14 CFR Part 43: What It Governs and Why It Matters for MROs and Propulsion Engineers

Prime Propulsion
20 hours ago
7 min read

14 CFR Part 43 is one of the most consequential regulations in the FAA's airworthiness framework, and one of the least discussed in the certification consulting space. Most content about FAA certification focuses on Parts 21, 23, 25, or 33 — the design approval regulations. Part 43 operates differently. It doesn't govern how you design a product. It governs how you maintain, repair, and alter it once it's in service — and what level of FAA authorization is required before that work goes back on an aircraft.

For MRO operators, repair stations, and propulsion engineers working on certificated engines, Part 43 is the regulatory foundation that determines whether a given repair or process requires formal FAA approval, what form that approval takes, and how test cell operations fit into the return-to-service framework. Getting Part 43 right is not optional. It's the line between a return-to-service that stands up to FAA scrutiny and one that doesn't.

What 14 CFR Part 43 Covers

Part 43 establishes the requirements for the maintenance, preventive maintenance, rebuilding, and alteration of certificated aircraft, airframes, aircraft engines, propellers, appliances, and component parts. It defines who is authorized to perform such work, what performance standards apply, what records must be kept, and what approval is required before a maintained article is returned to service.

The regulation applies to all U.S.-registered civil aircraft except those operating under an experimental certificate. For propulsion work specifically, Part 43 governs any maintenance or alteration performed on certificated aircraft engines — including turbofan, turbojet, turboprop, turboshaft, APU, and piston engines — by Part 145 certificated repair stations and other authorized maintenance entities.

The key sections for propulsion engineers and MRO operators are:

§43.2 — Return to Service establishes the requirement that any maintenance, alteration, or rebuilding performed under Part 43 must be performed by authorized individuals and meet the performance rules of §43.13. For engine overhaul facilities, this is also where the test cell requirement lives — an engine tested after overhaul must be tested in a facility that can demonstrate its data is accurate, which is the regulatory basis for test cell correlation.

§43.13 — Performance Rules specifies that maintenance must be performed using methods, techniques, and practices acceptable to the Administrator. This includes manufacturer's maintenance manuals, Instructions for Continued Airworthiness (ICA), and FAA-approved data. When a repair cannot be performed using standard accepted practices or available data, FAA-approved repair data is required — which is where repair specifications become necessary.

§43.15 — Additional Performance Rules for Inspections governs inspection requirements, including annual and 100-hour inspections, and the standards inspectors must apply.

Appendix A — Major Alterations, Major Repairs, and Preventive Maintenance defines which alterations and repairs are classified as major, which are minor, and what work constitutes preventive maintenance. This appendix is where most Part 43 compliance questions originate.

What Makes a Repair "Major" Under Part 43

The distinction between a major repair and a minor repair is one of the most practically significant determinations in aviation maintenance. Minor repairs can be performed and returned to service under the authority of the maintainer, using accepted practices. Major repairs require FAA approval before the article is returned to service.

Part 43 Appendix A defines a major repair as one that, if improperly done, might appreciably affect structural strength, flight characteristics, or other airworthiness qualities — or is not done according to accepted practices, or cannot be done by elementary operations.

For propulsion components specifically, major repairs typically include:

Repairs to engine cases, compressor housings, or turbine sections that involve welding, brazing, or material removal beyond ICA limits
Hot section component repairs using non-standard techniques or materials
Fuel system component repairs that affect flow characteristics or sealing integrity
Accessory gearbox repairs involving dimensional restoration beyond manufacturer limits
Any repair requiring engineering analysis to substantiate structural adequacy

The practical test is straightforward: if the repair methodology isn't clearly described in the manufacturer's ICA, CMM, or other approved data — or if performing it requires engineering judgment that goes beyond what those documents provide — it's almost certainly a major repair requiring formal FAA approval.

The Two Paths for Major Repair Approval Under Part 43

When a repair is classified as major, Part 43 provides two primary paths to FAA approval.

Form 337 — One-Time Field Approval

A Form 337 (Major Repair and Alteration) is a one-time approval for a specific major repair on a specific article. The form documents the nature of the repair, the data used, and the authorized individual who performed and inspected the work. It satisfies the return-to-service requirement for that one article, but it does not create reusable authority. The next time the same repair is performed on a different article, the approval process starts over.

For occasional or unique repairs, Form 337 is appropriate. For repairs performed repeatedly across a fleet of parts, it becomes administratively burdensome and inconsistent.

Repair Specification — Reusable FAA Approval

A repair specification is an FAA-approved document that defines the methodology for performing a specific major repair on a specific part number. Once approved, the repair specification can be applied to any article of that part number without obtaining new approval each time. It creates standardized, repeatable, auditable repair authority that scales with your operation.

For MRO facilities that perform the same repair repeatedly — turbine blade tip restoration, compressor blade blending, fuel nozzle reconditioning, gearbox case repair — a repair specification is the right long-term solution. It eliminates per-unit Form 337 submissions, standardizes the process, and provides the documented engineering foundation needed if the repair is later considered for conversion to a Parts Manufacturer Approval.

The relationship between a repair specification and a PMA is worth understanding. A repair specification authorizes your facility to perform the repair. A PMA authorizes you to manufacture and sell the repaired or replacement article to any operator. If your facility has been running a repair process under a specification and there's commercial demand for that capability beyond your current customer base, PMA conversion is often the logical next step. The engineering data developed for the repair specification becomes the foundation for the PMA substantiation package.

14 CFR §43.2 and Test Cell Correlation

Section 43.2 is the specific provision that governs the return-to-service of overhauled engines — and it's the regulatory basis for the FAA test cell correlation requirement that affects every MRO facility that tests engines after overhaul.

The requirement is straightforward in principle: an engine overhauled and tested before return to service must be tested in a facility that demonstrates it can produce accurate, reliable performance data. In practice, that demonstration is a test cell correlation — a systematic process of comparing your facility's test data against a reference cell (typically the OEM's master test cell) to confirm that your cell produces equivalent results within acceptable tolerances.

Without an approved correlation, an engine tested in your facility cannot be properly returned to service under Part 43. The correlation package — which includes instrumentation documentation, test procedures, raw test data, and statistical analysis — is what establishes that your cell's data is traceable to a known-good baseline.

This requirement applies across engine types. APU test cell correlation has seen significant growth as regional jet and business aviation fleets age and APU overhaul volume increases. Turbofan, turbojet, turboprop, and turboshaft test cells all require correlation documentation. The specific requirements vary by engine type and OEM standards, but the regulatory basis — §43.2 and the requirement for approved facilities — is consistent across all of them.

What Part 43 Means for Quality System Requirements

Part 43 work doesn't exist in isolation from quality system requirements. Part 145 certificated repair stations must maintain a quality system that covers the maintenance work performed, including major repairs. When a repair specification is in place, the quality system must be scoped to cover that specific repair process — inspection criteria, material controls, personnel qualification, and record retention.

This is an area where many MROs encounter gaps. The quality system that covers standard ICA-based maintenance may not adequately address the engineering controls required for a major repair under a formal repair specification. Closing those gaps typically involves adding specific procedures, inspection checkpoints, and documentation requirements — work that is usually incremental rather than a full quality system rebuild.

For MRO operators considering PMA conversion of an existing repair process, the Part 145 quality system also needs to be assessed against the Part 21.137 production quality system requirements that govern PMA holders. The two systems have different scopes and emphases, and the gap between them is one of the three primary items that need to be addressed in the conversion process alongside drawing formalization and substantiation reframing.

Practical Implications for Propulsion Engineers

For engineers working in propulsion maintenance and repair, Part 43 creates a specific framework for how repair work needs to be approached, documented, and approved. A few practical points worth keeping in mind:

When you encounter a repair that isn't covered by existing ICA data, the first determination is whether it's major or minor. Don't assume it's minor because it seems straightforward — the Part 43 Appendix A definition is functional, not complexity-based. A structurally significant repair performed routinely can still be major.

When a major repair is identified, consider whether this is a one-time situation (Form 337) or a recurring process (repair specification). The answer determines the appropriate investment in the approval process.

When a test cell correlation lapses — due to instrumentation changes, facility modifications, or the expiration of a previous correlation — §43.2 compliance is interrupted. Engines tested in a facility with a lapsed correlation are being tested outside the regulatory framework. This is an operational risk that affects every overhauled engine returned to service until the correlation is restored.

When a repair specification becomes commercially valuable — when operators outside your current customer base need the capability you've developed — PMA conversion is worth evaluating. The certification investment is incremental if the repair specification engineering data is solid.

How Prime Propulsion Supports Part 43 Programs

Prime Propulsion's core expertise sits at the intersection of Part 43 and the design approval regulations — the space where maintenance-side work connects to engineering approval requirements. Our founder holds active FAA delegations in Parts 23, 25, 27, 29, and 33, and brings firsthand experience as an FAA engineer, a Pratt & Whitney propulsion engineer, and a U.S. Air Force propulsion Subject Matter Expert.

We support MRO operators and propulsion engineers on repair specification development, test cell correlation packages across all gas turbine engine types, and PMA conversion of proven repair processes. Every engagement is led by the DER directly — no project managers, no handoff queues.

If you have a major repair that needs formal FAA approval, a test cell correlation that needs to be established or renewed, or a repair process that may be ready for PMA conversion, contact us for a free consultation. We'll give you a direct assessment of the right path before you commit to anything.

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