Two operators fly the same model of aircraft, with the same engine, at the same hours past the manufacturer's recommended TBO. One is flying perfectly legally. The other has grounded their aircraft. The engine is identical. What differs is the operating rule each one flies under — and that single fact decides whether the manufacturer's overhaul time is a hard regulatory limit or a non-binding recommendation.
TBO — time between overhaul — is published by engine and propeller manufacturers as the interval at which they recommend an overhaul. It is frequently misread as a federal expiration date. For most non-commercial Part 91 operations it is not. For Part 135 operators it usually is. The records you are legally required to keep — and how long — flow directly from which side of that line you are on.
This guide draws the line precisely against the primary regulations: 14 CFR §91.409 (inspection programs), §91.417 (records and retention), §43.10 (life-limited parts), and §135.421 (Part 135 manufacturer programs). We close with the time-since-overhaul records FileFlo helps you keep audit-ready.
Why the Same TBO Is Mandatory for One Operator and Advisory for Another
The manufacturer's TBO is a single published number. What gives it legal force — or doesn't — is the maintenance program the aircraft is required to follow, which in turn is driven by the operating rule. There is no regulation titled "engine TBO" that applies uniformly across all aircraft. Instead, the binding force of an overhaul time emerges from the interaction of three regulations.
| Operating Context | Governing CFR | Is TBO Binding? | Why |
|---|---|---|---|
| Part 91 non-commercial (piston) | §91.409 | Recommended | No rule makes a published TBO a hard limit; fly on condition if airworthy |
| Part 91 with AD-mandated overhaul | §39 + §91.409 | Mandatory | An AD or an Airworthiness Limitations Section item (§91.403(c)) can impose a hard overhaul/replacement time |
| Part 135, 9 or fewer pax seats | §135.411(a)(1), §135.421(a) | Mandatory | Must follow the manufacturer program (incl. its overhaul times) or an FAA-approved one |
| Part 135, 10 or more pax seats | §135.411(a)(2), §135.423–443 | Mandatory | Must follow a Continuous Airworthiness Maintenance Program (CAMP) |
| Life-limited part (any rule) | §43.10 | Mandatory (replace) | Hard replacement limit in type design / ICA / maintenance manual — not an overhaul |
The Phrase That Controls Everything: "Required to Be Overhauled on a Specified Time Basis"
14 CFR §91.417(a)(2)(iii) requires a record of "the time since last overhaul of all items installed on the aircraft which are required to be overhauled on a specified time basis." That conditional clause is the entire game. The record obligation attaches only to items that are required to be overhauled on a time basis. A Part 135 manufacturer program, a CAMP, or an AD can create that requirement — at which point time-since-overhaul becomes a mandatory permanent record. For a Part 91 piston engine flown on condition with no such requirement, that particular item is not "required to be overhauled on a specified time basis," so this specific clause is not what compels its TSO record. The overhaul time and the record obligation rise and fall together.
TBO Is Not a Life Limit — The §43.10 Distinction
Before going further, one distinction has to be locked in, because the entire industry routinely blurs it: an overhaul time (TBO) is not the same thing as a life limit. They come from different regulations, carry different legal force, and trigger entirely different records.
Overhaul / TBO
The interval at which a manufacturer recommends an engine or propeller be overhauled — disassembled, inspected, worn parts replaced, and restored to serviceable limits. The unit is returned to service, with its time-since-overhaul reset to zero. For Part 91 piston operation, TBO is generally advisory. Its record obligation under §91.417(a)(2)(iii) only attaches when the item is "required to be overhauled on a specified time basis."
Life-Limited Part (§43.10)
14 CFR §43.10 defines a life-limited part as "any part for which a mandatory replacement limit is specified in the type design, the Instructions for Continued Airworthiness, or the maintenance manual." At its limit the part must be removed and replaced — it cannot be overhauled and returned to service. "Life status" is the accumulated cycles, hours, or other limit. On removal, §43.10 requires the part be controlled by a tag/record/marking, and the record must accompany it on transfer.
Turbine engines have both — and the records are not interchangeable
A typical turbine engine carries life-limited rotating parts (compressor and turbine disks with mandatory cyclic limits under §43.10) and recommended hot-section and overhaul intervals. The cyclic life of a disk is a hard limit tracked in cycles; the hot-section interval is an overhaul-class event tracked in hours. Mixing them up — recording a hot-section as if it reset a disk's cyclic life, or treating a life-limited disk as "overhaulable" — is a documentation failure that surfaces immediately in a Part 135 surveillance audit and in pre-purchase inspections. Keep cyclic life-limit records and time-since-overhaul records as separate, clearly labeled tracks.
The practical takeaway: §91.417(a)(2) actually lists both as separate required status records — "(ii) the current status of life-limited parts of each airframe, engine, propeller, rotor, and appliance" and "(iii) the time since last overhaul of all items installed on the aircraft which are required to be overhauled on a specified time basis." Two distinct records, two distinct legal bases. A complete aircraft records set keeps them apart.
Part 91 "On Condition" — Flying Past Recommended TBO
For most privately operated, non-commercial aircraft under 14 CFR §91.409, there is no regulation that grounds an aircraft solely because its engine has exceeded the manufacturer's recommended TBO. The aircraft must pass its applicable inspection (annual under §91.409(a), or the program it is enrolled in), remain airworthy, and comply with all airworthiness directives — but a recommended TBO is not, by itself, a regulatory deadline. Operating in this manner is universally referred to as flying "on condition."
What §91.409 Does — and Does Not — Mandate
§91.409 prescribes inspection requirements and inspection-program options (§91.409(f) lists them: a Part 121/135 continuous airworthiness program, an inspection program approved under Part 135, a current manufacturer-recommended inspection program, or an operator program approved by the Administrator). §91.409(e) separately requires compliance with the replacement times for life-limited parts for large airplanes, turbojet multiengine airplanes, turbopropeller-powered multiengine airplanes, and turbine-powered rotorcraft. Nowhere does §91.409 convert a manufacturer's recommended overhaul interval into a hard limit for ordinary Part 91 piston operation. Inspection currency, life-limited-part replacement, and AD compliance are mandatory; recommended overhaul timing is not.
The Exceptions That Override "Recommended"
Two things turn a recommendation into a requirement even under Part 91. First, an airworthiness directive under Part 39 can mandate an overhaul or replacement at a specific time — some propeller ADs do exactly this, imposing hard overhaul intervals that are not optional. Second, the moment the aircraft is operated under Part 135, the manufacturer program required by §135.421(a) makes the overhaul time mandatory. Always check the AD list before treating any TBO as advisory; an AD-mandated overhaul is a hard limit regardless of operating rule.
"On condition" is a maintenance judgment, not a free pass
Flying past recommended TBO does not relieve the owner of the §91.7 responsibility to operate only an airworthy aircraft, or the §91.409 obligation to pass the required inspection. Insurers, lenders, and fractional-ownership programs frequently impose their own TBO limits by contract even where the FAR does not. The regulatory answer ("recommended") and the practical answer ("your insurer may still require it") can diverge — and your records need to satisfy whichever is stricter.
Part 135 — Where Overhaul Times Become Mandatory
Under Part 135, the recommendation/requirement question is answered by the maintenance program the operator is obligated to follow — and that program is determined by aircraft size under 14 CFR §135.411.
Nine or Fewer Passenger Seats — §135.411(a)(1) and §135.421(a)
Aircraft type-certificated for nine or fewer passenger seats (excluding pilot seats) are maintained under Parts 91 and 43 plus §§135.415, 135.417, 135.421, and 135.422. §135.421(a) requires the certificate holder to "comply with the manufacturer's recommended maintenance programs, or a program approved by the Administrator, for each aircraft engine, propeller, rotor, and each item of emergency equipment." §135.421(b) defines that program as the one set forth in the manufacturer's maintenance manual or instructions.
Here is the pivot: the manufacturer's maintenance program includes the manufacturer's overhaul and replacement times. Once §135.421(a) requires the operator to comply with that program, the overhaul times stop being advisory for that operator — they are the operator's required schedule, and the records proving compliance are mandatory program records, not optional logs.
Ten or More Passenger Seats — CAMP under §135.411(a)(2)
Aircraft type-certificated for ten or more passenger seats must be maintained under a Continuous Airworthiness Maintenance Program (CAMP) per §§135.415, 135.417, and 135.423 through 135.443. A CAMP is a comprehensive, FAA-accepted program with defined inspection and overhaul intervals, required-inspection-item (RII) controls, and a structured recordkeeping system. Under a CAMP, overhaul times are unambiguously mandatory program elements. See our CAMP recordkeeping guide for the full records architecture.
Single-Engine IFR Passenger Ops — §135.421(c) Engine Trend Monitoring
A specific and frequently missed requirement: §135.421(c) requires that for single-engine aircraft used in passenger-carrying IFR operations, the certificate holder incorporate either the manufacturer's recommended engine trend monitoring program (including oil analysis if appropriate) or an FAA-approved engine trend monitoring program with oil analysis at each 100-hour interval or the manufacturer's suggested interval, whichever is more frequent. Under §135.421(d), the certificate holder "records and maintains in the engine maintenance records the results of each test, observation, and inspection" required by that program. Trend-monitoring records are a distinct, mandatory engine record for these operators — separate from the overhaul track.
The Records Layer Is Where Part 135 Audits Are Won or Lost
In a Part 135 surveillance audit, an inspector does not simply ask "is the engine within TBO?" They ask to see the records proving the operator's required program — the manufacturer program or CAMP — has been followed: the time since last overhaul for each time-controlled item, the basis for that time, the §43.9 entries for the last overhaul, the life-limit status of life-limited parts, and (for single-engine IFR) the trend-monitoring results. A compliant engine with a documentation gap is treated as a finding. The compliance reality is documentary — which is exactly the layer FileFlo addresses. Review the full list in what records a Part 135 operator must keep.
The Time-Since-Overhaul Records You Must Keep
Whenever an overhaul time is binding — and for the permanent status records that apply regardless — specific documents are required. Two regulations govern: §43.9 controls the entry made when the overhaul is performed, and §91.417 controls the ongoing status record that tracks time since that overhaul.
The §43.9 Overhaul Entry — Required Content
Per 14 CFR §43.9(a), the person who performs the overhaul must make a maintenance-record entry containing:
- A description (or reference to data acceptable to the Administrator) of the work performed — i.e., that the engine or propeller was overhauled, to what data and standard
- The date of completion of the work performed
- The name of the person performing the work if other than the person who approves return to service
- The signature, certificate number, and kind of certificate held by the person approving the engine/propeller for return to service
An overhaul entry should make clear that the time-since-overhaul clock for that unit is now reset — this is what feeds the ongoing §91.417(a)(2)(iii) status record. A "rebuilt" engine furnished by the manufacturer is a distinct case under §91.421, which allows a new maintenance record using zero time since the unit was rebuilt by the manufacturer or an approved agency.
The §91.417(a)(2) Status Records — The Permanent Tier
Separate from the one-time §43.9 overhaul entry, §91.417(a)(2) requires the operator to maintain ongoing status records, including:
- Total time in service of the airframe, each engine, and each propeller (§91.417(a)(2)(i) context)
- Current status of life-limited parts of each airframe, engine, propeller, rotor, and appliance — §91.417(a)(2)(ii)
- Time since last overhaul of all items installed that are required to be overhauled on a specified time basis — §91.417(a)(2)(iii)
- Current inspection status and current AD compliance status, including method, date, and recurring-AD next-due time — §91.417(a)(2)(iv)–(v)
These records do not live in isolation. The overhaul-time record sits beside the AD compliance record, the inspection status record, and — when a Part 145 shop does the work — the repair station's return-to-service documentation. Inspectors and buyers read them together.
Retention & Transfer — How Long, and to Whom
14 CFR §91.417(b) sets two retention tiers, and the difference between them is the most misquoted fact in aircraft recordkeeping.
| Record Tier | CFR | Retention |
|---|---|---|
| Routine maintenance records (§43.9 entries for maintenance, preventive maintenance, alterations) | §91.417(a)(1), (b)(1) | Until the work is repeated or superseded, or 1 year after performed |
| Status records — total time, life-limit status, time since overhaul, inspection status, AD status | §91.417(a)(2), (b)(2) | Retained and transferred with the aircraft when it is sold (permanent) |
The "1 year" floor is for routine records — not for overhaul status
A common error is to apply a single retention number to all maintenance paperwork. The §43.9 entry for a discrete task has a one-year floor (or until superseded) under §91.417(b)(1). But the time-since-overhaul status record under §91.417(a)(2)(iii) is in the permanent tier — §91.417(b)(2) requires it to be retained and transferred with the aircraft on sale. In practice, keep every overhaul work-order package indefinitely: even though the underlying §43.9 line item has only a one-year minimum, the overhaul is what establishes the permanent time-since-overhaul record that the next owner is entitled to receive.
Because §91.417(b)(2) ties the status records to the aircraft rather than the owner, an aircraft sale is a records-transfer event. A buyer who cannot obtain the engine and propeller time-since-overhaul history, the life-limit status, and the AD status cannot establish the airworthiness baseline — which is why incomplete overhaul records are one of the most common reasons an aircraft transaction stalls or the price drops. The records are an asset that travels with the airframe.
Keep every time-since-overhaul record audit-ready — and never lose the transfer set
FileFlo is a compliance document intelligence platform — a read-only proof layer that classifies and tracks the records overhauls and time-controlled components generate. Upload your engine and propeller logbook entries, overhaul work orders, life-limited-part tags, AD compliance sheets, and trend-monitoring results, and FileFlo:
- Classifies each document against the governing CFR (§43.9, §43.10, §91.417, §135.421) so the right record is filed under the right requirement
- Extracts the overhaul date and time-in-service so your time-since-overhaul status is always current and exportable
- Tracks life-limited-part status separately from overhaul times — so the §43.10 hard limits never get conflated with advisory TBO
- Assembles a transfer-ready records set for a sale, pre-purchase inspection, or FAA surveillance audit, organized by CFR section
FileFlo sits alongside your maintenance provider, your logbooks, and your maintenance-tracking system — it does not perform maintenance, calculate airworthiness, run your CAMP, or replace an A&P, IA, or Part 145 repair station. It keeps the documents that prove your compliance organized, current, and audit-ready. Starter $89/mo · Professional $299/mo · 5-day free trial, no credit card required.
Frequently Asked Questions
Is engine TBO (time between overhaul) mandatory or just a recommendation?
It depends entirely on how you operate the aircraft. Under Part 91, a manufacturer-published TBO is a recommendation, not a regulatory limit. 14 CFR §91.409 sets inspection-program requirements and §91.409(e) requires compliance with replacement times for life-limited parts on large airplanes, turbojet multiengine airplanes, turbopropeller-powered multiengine airplanes, and turbine-powered rotorcraft, but neither makes a piston-engine or propeller TBO a hard limit for non-commercial Part 91 operation. A privately operated aircraft can legally fly "on condition" past published TBO as long as the engine remains airworthy and passes its required inspections. Under Part 135, the calculus changes: 14 CFR §135.421(a) requires certificate holders operating aircraft with nine or fewer passenger seats to comply with the manufacturer's recommended maintenance programs (which include the manufacturer's overhaul/replacement times) or an FAA-approved program. Once that manufacturer program is the operator's required program, those overhaul times become mandatory for that operator. The TBO number does not change — its legal force does.
What does 14 CFR §91.417 actually require me to keep for time-since-overhaul?
14 CFR §91.417(a)(2)(iii) requires the owner or operator to keep a record of "the time since last overhaul of all items installed on the aircraft which are required to be overhauled on a specified time basis." The operative phrase is "required to be overhauled on a specified time basis." If an item is required to be overhauled on a time basis — for example, because the operator is on a Part 135 manufacturer program, or a propeller AD or service-life document mandates it — then time-since-overhaul (TSO/TSMOH) is a mandatory record. Under §91.417(b)(2), these (a)(2) records must be retained and transferred with the aircraft when it is sold. For a purely on-condition Part 91 piston engine that is not required to be overhauled on a time basis, this specific TSO record requirement is not triggered for that item — though §43.9 maintenance entries for any overhaul actually performed are still required.
What is the difference between TBO and a life-limited part?
They are legally distinct, and conflating them is the single most common documentation error in this area. A life-limited part has a mandatory replacement limit fixed in the type design, the Instructions for Continued Airworthiness, or the maintenance manual — 14 CFR §43.10 defines it as "any part for which a mandatory replacement limit is specified." When a life-limited part reaches its limit (cycles, hours, or calendar), it must be removed and replaced — it cannot be overhauled and returned to service. TBO, by contrast, is the manufacturer's recommended interval at which an engine or propeller should be overhauled — disassembled, inspected, and restored to serviceable limits — not scrapped. Many turbine engines have both: life-limited rotating parts (hard time, §43.10 disposition rules apply) and a recommended hot-section or overhaul interval. A piston engine TBO is generally a recommendation; the life-limited-part framework of §43.10 does not apply to it unless a specific component carries a published hard-time limit.
Can I fly a Part 91 aircraft past published engine TBO?
For most non-commercial Part 91 operations, yes. There is no general FAA regulation that grounds a privately operated, non-commercial aircraft solely because its engine has exceeded the manufacturer-recommended TBO. The engine must still pass its required inspections under §91.409 (annual, or the applicable program) and remain airworthy, and any applicable airworthiness directives must be complied with. This is commonly called operating "on condition." Two important exceptions: (1) if the aircraft is operated under Part 135, the manufacturer program required by §135.421(a) can make the overhaul time mandatory; and (2) some propellers and a few engines carry overhaul or replacement requirements imposed by an airworthiness directive or an Airworthiness Limitations Section item (§91.403(c)), which override the "recommendation" status. Always check for an AD-mandated overhaul before treating any TBO as advisory.
How does Part 135 change engine and propeller overhaul obligations?
Part 135 splits on aircraft size. Under 14 CFR §135.411(a)(1), aircraft type-certificated for nine or fewer passenger seats are maintained under Parts 91 and 43 plus §§135.415, 135.417, 135.421, and 135.422. Section §135.421(a) requires those operators to comply with the manufacturer's recommended maintenance programs — which incorporate the manufacturer's overhaul and replacement times — or an FAA-approved alternative. Under §135.411(a)(2), aircraft with ten or more passenger seats must be maintained under a Continuous Airworthiness Maintenance Program (CAMP) per §§135.415, 135.417, and 135.423 through 135.443. Either way, once the operator adopts a program that specifies overhaul times, those times are no longer advisory for that operator — they are the operator's required maintenance schedule, and the records proving compliance become mandatory program records.
What does a compliant engine or propeller overhaul logbook entry need to contain?
Under 14 CFR §43.9(a), the person who performs the overhaul (the maintenance, in regulatory terms) must make an entry containing: (1) a description, or reference to acceptable data, of the work performed; (2) the date of completion; (3) the name of the person performing the work if other than the person approving it; and (4) the signature, certificate number, and kind of certificate held by the person approving the aircraft, engine, propeller, or component for return to service. For an overhaul specifically, the entry should establish the maintenance time basis being reset — that the engine or propeller was overhauled and the time-since-overhaul clock is now zero — because §91.417(a)(2)(iii) requires the operator to track time since last overhaul going forward. A "rebuilt" engine entry by the manufacturer carries additional meaning under §91.421 (a rebuilt engine may use a new maintenance record and zero time), which is distinct from a field overhaul.
Do I have to keep overhaul records after the aircraft is sold?
The records travel with the aircraft, not the seller. Under 14 CFR §91.417(b)(2), the (a)(2) records — which include total time in service, current life status of life-limited parts, time since last overhaul of items required to be overhauled on a specified time basis, current inspection status, and current AD compliance status — must be retained and transferred with the aircraft at the time it is sold. So the overhaul history is not the seller's to keep or discard; it is a permanent record set that follows the airframe, engine, and propeller through every ownership change. A buyer who cannot obtain the time-since-overhaul history and the AD/life-limit status cannot establish the aircraft's airworthiness baseline, which is why missing overhaul records materially reduce an aircraft's value and are a recurring pre-purchase deal-killer.
How long do routine engine and propeller maintenance records have to be kept?
Two retention tiers apply under 14 CFR §91.417(b). Routine maintenance records under §91.417(a)(1) — the §43.9 entries for individual maintenance, preventive maintenance, and alterations — must be retained until the work is repeated, superseded by other work, or for one year after the work is performed, whichever comes first. This is the often-misquoted figure: it is one year (or until superseded), not 24 months. The §91.417(a)(2) status records, including time since last overhaul of time-controlled items, are the permanent tier — retained and transferred with the aircraft on sale, effectively for the life of the aircraft. In practice, prudent operators keep all overhaul work-order packages indefinitely even though the underlying §43.9 entry has only a one-year floor, because the overhaul itself feeds the permanent §91.417(a)(2)(iii) time-since-overhaul record.
Related Aviation Compliance Guides
Part 91 Aircraft Records Requirements
The full §91.417 records framework — total time, life-limit status, inspection and AD status
Airworthiness Directive Compliance Records
How an AD can convert a recommended overhaul into a mandatory hard time, and the Part 39 tracking obligation
What Records a Part 135 Operator Must Keep
The complete Part 135 records set, including the maintenance and overhaul status records inspectors ask for
Part 135 CAMP Maintenance Recordkeeping
Continuous Airworthiness Maintenance Program records architecture for 10-or-more-seat aircraft
Part 91.409 Inspection Requirements
Annual, 100-hour, progressive, and AAIP — the inspection programs that frame overhaul timing
§43.9 Maintenance Record Entry Requirements
Exactly what every overhaul and maintenance logbook entry must contain to be compliant
Part 145 Repair Station Recordkeeping
What a repair station documents when it overhauls your engine or propeller
Preparing for a Part 135 FAA Surveillance Audit
How inspectors test time-since-overhaul and life-limit records — and the gaps that become findings
Prove your overhaul and life-limit status on demand
FileFlo classifies your engine and propeller records against 14 CFR §43.9, §43.10, §91.417, and §135.421, keeps time-since-overhaul and life-limit status current and separate, and assembles a transfer-ready record set for any sale, pre-purchase inspection, or FAA surveillance audit. Know exactly where every time-controlled component stands — before an inspector or a buyer asks.
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