14 CFR Part 91 — General Operating and Flight Rules Affecting Maintenance

Why Part 91 Matters to Maintenance Personnel

14 CFR Part 91 is the foundational set of federal operating rules for general aviation aircraft in the United States. Although it primarily addresses pilots and operators, large portions of Part 91 directly govern maintenance requirements, inspection intervals, and airworthiness responsibilities. As an aviation maintenance technician, you must know these sections thoroughly because your signature in a logbook—or your decision to release an aircraft to service—carries legal weight under these rules.

Owner and Operator Responsibility

Part 91.403 states plainly that the owner or operator of an aircraft is primarily responsible for maintaining that aircraft in an airworthy condition. This does not remove your liability as a technician, but it establishes who is ultimately accountable. Part 91.405 requires the owner or operator to ensure all required inspections are completed and that the aircraft is repaired according to the applicable regulations before further flight. Part 91.407 prohibits anyone from operating an aircraft that has undergone maintenance unless it has been approved for return to service by an authorized person and the maintenance entry has been made in the appropriate records.

Required Inspections and Their Intervals

Annual Inspection (91.409(a)): No person may operate an aircraft in any operation governed by Part 91 unless it has been inspected in the preceding 12 calendar months. An annual inspection must be performed by a certificated airframe and powerplant (A&P) mechanic who holds an Inspection Authorization (IA), or by an appropriately rated repair station or the aircraft manufacturer.

100-Hour Inspection (91.409(b)): Any aircraft carrying persons for hire, or any aircraft used to give flight instruction for hire, must have a 100-hour inspection in addition to the annual inspection. The 100-hour inspection may be performed by an A&P mechanic without an IA. The 100-hour limit may be exceeded by up to 10 hours when necessary to fly to a location where the inspection can be done, but those additional hours count against the next 100-hour interval. Note: an annual inspection may be substituted for a required 100-hour inspection, but a 100-hour inspection cannot substitute for an annual.

Altimeter and Static System Tests (91.411): Aircraft operated under Instrument Flight Rules (IFR) must have had the altimeter, altitude-reporting equipment, and related static pressure systems tested and inspected within the preceding 24 calendar months. This test must be performed by the manufacturer, a properly rated repair station, or a certificated mechanic with the appropriate ratings.

ATC Transponder Tests (91.413): All ATC transponders installed in aircraft must be tested and inspected within the preceding 24 calendar months, regardless of whether the aircraft is flown IFR or VFR. The test must be performed by an appropriately rated repair station or by a holder of a continuous airworthiness maintenance program approval.

ELT Inspection (91.207): Emergency Locator Transmitters (ELTs) must be inspected within 12 calendar months of the last inspection for proper installation, battery corrosion, operation of the controls and crash sensor, and the presence of a decal showing the next inspection date. Batteries must be replaced (or recharged for rechargeable types) when the transmitter has been in use for more than one cumulative hour, or when 50 percent of the battery's useful life has expired.

RVSM Authorization (91.180 and Appendix G): Operations within Reduced Vertical Separation Minimum (RVSM) airspace—generally flight level 290 through 410—require that the aircraft's altimetry and autopilot systems meet specific performance standards and that the operator hold RVSM authorization. Maintenance personnel must be aware that any repair or modification to RVSM-related systems requires re-verification of RVSM compliance before the aircraft re-enters RVSM airspace.

Inoperative Equipment — Part 91.213

One of the most frequently applied sections in maintenance practice is 91.213, which governs inoperative instruments and equipment. The section creates a structured decision process:

1. If the aircraft has an FAA-approved Minimum Equipment List (MEL), the operator may defer certain inoperative items per the MEL's conditions and placarding requirements.

2. If no MEL exists, the technician and operator must determine whether the inoperative equipment is required by the aircraft's type certificate data sheet (TCDS), the aircraft's equipment list, or an applicable FAR (including Part 91 itself and Parts 23, 25, etc.). If it is required by any of these, the aircraft cannot fly until repaired.

3. If the inoperative equipment is not required by any of those sources, it may be deactivated or removed, the item must be placarded INOPERATIVE, and the maintenance action must be entered in the maintenance record.

Understanding 91.213 protects both the operator and you from incorrectly grounding or—equally dangerous—incorrectly releasing an aircraft.

Safety Reminders for Maintenance Personnel

Whenever you work on inspection-related systems such as static ports, pitot tubes, or transponder antennas, follow lockout/tagout procedures before opening any pressurized line or disconnecting electrical components. Wear appropriate PPE—safety glasses when probing static lines, and hearing protection near running engines during transponder run-up checks. Never clear an aircraft for return to service based on memory alone; always verify dates and hours in the actual logbook records against the applicable regulatory requirement.