4.1 14 CFR Part 43 Maintenance Records and Return to Service

4.1 14 CFR Part 43 Maintenance Records and Return to Service

Core Aviation Standard: In civil aviation maintenance, physical craftsmanship and regulatory documentation carry equal legal standing. Under Title 14 of the Code of Federal Regulations (14 CFR), an unrecorded maintenance action or improper return-to-service entry renders an aircraft legally unairworthy, regardless of the physical quality of the installation. Avionics technicians must strictly execute maintenance record entries, understand the scope of technician certification, identify major alterations, and process FAA Form 337 documents in compliance with federal statutes.

Every avionics installation, repair, inspection, or software update performed on a certified aircraft must be substantiated by permanent records. These records provide the legal audit trail required for aircraft owners, future technicians, and Federal Aviation Administration (FAA) inspectors to verify continuous compliance with applicable airworthiness standards.

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14 CFR § 43.9 Maintenance Record Entries

Title 14 CFR § 43.9 governs the recording of maintenance, preventive maintenance, rebuilding, and alterations. Any technician or certified entity performing work on an aircraft, airframe, aircraft engine, propeller, appliance, or component part must enter specific information into the permanent maintenance records of that equipment.

The Four Mandatory Elements of 14 CFR § 43.9(a)

Under § 43.9(a), each maintenance entry must contain four distinct, non-negotiable items:

1. Description of Work Performed: A detailed technical description of the work accomplished, or an explicit reference to technical data acceptable to the FAA Administrator (such as an OEM Component Maintenance Manual [CMM], Aircraft Maintenance Manual [AMM], or Supplemental Type Certificate [STC] installation instructions).
2. Date of Completion: The calendar date on which the maintenance action was completed.
3. Name of Person Performing Work: The name of the individual who physically performed the work, if that person is different from the certified individual approving the work for return to service (for example, when an uncertificated apprentice works under the direct supervision of a certificated mechanic or repairman).
4. Approval for Return to Service: The signature, certificate number, and kind of certificate held by the person approving the aircraft, airframe, or appliance for return to service.

+--------------------------------------------------------------------------+
|                 14 CFR § 43.9 MAINTENANCE ENTRY STRUCTURE                |
+--------------------------------------------------------------------------+
| [1] DESCRIPTION: Removed defective VHF COMM transceiver P/N 011-01060-00 |
|     S/N 12345. Installed serviceable unit P/N 011-01060-00 S/N 67890     |
|     in accordance with Cessna 172 Maintenance Manual Chapter 23-10-00.   |
|     Performed post-installation ground functional test and ramp check    |
|     per AMM 23-10-00; operational checks normal.                         |
|                                                                          |
| [2] DATE: October 3, 2026                                                |
| [3] PERFORMED BY: J. Doe (under supervision)                             |
| [4] RETURN TO SERVICE: [Signature] R. Smith, A&P 3456789 IA              |
+--------------------------------------------------------------------------+

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14 CFR § 43.11 Inspection Records

While § 43.9 governs routine maintenance, repair, and alteration entries, 14 CFR § 43.11 establishes the legal requirements for recording inspections. It covers inspections performed under Part 91 (such as annual, 100-hour, and progressive inspections), Part 125, and certain Part 135 inspections. The 24-month tests under § 91.411 (altimeter and static system) and § 91.413 (ATC transponder) work differently: Part 43 Appendix E paragraph (d) and Appendix F paragraph (k) say to record them under § 43.9. The person testing an altimeter also marks on it the test date and the maximum altitude to which it was tested.

Mandatory Elements of an Inspection Entry (§ 43.11(a))

An inspection record entry must include:

• Type of Inspection: The specific inspection performed and a brief description of the scope (e.g., "Annual Inspection" or "100-Hour Inspection").
• Date and Total Time in Service: The exact date of completion and the aircraft total flight time in service.
• Airworthiness Certification Statement: Specific regulatory phrasing depending on whether the aircraft is approved or unairworthy:
  • Approved: "I certify that this aircraft has been inspected in accordance with [type of inspection] and was determined to be in airworthy condition."
  • Discrepancies / Unairworthy: "I certify that this aircraft has been inspected in accordance with [type of inspection] and a list of discrepancies and unairworthy items dated [date] has been provided for the aircraft owner or operator."
• Signature and Certificate: The signature, certificate number, and kind of certificate held by the authorized inspector.

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Authorization to Perform vs. Approve for Return to Service

A critical distinction in federal aviation regulations exists between who is authorized to perform maintenance and who is legally authorized to approve for return to service.

Persons Authorized to Perform Maintenance (14 CFR § 43.3)

Under § 43.3, maintenance, preventive maintenance, and alterations may be performed by:

• A certificated mechanic holding appropriate airframe and/or powerplant ratings (A&P).
• A certificated repairman holding appropriate privileges under 14 CFR Part 65.
• A person working under the supervision of a certificated mechanic or repairman, if the supervisor personally observes the work to the extent necessary to ensure it is done properly and is readily available, in person, for consultation. This does not authorize any Part 91 or Part 125 inspection, or any inspection after a major repair or alteration (§ 43.3(d)).
• A certificated Part 145 repair station within its approved ratings.
• A Part 121 or 135 certificate holder, as its operating rules provide (§ 43.3(f)).
• The manufacturer of the aircraft, aircraft engine, propeller, or appliance.
• A pilot certificate holder (sport pilots are limited to light-sport aircraft), performing the preventive maintenance listed in Part 43 Appendix A, paragraph (c), on an aircraft the pilot owns or operates that is not used under Part 121, 129, or 135 (§ 43.3(g)).

Persons Authorized to Approve for Return to Service (14 CFR § 43.7)

Under § 43.7, the authority to approve an aircraft, airframe, or appliance for return to service is restricted to:

• The holder of a mechanic certificate or inspection authorization, as provided in Part 65.
• The holder of a repair station certificate, as provided in Part 145. The repair station acts through the people it authorizes, who may include its certificated repairmen.
• A manufacturer, for work it performed under § 43.3(j). Except for minor alterations, that work must follow FAA-approved data.
• A Part 121 or 135 certificate holder, as its operating rules provide.
• A person holding at least a private pilot certificate, after performing preventive maintenance under § 43.3(g). Sport pilots and light-sport repairmen have parallel privileges for light-sport aircraft.

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Part 65 Repairman Privileges and Limitations

Avionics technicians working in specialized repair facilities often hold an FAA Repairman Certificate issued under 14 CFR Part 65 Subpart E (§ 65.101 through § 65.105).

Eligibility Requirements (§ 65.101)

To qualify for a Repairman certificate, an applicant must:

• Be at least 18 years of age.
• Be specially qualified to perform maintenance on aircraft or components appropriate to the job for which employed.
• Be employed for a specific job requiring those special qualifications by a certificated repair station, or by a certificated commercial operator or air carrier that must provide a continuous airworthiness maintenance program.
• Be recommended for certification by the employing repair station or air carrier.
• Have at least 18 months of practical experience in the procedures, practices, inspection methods, materials, tools, and equipment used in the assigned job (§ 65.101(a)(5)(i)), or have completed formal training that is acceptable to the Administrator and specifically designed to qualify the applicant for that job (§ 65.101(a)(5)(ii)).
• Be able to read, write, speak, and understand English, unless employed outside the United States, in which case the certificate is endorsed "Valid only outside the United States."

Privileges and Limitations (§ 65.103)

1. Employer-Specific Privilege: Under § 65.103(a), a repairman may perform or supervise maintenance appropriate to the job for which they were employed and certificated, but only in connection with duties for the certificate holder that employed and recommended them.
2. Does Not Travel With the Technician: The privileges cannot be used for another employer or for independent work. A technician who changes employers needs a new recommendation and certificate for the new job. A mechanic certificate, by contrast, belongs to the individual.
3. Facility Limitations: A repairman cannot perform freelance maintenance, issue independent sign-offs on private aircraft, or approve work for return to service outside the employing repair station's quality control system.
4. Current Instructions: Under § 65.103(b), a repairman may not use the certificate's privileges without understanding the employer's current instructions and the manufacturer's Instructions for Continued Airworthiness for that work. A repairman certificate also carries no inspection authorization; an IA can only be issued to a certificated mechanic (§ 65.91).

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Major Alterations and Major Repairs (14 CFR Part 43 Appendix A)

Federal regulations divide aircraft maintenance and modifications into minor and major categories. The classification dictates whether standard maintenance manual procedures are sufficient or whether approved technical data and formal FAA reporting are mandatory.

Regulatory Definitions (14 CFR § 1.1)

• Major Alteration: An alteration not listed in the aircraft, aircraft engine, or propeller specifications (Type Certificate Data Sheet [TCDS]):
  1. That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other airworthiness qualities; or
  2. That is not done according to accepted practices or cannot be done by elementary operations.
• Major Repair: A repair that, if improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other airworthiness qualities; or is not done according to accepted practices or cannot be done by elementary operations.

What Part 43 Appendix A Actually Lists

Appendix A does not list avionics installations by name. It lists the parts and types of alteration that are major when they are not listed in the aircraft specifications, including:

For anything else, apply the § 1.1 definition above: would the change appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other airworthiness qualities, or can it not be done by elementary operations? FAA Order 8300.16 (Major Repair and Alteration Data Approval) gives the FAA's classification guidance. An installation done under an STC is normally a major alteration with the STC as its approved data. A direct replacement with an identical or approved interchangeable unit, using the existing wiring and mounting, is normally a minor alteration recorded under § 43.9.

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FAA Form 337: Execution, Routing, and Field Approvals

Every major repair and major alteration performed on a civil aircraft must be recorded on FAA Form 337 (Major Repair and Major Alteration: Airframe, Powerplant, Propeller, or Appliance).

Form 337 Execution and Routing (14 CFR Part 43 Appendix B)

Under Part 43 Appendix B, the person or facility performing a major alteration or major repair must:

1. Execute Form 337 in at least duplicate.
2. Give a signed copy to the aircraft owner, who keeps it with the aircraft records (14 CFR 91.417(a)(2)(vi)).
3. Forward a copy to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours after the aircraft, airframe, engine, propeller, or appliance is approved for return to service.

Older references, and some shop habits, still say to send the 337 to the local FSDO. The current Appendix B names the Aircraft Registration Branch. For major repairs (not alterations) made under a manual or specifications acceptable to the Administrator, a certificated repair station may instead use the customer's work order and give the owner a signed maintenance release (Appendix B(b)).

                     FAA FORM 337 WORKFLOW & ROUTING:

    +-----------------------------------------------------------+
    |                  AVIONICS MAJOR ALTERATION                |
    |          (e.g., Autopilot or Glass Cockpit STC)           |
    +-----------------------------+-----------------------------+
                                  |
                                  v
    +-----------------------------------------------------------+
    |                 EXECUTE FAA FORM 337                      |
    |  - Block 1-7: Airframe, Owner, and Facility Info          |
    |  - Block 8: Technical Narrative, STC & ICA References,   |
    |             Weight & Balance, Electrical Load Analysis    |
    +-----------------------------+-----------------------------+
                                  |
                                  v
    +-----------------------------------------------------------+
    |             APPROVE FOR RETURN TO SERVICE                 |
    |      (A&P with IA, Part 145 Repair Station, or Mfr)       |
    +-----------------------------+-----------------------------+
                                  |
                   +--------------+--------------+
                   |                             |
                   v                             v
        +---------------------+       +---------------------+
        |      ORIGINAL       |       |   DUPLICATE COPY    |
        +---------------------+       +---------------------+
        | Delivered to Owner/ |       | Forwarded to FAA    |
        | Aircraft Operator   |       | Registry (OKC) in   |
        | (Permanent Records) |       | 48 hours of RTS     |
        +---------------------+       +---------------------+

Block 8: Description of Work Accomplished

Block 8 on the reverse side of Form 337 requires a comprehensive technical narrative. A legally compliant Block 8 narrative must state:

• The exact nature of the work performed, identifying components by manufacturer, model, part number, and serial number.
• The specific approved technical data supporting the alteration (e.g., Supplemental Type Certificate number, FAA-approved drawing revisions, or FAA Form 8110-3 engineering approval).
• Reference to Instructions for Continued Airworthiness (ICA) developed for the installation.
• Updated weight and balance calculations, including the new empty weight, center of gravity (CG), and useful load.
• An electrical load analysis when the alteration adds load. AC 43.13-1B paragraph 11-35 uses a rule of thumb that flags continuous loads able to exceed 80 percent of generator output.

Block 3: FAA Field Approval

When an avionics major alteration is performed without an existing Supplemental Type Certificate (STC), the alteration requires an FAA Field Approval:

• Prior to return to service, technical data (wiring schematics, structural doubler analyses, antenna radiation patterns) must be submitted to the local FAA FSDO.
• An FAA Aviation Safety Inspector (ASI) reviews the technical data, inspects the physical aircraft, and approves the data by physically signing and dating Block 3 (For FAA Use Only) on Form 337.
• Once Block 3 carries the FAA inspector's approval signature, the data becomes FAA-approved data, allowing the technician or repair station to approve the aircraft for return to service.

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Avionics Shop Worked Scenario: ADS-B Out Transponder Major Alteration

Scenario: An avionics repair station installs an L3Harris Lynx NGT-9000 Mode S / ADS-B Out transponder system on a Piper PA-28-181 Archer. The installation replaces an obsolete Narco AT-150 Mode C transponder and requires adding an active WAAS GPS antenna on the fuselage crown, a directional traffic antenna on the fuselage bottom, and connecting an RS-232 serial altitude encoder feed.

Step 1: Determine Alteration Classification

• The shop installs the ADS-B Out system under the manufacturer's STC, so the job is handled as a major alteration with the STC as its approved data.
• Action: A Form 337 is completed for the alteration.

Step 2: Establish Technical Data Baseline

• The installation is performed under the manufacturer's Approved Model List (AML) STC. The shop confirms that the PA-28-181 appears on the current AML, and that the owner has the STC holder's written permission to use the STC (14 CFR 91.403(d)).
• The manufacturer's STC Installation Manual provides FAA-approved structural doubler drawings and wiring interconnection schematics.
• Because approved STC data is utilized, no Block 3 Field Approval from the FSDO is required prior to installation.

Step 3: Perform Engineering Calculations

• Calculate weight and balance change: $+4.2\ \text{lbs}$ at fuselage station $+72.5\ \text{inches}$. Recalculate aircraft empty weight and CG.
• Perform continuous electrical load analysis: Transponder continuous draw is $1.8\ \text{A}$ at 14 VDC. Verify bus load remains below 80% of alternator 60A rating.

Step 4: Execute Documentation and Routing

• Complete Form 337 Blocks 1 through 7.
• In Block 8, record exact part numbers, antenna co-axial cable types (RG-400), reference the STC number, document revised empty weight and electrical load, and reference the STC's Instructions for Continued Airworthiness.
• Perform Part 43 Appendix F transponder tests and Part 43 Appendix E encoder correlation tests.
• Sign Block 7 approving the alteration for return to service under the repair station certificate.
• Give a signed copy of Form 337 to the owner, forward a copy to the FAA Aircraft Registration Branch within 48 hours, and make the § 43.9 maintenance record entry referencing the Form 337.