100-Hour Inspections: When Are They Required and What Do They Involve?

Aircraft ownership and operation come with a vital responsibility: maintaining airworthiness.1 This isn’t just a suggestion; it’s a legal and safety imperative. For single-engine and twin-piston aircraft, two inspections are commonly discussed: the annual inspection and the 100-hour inspection. While similar in scope, their application and requirements differ significantly. This article will delve into the intricacies of the 100-hour inspection, outlining when it’s required and what it entails, ensuring your aircraft remains safe and compliant.

WHEN IS A 100-HOUR INSPECTION REQUIRED?

The Federal Aviation Administration (FAA) mandates 100-hour inspections under Federal Aviation Regulation (FAR) 91.409(b).2 This regulation specifies that a 100-hour inspection is required for aircraft that are:

  • Carrying any person (other than a crew member) for hire: This primarily applies to commercial operations, such as air taxi services, skydiving operations, or banner towing.3 If you’re charging someone to be a passenger in your aircraft, this inspection is likely required.
  • Provided by any person giving flight instruction for hire: This is a crucial point for flight schools and independent flight instructors. If you provide an aircraft for flight training where you are compensated for the instruction, a 100-hour inspection is necessary. It’s important to note that if a student pilot provides their own aircraft for instruction, the 100-hour inspection is typically not required, though an annual inspection always is.

Unlike an annual inspection, which is required every 12 calendar months regardless of flight hours, the 100-hour inspection is based on time in service. This means if your aircraft is used for hire, it could require multiple 100-hour inspections within a single calendar year if it accrues enough flight time.

There’s a slight flexibility with the 100-hour rule: you can exceed the 100-hour limitation by not more than 10 hours while en route to a place where the inspection can be done.4 However, this excess time must be included in computing the next 100 hours of time in service. For example, if you fly 105 hours before getting the inspection, your next 100-hour inspection will be due at 195 hours (100 + 95).

WHO CAN PERFORM A 100-HOUR INSPECTION?

A key difference between an annual and a 100-hour inspection lies in who is authorized to perform them.5 While an annual inspection must be performed by an Airframe and Powerplant (A&P) mechanic holding an Inspection Authorization (IA), a 100-hour inspection can be performed by any FAA-certified A&P mechanic.6 This offers more flexibility for operators needing to meet this requirement. It’s also important to remember that an annual inspection can satisfy the 100-hour requirement if performed within the preceding 100 hours of time in service. However, a 100-hour inspection cannot substitute for an annual inspection.

WHAT DOES A 100-HOUR INSPECTION INVOLVE?

The scope and detail of a 100-hour inspection are comprehensive, mirroring that of an annual inspection. The FAA’s Part 43 Appendix D outlines the specific items to be included.7 Generally, the inspection process can be broken down into several key areas:

PRE-INSPECTION RUN-UP AND TAXI

The inspection often begins with a thorough engine run-up and a short taxi.8 During this phase, the mechanic observes engine performance, flight control smoothness, brake effectiveness, and general system functionality. Any anomalies are noted for further investigation. This initial operational check provides valuable insights into the aircraft’s overall health before deeper examination.

AIRFRAME INSPECTION

This is a detailed visual and tactile examination of the aircraft’s structure. It involves removing various inspection plates, access doors, fairings, and cowlings to gain access to internal components.9 Mechanics meticulously inspect for:

  • Fabric and Skin: Deterioration, distortion, cracks, dents, and secure attachment of fittings.10
  • Fuselage and Empennage: Structural integrity, corrosion, loose rivets, and proper alignment.11
  • Wings and Control Surfaces: Condition of spars, ribs, skin, hinges, cables, and push-rods. Control surface movement and balance are also checked.
  • Landing Gear: Condition of struts, linkages, tires, wheels, bearings, brakes, and retraction mechanisms (for retractable gear aircraft). Hydraulic lines and electrical systems associated with the landing gear are also scrutinized for leaks, chafing, and proper operation.

ENGINE AND PROPELLER INSPECTION

The powerplant is the heart of the aircraft, and its inspection is critical.12 This includes:

  • External Examination: Visual checks for leaks (oil, fuel, hydraulic), loose or cracked components, and proper safetying.
  • Internal Engine: Compression checks of cylinders, inspection of spark plugs, and examination of sump drain plugs and screens for metal particles that could indicate internal wear.
  • Induction and Exhaust Systems: Cracks, leaks, and proper attachment of components.
  • Engine Controls: Proper travel, security, and condition of throttle, mixture, and propeller controls.
  • Propeller Assembly: Cracks, nicks, bends, oil leakage, and security of attachment.13 Anti-icing devices, if installed, are also checked.

SYSTEMS AND COMPONENTS

Beyond the airframe and engine, various systems and components are thoroughly inspected:

  • Fuel System: Tanks, lines, pumps, filters, and gauges for leaks, contamination, and proper operation.14
  • Oil System: Lines, coolers, and filters.
  • Electrical System: Wiring, conduits, switches, circuit breakers, and batteries for proper installation, condition, and charge.
  • Avionics and Instruments: Proper mounting, marking, and (where practical) operation of instruments and electronic equipment. Bonding and shielding are also checked.
  • Cabin and Cockpit: Seats, safety belts, windows, windshields, and overall cleanliness.15 Flight and engine controls are checked for proper installation and operation.16
  • Miscellaneous Items: Any other installed equipment not specifically covered by the above categories is inspected for proper installation and operation.17

POST-INSPECTION AND DOCUMENTATION

Once the inspection and any necessary repairs are completed, the aircraft is serviced, which typically includes an oil change and often servicing of other components like landing gear bearings. Finally, a meticulous logbook entry is made, detailing the inspection, any discrepancies found, and the corrective actions taken.18 This entry, signed by the authorized A&P mechanic, is essential for demonstrating the aircraft’s continued airworthiness.

The 100-hour inspection, while seemingly frequent for high-utilization aircraft, is a cornerstone of aviation safety.19 It ensures that components prone to wear and tear from regular operation are identified and addressed before they become serious issues, ultimately contributing to safer flights for all.20

FOOTNOTES

  1. Federal Aviation Administration (FAA) – 14 CFR Part 91.409 – Inspections: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F/part-91/subpart-E/section-91.409
  2. Federal Aviation Administration (FAA) – 14 CFR Appendix D to Part 43 – Scope and Detail of Items: https://www.law.cornell.edu/cfr/text/14/appendix-D_to_part_43
  3. AOPA – Guide to Aircraft Inspections: https://www.aopa.org/go-fly/aircraft-and-ownership/maintenance-and-inspections/aircraft-inspections

The Aero Center is located at William J. Fox Airfield KWJF | Lancaster, CA.. Contact us at 209.885.6950 for any questions or to schedule an appointment.

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