When an aviation maintenance technician (AMT) overhauls an aircraft engine, every measurement taken, every tolerance checked, and every procedure followed must trace back to an FAA-approved data source. Using unapproved data — even data that seems technically accurate — can render the work legally and airworthiness-invalid. The FAA is explicit about this: maintenance must be performed in accordance with the manufacturer's current instructions or other FAA-approved data. For the Powerplant knowledge test and oral exams, you need to know exactly which sources qualify, how they are approved, and how to choose between them when they appear to conflict.
This article walks through every category of approved overhaul data, explains the regulatory basis in 14 CFR and the relevant handbooks, and highlights the practical and exam-critical details you need to carry into the shop and into the testing center.
The Regulatory Foundation
14 CFR Part 43 is the governing regulation for maintenance, preventive maintenance, rebuilding, and alteration of aircraft. Section 43.13 requires that a person performing maintenance use methods, techniques, and practices acceptable to the Administrator. It specifically requires that the work be done in accordance with the current manufacturer's maintenance manual or Instructions for Continued Airworthiness (ICA), or in a manner that is at least equal to the original design. Part 43 Appendix A lists the work that qualifies as a major repair or major alteration, and engine overhaul-related work falls within that list — but the term "overhauled" itself is separately defined by 14 CFR 43.2, which sets the standard an engine must meet (disassembled, cleaned, inspected, repaired as necessary, reassembled, and tested to manufacturer's tolerances and limits using approved standards and technical data) before it can be described as overhauled on a maintenance record. Because engine overhaul work falls under Appendix A's major repair criteria, the data used must be FAA-approved data — not just acceptable data.
The distinction between approved data and acceptable data is one of the most commonly tested concepts on the AMT Powerplant exam. Approved data has received direct FAA approval (a signature or equivalent regulatory endorsement). Acceptable data has not been individually approved but meets a standard the FAA finds satisfactory. For a major repair such as an engine overhaul, approved data is required.
Primary Approved Data Sources
Type Certificate Data Sheets (TCDS)
The Type Certificate Data Sheet is the foundational legal document for any certificated engine. Issued by the FAA as part of the type certification process under 14 CFR Part 33, the TCDS lists the engine model, approved operating limits (horsepower, RPM, oil pressure, temperatures), fuel and oil grades, and time between overhaul (TBO) recommendations. The TCDS does not usually contain step-by-step overhaul procedures, but it establishes the limits that all overhaul data must respect. Any overhaul procedure that would produce an engine operating outside the TCDS limits is automatically invalid. TCSDs are publicly available through the FAA's online database and are considered FAA-approved data.
Manufacturer's Overhaul Manuals (Instructions for Continued Airworthiness)
For engines certificated after 1980, 14 CFR Part 33.4 requires manufacturers to provide Instructions for Continued Airworthiness (ICA). For older engines, manufacturers typically provided equivalent overhaul manuals or maintenance manuals. These documents contain the dimensional tolerances, fits and clearances, step-by-step disassembly and reassembly procedures, inspection criteria, test-cell run-in procedures, and parts lists that an AMT needs to perform a complete overhaul. When the manufacturer produces these documents and they are incorporated into the type certificate basis, they carry FAA-approved status. Always use the current revision of the manual — an outdated revision may reference superseded part numbers or tolerances, and using it could compromise airworthiness.
Airworthiness Directives (ADs)
Airworthiness Directives are legally mandatory documents issued under 14 CFR Part 39. During an engine overhaul, the AMT must research and comply with all applicable ADs for that engine model, its components, and accessories. ADs are FAA-approved data by definition — they are published in the Federal Register and carry the force of law. Importantly, an AD may modify or supersede a manufacturer's overhaul manual instruction. When a conflict exists, the AD takes precedence. Failing to comply with an applicable AD during an overhaul is a serious airworthiness violation and a common exam trap.
FAA-Approved Supplemental Type Certificate (STC) Data
If an engine has been modified under a Supplemental Type Certificate, the STC holder's data package becomes FAA-approved data for that specific modification. For example, if an STC authorizes the use of an alternative fuel system or a different ignition timing setting, the AMT must follow the STC data when overhauling or maintaining that feature. The original manufacturer's manual still governs all areas not addressed by the STC. The AMT must ensure both sets of data are compatible and that neither is violated.
FAA Advisory Circulars Referenced as Approved Data
Advisory Circulars (ACs) are generally acceptable data rather than approved data, but some ACs are specifically incorporated by reference into regulations or type certificates, elevating them to approved status. AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair) is the most widely cited, but its title reveals its status: it provides acceptable methods. For engine overhaul — a major repair — AC 43.13-1B alone is not sufficient. However, portions of overhaul procedures that are not covered by manufacturer data may reference AC 43.13-1B as a supplementary acceptable method, provided the overall work remains consistent with approved data.
FAA Field Approval Data (FAA Form 337)
When no existing approved data covers a specific overhaul situation — for example, an unusual repair to an engine case — an AMT or IA (Inspection Authorization holder) can obtain a field approval from the local FSDO. The approved data is then documented on FAA Form 337 (Major Repair and Alteration). Once signed off by an FAA representative, the Form 337 data becomes FAA-approved data for that specific aircraft and engine. The Form 337 must be retained in the aircraft records. This is a last resort; the preference is always to use existing approved data from the manufacturer or an STC.
Time Between Overhaul (TBO) and Its Data Source
TBO figures are among the most misunderstood items in engine overhaul. For most general aviation piston engines, the manufacturer publishes a recommended TBO — commonly ranging from 1,200 to 2,000 hours depending on the engine model — in the overhaul manual or a separate service document. It is critical to understand that for non-commercial operations under 14 CFR Part 91, manufacturer TBO is a recommendation, not a legal requirement. However, for air carrier and commercial operations under Parts 121 and 135, TBO limits may be legally mandated in the operator's approved maintenance program. Always verify which regulatory part governs the aircraft's operation before advising on TBO compliance.
Why This Matters for Airworthiness and Liability
Using unapproved data during an engine overhaul creates a chain of serious consequences. First, if the overhaul is performed with non-approved data, the engine may not legally be returned to service, because the work does not comply with 14 CFR Part 43. Second, the AMT who signed off the work bears personal legal and certificate liability. Third, if an accident occurs, insurance coverage may be voided and civil liability can fall on the mechanic. Fourth, and most importantly, incorrect tolerances or procedures can result in engine failure and loss of life. The FAA's insistence on approved data is not bureaucratic formality — it is the engineering chain of accountability that keeps aircraft airworthy.
Key Numbers and Rules
- 14 CFR 43.13 — requires approved data for major repairs; engine overhaul work falls under the major repair/alteration criteria of Appendix A of Part 43, while 14 CFR 43.2 separately defines the standard an engine must meet to be recorded as "overhauled."
- 14 CFR Part 33.4 — requires engine manufacturers to provide ICAs as part of type certification.
- 14 CFR Part 39 — makes ADs legally mandatory; ADs supersede conflicting manufacturer data.
- FAA Form 337 — documents field-approved data for major repairs/alterations with no existing approved source.
- TCDS — sets the legal operating limits that all overhaul work must respect; freely searchable in the FAA online database.
- Approved vs. Acceptable — engine overhaul requires approved data; AC 43.13-1B is acceptable, not approved, for major repairs.
- Current revision required — always verify you have the latest revision of any manufacturer manual or ICA before beginning overhaul work.
Common Test Traps
- Confusing approved and acceptable data. The exam frequently offers AC 43.13-1B as an answer choice for engine overhaul data. It is acceptable data, not approved data, and is insufficient by itself for a major repair like an overhaul.
- Ignoring ADs during overhaul. An AMT who follows only the manufacturer's manual without checking for applicable ADs can miss a mandatory modification. ADs supersede the manual when they conflict.
- Treating TBO as universally mandatory. Manufacturer TBO is recommended for Part 91 operations but may be legally required for Part 121/135. The exam may test whether you know the difference.
- Using an outdated revision. An old edition of an overhaul manual may contain superseded tolerances. Always confirm you have the current revision — older editions are not valid approved data.
- Assuming STC data replaces all manufacturer data. An STC only covers what it specifically addresses. The original type certificate data and manufacturer manual remain in force for everything else. Both must be consulted and must not conflict.