What is the difference between an annual inspection and a 100-hour inspection?

An annual inspection is required every 12 calendar months for all U.S.-registered civil aircraft and must be performed by an FAA-certificated Airframe and Powerplant mechanic holding an Inspection Authorization (IA). A 100-hour inspection is required under 14 CFR 91.409 only when an aircraft is operated for hire or flight instruction for hire, and it may be performed by any appropriately certificated A&P mechanic without an IA. Both inspections cover the same scope of items, but the 100-hour limit can be exceeded by up to 10 hours when flying to a maintenance facility to get the inspection done.

What is an Airworthiness Directive and does it apply to my aircraft?

An Airworthiness Directive (AD) is a legally enforceable regulation issued by the FAA under 14 CFR Part 39 to correct an unsafe condition found in an aircraft, engine, propeller, or appliance. ADs apply to any aircraft of the affected type certificate, serial number, or part number specified in the directive, so you must check whether each AD applies to your specific aircraft. Compliance with applicable ADs is mandatory, and operating an aircraft in violation of an AD renders it unairworthy. AD compliance records must be maintained in the aircraft maintenance logs per 14 CFR 91.417.

How do you know if an aircraft's annual inspection is still current?

You can verify the currency of an annual inspection by checking the aircraft's maintenance records, where the IA must have entered a signed and dated logbook entry indicating the aircraft was inspected in accordance with 14 CFR 91.409 and found airworthy. The inspection remains valid through the last day of the 12th calendar month following the month in which it was completed; for example, an annual completed on March 15 is valid through March 31 of the following year. An aircraft with an expired annual inspection is not considered airworthy and may not be operated, except in certain limited circumstances such as flying to a maintenance facility under 14 CFR 91.409(b).

Required Aircraft Inspections: Annual, 100-Hour, and ADs

Before any flight, a pilot must be reasonably confident the aircraft is legally airworthy. But airworthiness is not something you simply assume — it is maintained through a system of required inspections and compliance documents established by the FAA under Title 14 of the Code of Federal Regulations (14 CFR). For the Private Pilot knowledge test and, more importantly, for safe real-world operations, you need to understand three cornerstone requirements: the annual inspection, the 100-hour inspection, and Airworthiness Directives (ADs). Each serves a distinct purpose, and knowing when each applies — and who can perform it — is essential knowledge every pilot must carry.

The governing regulations live primarily in 14 CFR Part 91 (general operating and flight rules) and 14 CFR Part 43 (maintenance, preventive maintenance, rebuilding, and alteration). Together, these parts spell out the inspection requirements, who is authorized to sign off on them, and what the consequences are for flying an aircraft that is out of compliance.

The Annual Inspection

Under 14 CFR §91.409(a), no person may operate a civil aircraft in the United States unless it has been inspected and approved for return to service within the preceding 12 calendar months. This is the annual inspection requirement. The phrase "12 calendar months" is critical: it means the inspection is valid through the last day of the twelfth month after the month in which the inspection was performed. For example, if the annual is signed off on March 15, 2024, it remains valid through March 31, 2025 — not just 365 days from the date of signature. This calendar-month accounting actually gives the owner a small buffer and is a commonly tested nuance.

Only an FAA-certificated Airframe and Powerplant (A&P) mechanic who also holds an Inspection Authorization (IA) may conduct and approve an aircraft for return to service after an annual inspection. A standard A&P without an IA cannot sign off an annual. The IA designation is an additional FAA authorization that requires experience, testing, and regular renewal. The inspection itself is comprehensive, covering the airframe, powerplant, propeller, and all systems, following a checklist that meets or exceeds the standards in 14 CFR Part 43, Appendix D.

The 100-Hour Inspection

The 100-hour inspection, governed by 14 CFR §91.409(b), applies to a specific and narrower category of aircraft: those used for hire or used to give flight instruction for hire. If you rent a Cessna 172 from an FBO or take dual instruction in an aircraft that the school charges for, that aircraft must have received a 100-hour inspection within the preceding 100 hours of flight time. Privately owned aircraft flown only for personal use do not need a 100-hour inspection — only the annual applies.

Mechanically, the 100-hour inspection covers the same scope as an annual inspection and follows the same checklist standards. The key differences are who can perform it and how the interval is measured. Unlike an annual, a 100-hour inspection may be performed and approved by any certificated A&P mechanic — an Inspection Authorization is not required. The interval is measured in aircraft flight hours, not calendar time.

The 10-Hour Overrun Allowance

Practical operations sometimes make it impossible to land exactly at the 100-hour mark. The FAA addresses this in 14 CFR §91.409(b) by allowing an aircraft to exceed the 100-hour limit by up to 10 hours, but only when doing so is necessary to fly the aircraft to a location where the inspection can be performed. Those extra hours are not "free" — they count against the next 100-hour interval. In other words, if you flew 107 hours since the last 100-hour inspection to reach the maintenance shop, the next inspection is due at 100 hours minus 7, or 93 hours of additional flight time. This prevents pilots from simply treating the overrun as a bonus every cycle.

Airworthiness Directives (ADs)

An Airworthiness Directive is a legally enforceable regulation issued by the FAA under 14 CFR Part 39 when an unsafe condition is found to exist in an aircraft, aircraft engine, propeller, or appliance. Compliance with applicable ADs is not optional — operating an aircraft in violation of an AD makes it not airworthy, regardless of whether it just passed an annual inspection. ADs take precedence over the Type Certificate and are part of the aircraft's continuing airworthiness requirements.

ADs come in two major forms. A one-time AD requires a specific action to be accomplished once (such as inspecting a part, modifying a component, or replacing it), after which compliance is complete. A recurring AD requires action at specified intervals — every 100 hours, every annual, every certain number of flight cycles, or on some other schedule — meaning the operator must track and repeat the required action indefinitely throughout the aircraft's service life.

The FAA publishes ADs in the Federal Register, and they can be searched through the FAA's online AD database by aircraft make, model, and serial number. It is the aircraft owner's or operator's responsibility under 14 CFR §91.403 to ensure the aircraft complies with all applicable ADs. Mechanics record AD compliance in the aircraft's maintenance records, noting the AD number, the method of compliance, the date, aircraft total time, and the mechanic's certificate number and signature. Pilots performing a preflight document review should confirm that AD compliance entries are current, particularly for recurring ADs.

Other Inspections Worth Knowing

While the annual and 100-hour inspections dominate the knowledge test, several additional inspections are required by regulation and are worth understanding:

Transponder inspection (14 CFR §91.413): Aircraft transponders used in controlled airspace must be tested and inspected every 24 calendar months. This applies to both Mode C and Mode S transponders.
ELT inspection (14 CFR §91.207): Emergency Locator Transmitters must be inspected within 12 calendar months after the last inspection. Battery replacement is required when the ELT has been in use for more than one cumulative hour, or when 50% of the battery's useful life has expired.
Altimeter and pitot-static system (14 CFR §91.411): Aircraft operated in controlled airspace under IFR must have the altimeter and static system inspected and tested within the preceding 24 calendar months. This requirement does not apply to VFR-only operations.
VOR receiver check (14 CFR §91.171): For IFR flight, VOR equipment must be checked within the preceding 30 days. This is a pilot-performable check with several acceptable methods outlined in the regulation.

Why These Requirements Matter

These inspections are not bureaucratic box-checking exercises — they exist because mechanical failures cause accidents. An aircraft may look and feel fine to a non-expert pilot during preflight, while harboring a hidden structural crack, a fatigued brake component, or a fuel system deficiency. The annual and 100-hour inspection processes bring trained, certificated mechanics into contact with the aircraft at regular intervals specifically to find problems that pilots are not qualified to detect. ADs exist because manufacturers and the FAA have identified real-world failures across fleets — sometimes after fatal accidents — and compliance ensures the entire active fleet addresses those known hazards.

For the student pilot renting aircraft, the practical implication is straightforward: check the maintenance logbooks before flight. Confirm the annual inspection date, confirm that the 100-hour is current if the aircraft is used for hire, and verify that recurring ADs have been complied with. This is not an act of distrust toward the school or FBO — it is a fundamental pilot responsibility enshrined in 14 CFR §91.7, which places the final determination of airworthiness squarely on the pilot in command.

Key Numbers and Rules

Annual inspection: Required every 12 calendar months; valid through the last day of the 12th month; must be signed off by an A&P with an IA.
100-hour inspection: Required every 100 hours of flight time for aircraft used for hire or flight instruction for hire; an IA is not required — a standard A&P may sign off.
100-hour overrun: Up to 10 additional hours allowed to fly to an inspection facility; those hours are deducted from the next 100-hour interval.
ADs: Legally binding under 14 CFR Part 39; one-time or recurring; non-compliance = not airworthy; owner/operator is responsible for compliance.
Transponder check: Every 24 calendar months (§91.413).
ELT inspection: Every 12 calendar months (§91.207).
Altimeter/pitot-static: Every 24 calendar months for IFR in controlled airspace (§91.411).
VOR check: Every 30 days for IFR operations (§91.171).

Common Test Traps

Calendar months vs. hours: The annual is measured in calendar months (not flight hours); the 100-hour is measured in flight hours (not calendar time). Mixing these up is the most common error.
Who can sign off what: An IA is required for an annual; a plain A&P (no IA) can sign off a 100-hour. The test loves to ask whether a specific person is authorized for a specific inspection type.
The 10-hour overrun is not a bonus: Many students think those extra hours are free. They are not — they reduce the next 100-hour interval by the amount of the overrun.
100-hour applies only to for-hire operations: A privately flown aircraft does not need a 100-hour inspection, only an annual. If a question describes a "privately owned aircraft," the 100-hour rule does not apply.
AD non-compliance voids airworthiness: An aircraft that just had an annual inspection is still not airworthy if an applicable AD has not been complied with. Students sometimes assume a fresh annual covers everything — it does not override outstanding ADs.