ATA chapter numbers, explained

ATA chapter numbers, named for the Air Transport Association of America (ATA), are the two-digit system codes that commercial aircraft maintenance information is filed under: 21 for air conditioning, 32 for landing gear. The numbering belongs to ATA iSpec 2200, which Airlines for America (A4A) describes as including the industry-wide standard for numbering aircraft systems. A4A says ATA Spec 100 and Spec 2100 were incorporated into iSpec 2200 in 2000 and will not be updated beyond their 1999 revision level. The Federal Aviation Administration (FAA) uses the same numbers, both in a code table for safety reports and in the subject line of airworthiness directives.

Where the numbering comes from

A4A, legally the Air Transport Association of America doing business as Airlines for America, sells iSpec 2200 as a global standard for the content, structure, and electronic exchange of aircraft engineering and maintenance information. The product page describes a suite of data specifications for maintenance requirements and procedures and configuration control, with document type definitions included. The revision history runs from 2000.1, the original issue, to 2025.1. Members of the ATA e-Business Program get the specification free.

The chapter numbers are also sold alone. A4A's numbering extract provides the standard for numbering aircraft systems, "often referred to as system or chapter numbers." The extract was first titled "ATA iSpec 2200: Aircraft Systems." Revision 2024.1 removed the printed table of definitions of aircraft groups, systems, and subsystems and replaced it with an Excel file in the download package.

Because the specification is sold, the public detail of what each chapter covers comes from the FAA's derivative table below.

How the chapters read

The FAA's Joint Aircraft System/Component (JASC) code table, last updated October 27, 2008, calls itself a modified version of the ATA Specification 100 code. The four-digit JASC code was introduced in May 1991 to classify Service Difficulty Reports. In some cases it matches the first three digits of the ATA code, which identify systems; the ATA code does not use the fourth digit, so JASC uses it for components.

The table groups the chapters. Aircraft topics include 11 placards and markings and 12 servicing. Airframe systems run from 21 air conditioning through 57 wings, with 24 electrical power, 27 flight controls, 29 hydraulic power, 32 landing gear, and 34 navigation among them. Propeller and rotor systems sit in the 60s and powerplant in 71 through 85.

Inside a chapter the digits narrow. Code 2100 is the air conditioning system, 2120 air distribution, and 2121 the air distribution fan. Code 3200 covers the units that support and steer the aircraft on the ground or water and let the gear retract, while the door structure belongs to chapter 52, doors.

JASC departs from ATA Spec 100 in places. Turbine engines are in chapter 72 and reciprocating engines in chapter 85, and stall warning moved to 3418 under navigation. The FAA counts 488 JASC codes, down from 568 in its older eight-digit code.

Chapter numbers in FAA rules

JASC starts at chapter 11, but the ATA scheme also has chapter 05, and FAA airworthiness directives (ADs) use it. Paragraph (d) of these ADs names the subject by ATA code. AD 2025-17-09, for ATR72 airplanes and effective September 29, 2025, gives "Air Transport Association (ATA) of America Code 05, Time Limits/Maintenance Checks." The AD requires operators to revise the maintenance or inspection program to add new or more restrictive airworthiness limitations. It adopts European Union Aviation Safety Agency (EASA) AD 2024-0159, dated August 16, 2024, which the FAA says sets limitations for airplane structures and safe life limits. The unsafe condition it names is failure of the chemical oxygen mask release system to release the mask when necessary.

The same field carries system chapters. AD 2017-01-09, for Boeing 767-300 and 767-300F series airplanes and effective February 21, 2017, gives its subject as Code 21, Air Conditioning.

Where military manuals meet the scheme

The link to defense manuals is older than iSpec 2200. The About S1000D page says that in the early 1980s most civil airline projects were documented to ATA 100 while military projects followed national specifications. A European industry working group found ATA 100 the only internationally accepted aerospace documentation specification and set out to harmonize civil and military documentation with ATA 100 as a source document. The ATA e-Business Program is now one of the three organizations that produce S1000D.

Commercial derivative aircraft are where Navy maintenance meets commercial practice directly. The Navy's Naval Aviation Maintenance Program (NAMP), dated October 28, 2024, says commercial derivative aircraft that apply FAA-approved civil maintenance plans offer the Navy and Marine Corps significant life-cycle cost savings. Where an approved manufacturer's plan exists, the program office decides whether commercial practice replaces the NAMP. FAA ADs, which carry an ATA code, are always mandatory, but major changes to commercially controlled components still need Naval Air Systems Command (NAVAIR) board approval and a technical directive.

What to check

• Which iSpec 2200 revision does the manual follow? A4A lists 2025.1 as the newest, and Spec 100 stopped at its 1999 level.
• Does a code follow ATA Spec 100 or the FAA's JASC table, which moves reciprocating engines to chapter 85 and stall warning to 3418?
• For a landing gear door, is the item filed under chapter 32 for function or chapter 52 for structure?
• Does an AD's ATA code in paragraph (d) match the chapter of the manual task it changes?
• For a commercial derivative aircraft, which parts of the NAMP has the program office replaced with the manufacturer's plan?

Azcuna's TRACE is one offline viewer for S1000D, MIL-STD-3001 XML, and PDF technical manuals. It keeps the source location and the markings of everything it shows and never modifies the source. It was proposed to NAVAIR under Navy SBIR Phase I topic DON26BZ05-NV078, and the proposal was submitted on September 23, 2026.

Sources

1. 1iSpec 2200: Information Standards for Aviation Maintenance, Revision 2025.1 · Airlines for America (A4A) Publications
2. 2iSpec 2200 Extract: ATA Standard Numbering System, Revision 2024.1 · Airlines for America (A4A) Publications
3. 3Joint Aircraft System/Component Code Table and Definitions · Federal Aviation Administration, Flight Standards Service, AFS-620, last updated October 27, 2008
4. 4Airworthiness Directives; ATR-GIE Avions de Transport Régional Airplanes (AD 2025-17-09) · Federal Aviation Administration, Federal Register, August 25, 2025
5. 5Airworthiness Directives; The Boeing Company Airplanes (AD 2017-01-09) · Federal Aviation Administration, Federal Register, February 16, 2017 (final rule; correction)
6. 6About S1000D · S1000D Council and Steering Committee (s1000d.org)
7. 7The Naval Aviation Maintenance Program (NAMP), COMNAVAIRFORINST 4790.2E, 28 Oct 2024 · Commander, Naval Air Forces, posted by Naval Air Systems Command

Where this shows up in our work

• TRACE, the offline S1000D and IETM viewer opens S1000D, MIL-STD-3001 and PDF technical manuals on a tablet with every marking intact.
• ELLMENT, the offline LLM for E-2D crews answers a crew's question from the aircraft's own manuals and cites the page, with no network.