14 CFR Parts 121 and 135 — Air Carrier Maintenance Programs

Overview: Why Commercial Operators Follow Different Rules

When an airline or charter company puts an aircraft into revenue service, the stakes change. Hundreds of passengers may depend on a single maintenance decision, and the operator flies far more hours per year than a typical private owner. Congress and the FAA responded by creating separate regulatory frameworks for commercial operators. 14 CFR Part 121 covers domestic, flag, and supplemental air carriers (think scheduled airlines). 14 CFR Part 135 covers commuter and on-demand operations (think regional turboprop commuters and charter operators). 14 CFR Part 91 remains the baseline for general aviation. Understanding where these rules differ — and overlap — is essential for any A&P who works in commercial aviation or as a contract shop serving those operators.

Part 91: The General Aviation Baseline

Part 91 sets the minimum maintenance standards that apply to almost every civil aircraft. The registered owner or operator is responsible for airworthiness. Inspections follow either an annual/100-hour schedule or a manufacturer-approved progressive inspection program. Records must document each maintenance action, but the format requirements are relatively straightforward. An A&P certificated under 14 CFR Part 65 can approve an aircraft for return to service after most maintenance tasks, and an Inspection Authorization (IA) holder approves annual inspections. Approved data (manufacturer manuals, Airworthiness Directives, and FAA-approved data) must be used, but the owner has flexibility in selecting who performs the work.

Part 121: Air Carrier Maintenance Programs

Part 121 operators must have an FAA-approved maintenance program before they can carry passengers. This program is not a suggestion — it is a legal document incorporated into the operator's Operations Specifications (OpSpecs). Key features include:

• Continuous Airworthiness Maintenance Program (CAMP): Rather than a fixed annual inspection, a CAMP divides maintenance into scheduled tasks performed at defined intervals throughout the aircraft's operating life. Tasks are derived from the aircraft manufacturer's Maintenance Review Board (MRB) report and are approved by the FAA.
• Maintenance Manual: The operator publishes a General Maintenance Manual (GMM) or equivalent document that governs how all maintenance is planned, performed, recorded, and inspected. Every procedure must trace back to approved data.
• Record Requirements: Part 121 record-keeping is more detailed than Part 91. Records must include the identity of the person performing the work, the person approving the return to service, and the approved data used. Records for major structural inspections may need to be retained for years beyond those required under Part 91.
• Inspection Personnel: Part 121 operators designate specific individuals as authorized inspectors. Not every A&P on the floor may sign off an aircraft for return to service — authority flows through the company's approved quality system.
• FAA Surveillance: The FAA's Flight Standards District Office (FSDO) and, for larger carriers, the Certificate Management Office (CMO) conduct ongoing surveillance of the carrier's maintenance program, including unannounced ramp inspections and record audits.

Part 135: On-Demand and Commuter Operations

Part 135 operators also need FAA approval of their maintenance program, but the specific requirements scale with aircraft size and operation type. Commuter operations (scheduled service with aircraft of 9 or fewer passenger seats) and on-demand charter operations each carry somewhat different obligations, but both share these core features:

• An approved aircraft inspection program (AAIP) or use of the manufacturer's inspection program, accepted by the FAA.
• Record-keeping requirements that are more rigorous than Part 91 but may be less complex than a large Part 121 carrier.
• Personnel authorization requirements specifying who may return an aircraft to service.
• Operations Specifications that define approved maintenance providers and procedures.

Because Part 135 operators range from single-aircraft charter companies to large regional airlines, their programs vary widely, but all must be FAA-approved and followed exactly.

Contract Maintenance Shops and Air Carrier Programs

Air carriers routinely outsource maintenance to Certificated Repair Stations (CRS) certificated under 14 CFR Part 145, or to individual certificated mechanics. This is where the concept of a contract maintenance agreement becomes critical. When a repair station or A&P performs work for a Part 121 or Part 135 operator, they are working within the operator's approved maintenance program, not just under Part 91 rules. The practical implications are:

1. The carrier's procedures govern. The contract shop must follow the carrier's GMM and approved data, even if those procedures are more restrictive than the shop's own repair station manual.

2. Record formats must meet carrier requirements. A work order that satisfies Part 91 may not satisfy the carrier's record requirements. The A&P must use the carrier-specified forms or data entry system.

3. Authorization levels are defined by the carrier. The carrier's OpSpecs specify which personnel at the contract shop are authorized to return the aircraft to service. This authorization must be coordinated before work begins.

4. Tooling and parts must be traceable. Parts installed on Part 121 aircraft require complete traceability documentation (serviceable tags, 8130-3 forms for FAA-approved parts).

Safety reminder: Any time you work in an unfamiliar facility or on an unfamiliar aircraft type, review the applicable Maintenance Manual before touching the aircraft. Lockout/tagout procedures and electrical/hydraulic energy isolation requirements must follow the carrier's approved program, which may be more stringent than general shop practice. Always confirm PPE requirements with the carrier's maintenance supervisor before entering maintenance areas on commercial aircraft.

Why This Matters for Your Career

Most A&P jobs in commercial aviation involve interacting with an air carrier's approved maintenance program, even if you never work directly for an airline. Understanding what the regulations require — and that the carrier's manual is as binding as the regulation itself — protects you legally, protects passengers, and helps you communicate professionally with carrier quality assurance personnel.