Replacing an aircraft engine is one of the most significant maintenance events in the life of an airframe. Whether swapping out a tired powerplant for a factory-new unit, installing a rebuilt engine, or fitting an overhauled assembly, the physical work is only half the job. The other half — arguably just as important for legal and safety reasons — is creating an accurate, complete, and properly signed maintenance record. These records are not bureaucratic formalities; they are the legal evidence that the work was done correctly, by a qualified person, and that the aircraft is airworthy and approved for return to service.
This article walks through exactly what documentation is required after an engine replacement, how to structure logbook entries, who has the authority to approve an aircraft for return to service, and the critical distinctions between types of engine work that affect what paperwork must accompany the installation. Every requirement discussed is grounded in 14 CFR Part 43, Part 91, and FAA guidance in the Aviation Maintenance Technician Handbooks.
The Legal Framework: 14 CFR Part 43
Title 14 CFR Part 43 governs the maintenance, preventive maintenance, rebuilding, and alteration of civil aircraft. Section 43.9 establishes the baseline content requirements for maintenance record entries, and Section 43.11 establishes what must appear in the record when a major inspection — such as a 100-hour or annual — coincides with return to service. Section 43.7 defines who may approve an aircraft for return to service after maintenance, and this is critical when engine replacement is involved.
Under 14 CFR 43.9, every maintenance record entry for a completed work item must contain four elements: (1) a description of the work performed, (2) the date the work was completed, (3) the name of the person performing the work, and (4) the signature, certificate number, and kind of certificate held by the person approving the aircraft for return to service. If these four elements are missing or incomplete, the entry is legally deficient — and the aircraft is not properly approved to fly, even if the physical work was done perfectly.
Who May Approve Return to Service After Engine Replacement
Engine replacement almost always constitutes a major repair under 14 CFR Part 43, Appendix A, which lists engine removal and installation as work that may have an appreciable effect on structural strength, flight characteristics, or other airworthiness qualities. A certificated Airframe and Powerplant (A&P) mechanic may perform an engine replacement, but the authority to approve the aircraft for return to service after major repairs belongs specifically to an A&P mechanic holding an Inspection Authorization (IA), a certificated repair station with appropriate ratings, or the aircraft manufacturer — unless an FAA-authorized alternative applies.
An A&P mechanic without an IA may perform the actual removal and installation work and must sign the maintenance record entry describing the work done. However, that mechanic's signature alone does not constitute an approval for return to service after a major repair. The IA holder or repair station must separately review the work and provide the approval signature. This two-signature scenario is common and important — both the performing technician and the approving authority must be documented.
If the engine replacement also involves an alteration — for example, installing an engine of a different model not listed in the aircraft's Type Certificate Data Sheet (TCDS) — additional FAA approval via a Supplemental Type Certificate (STC) or a field approval (Form 337) may be required before any return-to-service entry can be made.
Constructing the Maintenance Record Entry
The maintenance record entry for an engine replacement should be made in the airframe logbook and the engine logbook (or engine records if the aircraft uses separate records for each). Many technicians also make a cross-reference entry noting where the complementary entry can be found. Here is what a thorough entry must include:
- Description of work performed: State clearly that the engine was removed and replaced. Identify the removed engine by make, model, serial number, and total time in service (TTIS) at removal. Identify the installed engine by make, model, serial number, and its time since new (TSN), time since overhaul (TSOH), or time since rebuild — whichever applies. Reference the applicable maintenance manual and revision level used.
- Compliance with manufacturer's instructions: Note which manufacturer's maintenance or installation manual procedures were followed, including any specific torque values, safety-wiring steps, operational checks, or test runs performed per the manual. If any service bulletins or airworthiness directives (ADs) were complied with during the installation, list them by number and state the method of compliance.
- Date of completion: The calendar date the work was completed and the approval for return to service was given. If work spanned multiple days, the date of final approval is used in the return-to-service entry.
- Aircraft total time and engine time: Record the aircraft total airframe time and the new engine's time at installation (commonly zero for a new or rebuilt engine, or carried-forward hours for an overhauled engine) so the historical record remains continuous.
- Technician signature and certificate information: The performing technician signs, prints their name, and lists their certificate number and type (e.g., A&P certificate number XXXX). The approving authority (IA or repair station) signs separately, with their authorization number.
Engine Logbook Entries and Continued Airworthiness Records
The engine logbook is a document that follows the engine itself, not the airframe. When the overhauled or new engine was last approved for return to service at the overhaul shop, that shop should have made an entry in the engine records specifying the work performed, the standard to which the overhaul was accomplished (e.g., manufacturer's overhaul manual, FAA-approved data), and the total time at overhaul (usually zero for a rebuilt engine).
Upon installation, the installing technician should make an entry in the engine logbook stating the date of installation, the airframe it was installed in (aircraft make, model, registration number N-number), and the total aircraft time at installation. This creates a complete chain of custody. When the engine is later removed — whether for another overhaul or retirement — the records will show exactly which airframes it operated in, for how many hours, and under what maintenance regime.
For rebuilt engines, 14 CFR 43.2(b) provides a special rule: only the manufacturer or an agency approved by the manufacturer may certify an engine as rebuilt. A rebuilt engine may use zero-time records. An overhauled engine, by contrast, retains its accumulated time and cannot reset its time to zero — a distinction that is heavily tested and critically important in practice.
FAA Form 337 for Major Repairs and Alterations
When an engine replacement involves work that qualifies as a major repair (as listed in Part 43 Appendix A), and the approving authority is an IA holder, the IA must complete FAA Form 337, Major Repair and Alteration. This two-copy form describes the major repair or alteration in detail, references the approved data used, and is signed by the IA. One copy is kept with the aircraft's permanent records; the other copy must be forwarded to the FAA within 48 hours of the aircraft's approval for return to service, per 14 CFR 43.9(d) and current Form 337 instructions directing submission to the FAA Aircraft Registration Branch in Oklahoma City.
Failure to submit Form 337 when required — or submitting it late — is a regulatory violation that can result in certificate action against the IA. More practically, an aircraft without the required Form 337 on file is not legally in compliance, and its airworthiness can be challenged during ramp checks or accident investigations.
Why Accurate Documentation Matters
Beyond regulatory compliance, accurate engine replacement records serve several vital safety and practical functions. Service life tracking depends entirely on unbroken records: if an engine's time since overhaul cannot be verified because an installation entry was skipped, the operator may unknowingly fly an engine that has exceeded its recommended TBO. Airworthiness Directive compliance is tracked by serial number — if the installed engine's serial number is never recorded in the airframe log, proving AD compliance becomes impossible during an FAA inspection. Resale value is directly tied to records quality; an aircraft with incomplete or suspicious maintenance logs loses significant market value. And in the event of an accident, investigators will scrutinize maintenance records in detail — a sloppy or missing return-to-service entry can shift legal liability onto the technician.
Key Numbers and Rules
- 14 CFR 43.9: Four required elements for every maintenance record entry — description, date, name, and signature/certificate.
- 14 CFR 43.7: Defines who may approve return to service — IA, certificated repair station, or manufacturer (for major repairs).
- 14 CFR 43.2(b): Only the manufacturer or manufacturer-approved agency may certify an engine as rebuilt; rebuilt engines may use zero-time records; overhauled engines may not.
- Form 337: Required for major repairs; one copy retained with the aircraft records, one copy forwarded to the FAA (Aircraft Registration Branch) within 48 hours of return to service, per 14 CFR 43.9(d).
- Part 43 Appendix A: Lists engine installation/removal as a major repair requiring IA or repair station approval for return to service.
- Engine logbook cross-entry: Always record the N-number of the airframe and the airframe total time at the time of installation in the engine's own records.
Common Test Traps
- A&P vs. IA authority: An A&P without an IA can perform an engine replacement but CANNOT sign the return-to-service approval for a major repair. Students often confuse performing work with approving it.
- Rebuilt vs. overhauled time: Only a rebuilt engine can start with zero time. An overhauled engine always carries its accumulated total time. Mixing these up is one of the most frequently missed questions on AMT knowledge tests.
- Form 337 timing: The 48-hour deadline for submitting the FAA copy of Form 337 is absolute. Students sometimes think the form only needs to be filed at the next convenient opportunity.
- Missing serial numbers: The installed engine's make, model, and serial number must appear in both the airframe and engine logbooks. Omitting the serial number makes the entry functionally useless for AD compliance tracking.
- STC or field approval oversight: Installing an engine not listed in the TCDS requires STC or Form 337 field approval BEFORE return to service — not after. Students sometimes assume the standard engine installation process covers non-listed engine models.