Comparing Annual and 100-Hour Inspections: Similarities and Differences

For single-engine and twin-piston aircraft owners and operators, understanding the nuances of aircraft inspections is paramount for safety, compliance, and maintaining the value of your asset. While both annual and 100-hour inspections are critical for ensuring airworthiness, they serve different purposes and have distinct requirements under Federal Aviation Regulations (FARs). This article will delve into the similarities and differences, helping you navigate these essential maintenance events.

THE FOUNDATION OF AIRWORTHINESS: FAR 91.409

At the heart of aircraft inspection requirements for general aviation is FAR 91.409.¹ This regulation dictates when and what type of inspections are necessary.² Essentially, all civilian aircraft in the United States must undergo an annual inspection every 12 calendar months to remain airworthy.³ The 100-hour inspection, however, is a more specific requirement tied to the commercial operation of an aircraft.

ANNUAL INSPECTION: THE COMPREHENSIVE OVERHAUL

The annual inspection is a thorough, nose-to-tail examination of the aircraft, mandated by the FAA for nearly all general aviation aircraft.⁴ This inspection must be performed by a mechanic holding an Inspection Authorization (IA). An IA is an Airframe and Powerplant (A&P) mechanic who has met additional experience and testing requirements, demonstrating a higher level of knowledge and skill in aircraft inspections.⁵

During an annual inspection, every major component of the aircraft is scrutinized. This includes, but is not limited to, the fuselage, wings, empennage, flight controls, landing gear, engine, propeller, and all associated systems (electrical, fuel, hydraulic, etc.). Panels and cowlings are removed to allow for a detailed visual inspection of internal structures, wiring, plumbing, and control linkages. The engine undergoes a compression check, and all fluids are typically drained and replaced.⁶ The mechanic will also verify compliance with all applicable Airworthiness Directives (ADs), which are mandatory modifications or inspections issued by the FAA to correct unsafe conditions. Logbooks are meticulously reviewed to ensure all previous maintenance and inspections are properly documented and current.⁷

The annual inspection is a significant undertaking, often requiring several days or even weeks to complete, depending on the aircraft’s condition and the findings of the inspection. The goal is to ensure the aircraft is in a condition for safe operation and fully compliant with its type certificate and all relevant regulations.⁸ Once the annual inspection is successfully completed, the IA mechanic makes an entry in the aircraft’s maintenance records, approving it for return to service, which is valid for 12 calendar months. For example, if an annual is completed on July 15, 2025, the next annual is due before August 1, 2026.

100-HOUR INSPECTION: FOR AIRCRAFT “FOR HIRE”

The 100-hour inspection is, in terms of scope and detail, virtually identical to an annual inspection. The critical difference lies in when and by whom it is required. A 100-hour inspection is mandatory for aircraft that are:

• Carrying any person (other than a crew member) for hire. This includes operations like aerial photography, certain types of charter, or other commercial uses where a person is paying to be transported in the aircraft.⁹
• Provided by any person giving flight instruction for hire. If a flight instructor provides the aircraft for paid instruction, that aircraft must undergo a 100-hour inspection every 100 hours of time in service. However, if a student provides their own aircraft for instruction, a 100-hour inspection is generally not required unless the aircraft is otherwise operated for hire.

Unlike an annual, a 100-hour inspection can be performed by any A&P mechanic, they do not need an Inspection Authorization.¹⁰ The 100-hour limit can be exceeded by up to 10 hours if the aircraft is en route to a place where the inspection can be accomplished.¹¹ However, this excess time must be included in computing the next 100 hours of time in service.

KEY SIMILARITIES AND DIFFERENCES

SIMILARITIES:

• Scope and Detail: Both inspections follow the same comprehensive checklist, as outlined in Appendix D to FAR Part 43, ensuring all critical components and systems are thoroughly examined.
• Airworthiness Focus: The primary objective of both inspections is to determine and verify the aircraft’s airworthiness. Any discrepancies found must be rectified before the aircraft can be returned to service.¹²
• Logbook Entry: Upon completion, a detailed logbook entry is required for both, documenting the inspection, any repairs made, and the aircraft’s approval for return to service.

DIFFERENCES:

• Frequency: An annual inspection is required every 12 calendar months, regardless of how much the aircraft is flown. A 100-hour inspection is required every 100 hours of time in service for specified commercial operations.
• Authority to Perform: An annual inspection must be performed by a mechanic holding an Inspection Authorization (IA). A 100-hour inspection can be performed by any certified A&P mechanic.¹³
• Interchangeability: An annual inspection can satisfy the 100-hour inspection requirement. If an annual inspection is performed, the 100-hour clock effectively resets. However, a 100-hour inspection cannot substitute for an annual inspection.¹⁴

PROGRESSIVE INSPECTIONS: AN ALTERNATIVE

For aircraft with high usage, such as those used by flight schools or busy charter operations, a progressive inspection program can be an alternative to the traditional annual and 100-hour inspections. This allows for the inspection to be broken down into smaller, more frequent phases, minimizing downtime.¹⁵ All items required for the annual and 100-hour inspections must still be covered within a specified period (typically 12 calendar months or 100 hours, depending on the operating limitations).

THE IMPORTANCE OF REGULAR MAINTENANCE

Regardless of whether your aircraft requires an annual, a 100-hour, or a progressive inspection, adherence to these regulatory requirements is crucial. These inspections are not just bureaucratic hurdles; they are vital for identifying wear, fatigue, corrosion, and other potential issues that could compromise safety. Regular, thorough maintenance by qualified professionals is the cornerstone of safe and reliable aircraft operation, protecting your investment and, most importantly, the lives of those on board.¹⁶

Footnotes:

1. Federal Aviation Administration (FAA) – 14 CFR Part 91.409, “Inspections”¹⁷
2. Federal Aviation Administration (FAA) – 14 CFR Part 43, Appendix D, “Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections”
3. Aircraft Inspection: Guide to Keeping Your Plane Safe – Pilot Mall. https://www.pilotmall.com/blogs/news/aircraft-inspection-guide-to-keeping-your-plane-safe
4. Guide to Aircraft Inspections – AOPA. https://www.aopa.org/go-fly/aircraft-and-ownership/maintenance-and-inspections/aircraft-inspections

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