When an aircraft is used for hire — carrying passengers for compensation or for flight instruction — 14 CFR Part 91 mandates that its airframe, engine(s), and propeller(s) receive a thorough examination every 100 hours of flight time. But completing the physical inspection is only half the job. Federal aviation regulations are equally demanding about what gets written down, where it gets written, and who signs it. Incomplete or missing documentation is itself a regulatory violation, and it can ground an aircraft just as effectively as a broken part. For the aspiring Aviation Maintenance Technician (AMT), mastering the paperwork is inseparable from mastering the work itself.
This article walks through the regulatory framework, the specific entries required, who is authorized to make them, and the practical details that separate a legally sound maintenance record from one that invites an FAA enforcement action.
Regulatory Foundation
Two sections of 14 CFR drive 100-hour inspection documentation. 14 CFR 91.409(b) establishes the inspection requirement itself: no person may operate an aircraft carrying passengers for hire or for flight instruction in an aircraft they provide unless the aircraft has been inspected within the preceding 100 hours and holds a current airworthiness certificate. 14 CFR 91.417 governs maintenance records — it spells out what owners and operators must keep and for how long. The actual content of each required maintenance entry is described in 14 CFR 43.9, which applies to all maintenance, preventive maintenance, rebuilding, and alteration records.
Together, these three regulatory sections form the backbone of 100-hour documentation. An AMT who understands all three can navigate virtually any recordkeeping scenario on the job or on the knowledge test.
Who May Perform and Sign Off a 100-Hour Inspection
A 100-hour inspection is classified as an inspection under Part 43, and it must be performed by one of the following individuals as specified in 14 CFR 43.3 and 14 CFR 91.409:
- A certificated Airframe and Powerplant (A&P) mechanic holding an Inspection Authorization (IA) — although an IA is not required to perform or approve a 100-hour inspection (unlike an annual, which requires IA sign-off), one is required to approve the aircraft for return to service after an annual. For a 100-hour, a plain A&P mechanic who performed or directly supervised the inspection may sign it off for return to service under 14 CFR 43.7.
- A certificated A&P mechanic with the appropriate rating (airframe or powerplant) working on the relevant system.
- A certificated repair station with the appropriate ratings under 14 CFR Part 145.
- The aircraft manufacturer, when performing work on its own products.
This is a commonly tested distinction: an IA is required to return an aircraft to service after an annual inspection, but a standard A&P mechanic who performed or supervised the work may complete and approve a 100-hour inspection. The scope of work may be identical, but the authorization requirements differ.
Required Content of the Maintenance Entry
Under 14 CFR 43.9(a), each person who performs maintenance, preventive maintenance, rebuilding, or an alteration — and each person who approves or disapproves an aircraft for return to service — must make an entry in the maintenance record. For a 100-hour inspection, that entry must include all of the following elements:
- Description of the work performed — a plain-language statement identifying what was done. For a 100-hour, this typically states that the inspection was performed in accordance with a specific checklist or inspection program, such as the aircraft manufacturer's maintenance manual or an FAA-accepted inspection checklist.
- Date the work was completed — the calendar date on which the inspection was finished and the aircraft was returned to service (or rejected from service).
- Aircraft total time in service — the total airframe hours accumulated at the time of the inspection. This is expressed in hours and tenths of hours and serves as the reference point for calculating the next 100-hour due time.
- Name of the person performing the work — if the individual holds a certificate, the certificate number must be included. If the work was performed by someone under supervision, the supervising certificated individual signs the entry.
- Signature — the certificated person approving or disapproving the aircraft for return to service must sign the entry. The signature constitutes a legal attestation that the work was completed as described.
- Certificate number and type — the mechanic's FAA certificate number and certificate type (e.g., A&P, Repairman, Part 145 Repair Station certificate number) must appear in the record.
- Approval or disapproval for return to service — the entry must explicitly state whether the aircraft is approved or disapproved for return to service. If disapproved, the specific discrepancy or reason must be documented.
Where Entries Are Made
14 CFR 91.417(a) requires owners and operators to maintain records of maintenance, preventive maintenance, alterations, and 100-hour or annual inspections. These records may take several acceptable forms:
- Dedicated airframe, engine, and propeller logbooks — the most common format for light general aviation aircraft.
- Separate inspection records or work order files maintained by a repair station.
- Electronic records, provided the system meets FAA acceptance criteria and the data can be reproduced in a legible printed form for inspection.
The 100-hour inspection entry must appear in the airframe maintenance record. If engine or propeller work was also performed (as it typically is during a thorough inspection), separate entries must appear in the respective engine and propeller records. There is no single universal form — the regulation prescribes content, not a specific government form number.
Retention Requirements
Under 14 CFR 91.417(b), records related to inspections — including 100-hour inspections — must be retained for at least 12 calendar months after the aircraft is approved for return to service. However, records that establish the current status of life-limited parts, the current inspection status, and time since overhaul of major components must be retained and transferred with the aircraft when ownership changes. This means a complete historical record of 100-hour inspections significantly affects an aircraft's marketability and resale value.
The 10-Hour Overrun Rule
One practical nuance worth understanding: under 14 CFR 91.409(b), the 100-hour limit may be exceeded by up to 10 hours if additional flight time is necessary to reach a place where the inspection can be performed. However — and this is critical — those excess hours count against the next 100-hour interval. If an aircraft flies 107 hours since its last inspection to reach a maintenance facility, the next inspection is due 100 hours after the actual inspection is performed — that is, at the 107-hour point — meaning the following interval is effectively shortened to 93 hours of flying before the next inspection comes due, not a full 100 hours. The mechanic documenting the inspection must note the actual total time at which the inspection took place, ensuring the record accurately reflects the overrun so the next due date is calculated correctly.
Returning an Aircraft to Service vs. Grounding It
A 100-hour inspection does not always end with an airworthy aircraft. When discrepancies are found, the mechanic must document them and mark the aircraft as disapproved for return to service. The entry should list each unairworthy condition found. The aircraft may not be flown until repairs are completed and a new, affirmative approval-for-return-to-service entry is made. Simply crossing out the disapproval entry is not acceptable — a separate, signed, dated approval entry is required after repairs.
Key Numbers and Rules
- 100 hours — maximum interval between inspections for hire operations under 14 CFR 91.409(b).
- 10-hour overrun — permitted to reposition to an inspection facility; counts against the next interval.
- 12 calendar months — minimum record retention period for inspection entries (14 CFR 91.417(b)).
- A&P (no IA required) — the mechanic who performed or supervised the work may approve a 100-hour inspection for return to service; an IA is only required for annual inspections and certain alterations.
- 14 CFR 43.9 — governs the required content of every maintenance record entry.
- 14 CFR 91.417 — governs retention and transfer of maintenance records.
Common Test Traps
- IA confusion: The FAA knowledge test frequently asks whether an Inspection Authorization is required for a 100-hour inspection. It is not — a standard A&P who performed or supervised the inspection can sign it off for return to service. An IA is required only for annual inspections and certain alterations.
- Missing total time: Students forget that the entry must include the aircraft's total time in service at the time of the inspection, not just the date. Both are required under 14 CFR 43.9.
- Overrun math: If an aircraft exceeds the 100-hour limit by 7 hours to reach a shop, the next 100-hour inspection is due at 100 hours from the actual inspection time — effectively 93 hours after the overrun flight, not 100 hours.
- Record location: Engine and propeller inspections must be entered in the engine and propeller records respectively, not only in the airframe logbook. Exam questions may test whether a single entry suffices.
- Disapproval is not optional: If the aircraft fails the inspection, the mechanic must make a written disapproval entry with reasons. Failing to document a rejection while allowing the aircraft to be operated is a serious regulatory violation.