS)
The Definition
The Mandatory Part of the ICA
Every design approval holder owes Instructions for Continued Airworthiness (ICA): the manuals that tell mechanics how to service, inspect, and repair the product. Most of that material is the manufacturer's recommendation, written to be acceptable to the FAA. One section is different. The rules for every certification part require the ICA to contain a section titled Airworthiness Limitations that is segregated and clearly distinguishable from the rest of the document.
Where the ICA run to several documents, the section must sit in the principal manual. And it must carry, in a prominent location, a statement that for transport airplanes reads: “The Airworthiness Limitations section is FAA-approved and specifies maintenance required under §§ 43.16 and 91.403 of the Federal Aviation Regulations, unless an alternative program has been FAA approved.”
That sentence explains the whole arrangement. The rest of the ICA must be acceptable to the FAA; the limitations section is approved by it, because it states maintenance the regulations make compulsory.
Who Must Comply
Not a Recommendation
Two rules give the section its force. 14 CFR 43.16 requires anyone performing an inspection or other maintenance specified in an Airworthiness Limitations section to do it in accordance with that section. 14 CFR 91.403(c) bars anyone from operating an aircraft whose manual or ICA contains an Airworthiness Limitations section unless the mandatory replacement times, inspection intervals, and related procedures in it have been complied with.
The rules allow two alternatives, and only two: inspection intervals and procedures in operations specifications approved under Part 121 or 135, or an inspection program approved under 91.409(e). An operator's own preference, or a mechanic's judgment that a part looks fine, is not one of them.
This is also why a maintenance planning document and an operator's maintenance program treat limitation items differently from ordinary scheduled tasks: the ordinary tasks can be escalated with evidence, the limitations cannot be changed without the approval of the authority behind them.
Contents
What the Section Must Contain
The list depends on what was certified. Appendix H to Part 25 asks the most of transport airplanes, which must set forth six kinds of limitation:
- Structural limitations
- Each mandatory modification time, replacement time, structural inspection interval, and related structural inspection procedure approved under the fatigue and damage tolerance rule, 25.571.
- Fuel tank system limitations
- Each mandatory replacement time, inspection interval, and related inspection procedure for the fuel tank system, plus all critical design configuration control limitations approved under 25.981.
- EWIS replacement times
- Any mandatory replacement time for electrical wiring interconnection system components.
- Limit of validity
- The limit of validity (LOV) of the engineering data behind the structural maintenance program, stated in flight cycles, flight hours, or both.
- Lightning protection limitations
- Each mandatory replacement time, inspection interval, and related inspection and test procedure, and each critical design configuration control limitation, for each lightning protection feature approved under 25.954.
- Certification maintenance requirements
- Each certification maintenance requirement established to comply with any applicable provision of Part 25.
The other certification parts are shorter. For normal category airplanes (Appendix A to Part 23) and rotorcraft (Appendix A to Parts 27 and 29), the section sets forth each mandatory replacement time, structural inspection interval, and related structural inspection procedure required for type certification. For engines (Appendix A to Part 33) and propellers (Appendix A to Part 35), it is each mandatory replacement time, inspection interval, and related procedure. Rotorcraft engines with 30-second and 2-minute one-engine-inoperative ratings must also prescribe the mandatory post-flight inspections and maintenance after any use of those ratings. For the full manual set behind an engine or propeller ICA, see engine and propeller manuals.
So when someone asks what airworthiness limitations are divided into, the honest answer is: whatever the applicable appendix requires, and for an airliner that means structures, fuel tank safety, wiring, the limit of validity, lightning protection, and certification maintenance requirements. Some manufacturers publish the section as several parts by subject.
The Vocabulary
Terms You Will Meet in an ALS
- AWL (airworthiness limitation)
- An item the certification process found critical, typically from a fatigue or damage tolerance assessment, with a mandatory inspection interval or replacement time. CASA's guidance says AWLs should be treated with the same weight as the strictest class of certification maintenance requirement.
- CMR (certification maintenance requirement)
- A scheduled task established during design certification as an operating limitation of the type certificate or STC, usually from the numerical safety analysis. CMRs are failure-finding tasks: they exist to catch hidden failures that, combined with another failure, could become hazardous or catastrophic.
- One-star and two-star CMRs
- Many manufacturers split CMRs into tasks whose intervals cannot change without the state of design's approval, and tasks whose intervals can be varied under an approved procedure and monitoring program. Some manufacturers define the stars the other way round, so read the introduction to the data you are using.
- CDCCL (critical design configuration control limitation)
- A design feature, such as wire separation, a panel gasket, or a bonding jumper resistance, that must be kept exactly as designed for the life of the aircraft. CDCCLs come from the fuel tank safety work under SFAR 88 and must be checked whenever maintenance, repairs, or alterations touch the area.
- LOV (limit of validity)
- The point, in flight cycles or flight hours, beyond which the engineering data supporting the structural maintenance program has not been shown to hold.
- Life-limited part
- Under 14 CFR 43.10, any part for which a mandatory replacement limit is specified in the type design, the ICA, or the maintenance manual.
CMRs and the MSG-3 tasks behind most scheduled maintenance come from different kinds of analysis, which is why they live in different documents. The FAA's maintenance review board guidance notes that candidate CMRs come from system safety assessments, and that some of them end up as CMRs listed in the product's Airworthiness Limitations section.
In the Manuals
Where the Section Lives
In the ATA iSpec 2200 chapter numbering used across commercial aviation, chapter 04 is Airworthiness Limitations, ahead of chapter 05, time limits and maintenance checks, in the aircraft maintenance manual and the documents around it. Many older aircraft carry their airworthiness limitations in the same maintenance manual chapter as the recommended overhaul periods, which is exactly the blending the segregation requirement exists to prevent in newer data.
Structural limitations connect to repair data too: a repair to a structure covered by an inspection program can bring inspection requirements of its own, which is one reason the structural repair manual and the limitations section need to be read together. For how all of these documents fit, see our overview of the types of aircraft manuals.
For Writers
Publishing a Section That Is Approved, Not Accepted
For a publications team, the approval status changes how the section has to be handled. It needs its own revision record, because a change to it is a change to approved data. It needs the prescribed statement, word for word, where a reader will see it. Its content must never be blended into ordinary maintenance tasks, even when the same inspection appears in both places, and every limitation should be traceable to the certification analysis that produced it.
Revisions also have to reach everyone who holds the ICA, quickly and provably: an operator working to a superseded limitation is out of compliance however well they maintain the aircraft. Our piece on keeping revisions current covers that problem, and it is one a delivery platform solves better than a shelf of binders.
FAQ
Questions We Hear
What is the Airworthiness Limitations section?
The Airworthiness Limitations section (ALS) is the part of an aircraft's, engine's, or propeller's Instructions for Continued Airworthiness that sets out the maintenance which is mandatory: replacement times, inspection intervals, and the related procedures required for type certification. The regulations require it to be segregated and clearly distinguishable from the rest of the ICA, placed in the principal manual where there are several documents, and to carry a statement that it is FAA-approved.
Are airworthiness limitations mandatory?
Yes. 14 CFR 43.16 requires anyone performing an inspection or other maintenance specified in an Airworthiness Limitations section to do it in accordance with that section, and 91.403(c) bars operating an aircraft unless its mandatory replacement times, inspection intervals, and related procedures have been complied with. The only alternatives the rules allow are intervals in operations specifications approved under Part 121 or 135, or an inspection program approved under 91.409(e).
What are airworthiness limitations divided into?
For a transport airplane, Appendix H to Part 25 lists six kinds: structural limitations from the fatigue and damage tolerance rule, fuel tank system limitations including critical design configuration control limitations, EWIS replacement times, the limit of validity, lightning protection limitations, and certification maintenance requirements. Normal category airplanes, rotorcraft, engines, and propellers have shorter lists centered on mandatory replacement times and inspection intervals. Some manufacturers publish the section as several separately revised parts by subject.
What is the difference between an AWL and a CMR?
An airworthiness limitation item usually comes from structural fatigue or damage tolerance analysis and sets a mandatory inspection or replacement. A certification maintenance requirement comes from the safety analysis of aircraft systems and is a failure-finding task meant to expose hidden failures. Both are mandatory where they appear in the ALS. CMRs also come from a different process than the MSG-3 analysis behind most scheduled maintenance tasks.
Where is the Airworthiness Limitations section in the manuals?
In the ATA iSpec 2200 numbering used by most commercial aircraft manuals, chapter 04 is Airworthiness Limitations. Some manufacturers publish the section in separate parts by subject, while many older aircraft carry their airworthiness limitations in the same maintenance manual chapter as recommended overhaul periods.
Does an STC need its own Airworthiness Limitations section?
If the modification introduces mandatory replacement times or inspections, they belong in an Airworthiness Limitations section in the STC holder's ICA, with the same segregation and approval statement. Where the change adds none, the ICA should say so clearly. An STC holder owes ICA under 14 CFR 21.50(b), and the limitations section is part of them.
Get In Touch
Publishing an ALS?
We structure airworthiness limitations as segregated, separately controlled content within your ICA, and deliver revisions to every holder with a record that they arrived.