Definition
A required maintenance inspection of an aircraft's airframe, engine, propeller, and systems performed by an authorized mechanic and recorded in the aircraft's maintenance logs. An annual inspection is required every 12 calendar months for most aircraft and must be performed by a mechanic holding an Inspection Authorization (IA). A 100-hour inspection is required for aircraft operated for hire or used for flight instruction provided by the aircraft owner, and must be completed every 100 hours of time-in-service; it can be performed by any certificated airframe and powerplant (A&P) mechanic. The two inspections cover the same scope of items.
Plain English
A thorough check-over of the aircraft that has to be done either once a year, or every 100 hours of flying when the aircraft is used for hire or instruction. It's the airplane's required physical exam to confirm it's still safe and legal to fly.
Context Anchor
You’ll see this in aircraft logbooks, maintenance records, and weight-and-balance paperwork when checking whether the aircraft is legal and current to fly.
Derivation
Annual comes from the Latin word annus, meaning year. That helps because an annual inspection is the yearly required inspection. 100-hour is plain wording: it points to an inspection interval based on 100 hours of aircraft use.
Why Pilots Care
The aircraft is not legal to fly without a current inspection; the pilot in command is responsible for confirming it has been completed before flight.
Intuition Check
Do not read annual as just any yearly look-over, and do not read 100-hour as an informal check near 100 hours. In this aviation use, both mean specific required inspections that must be properly completed and recorded.
Example Sentence 1
Before renting the Cessna, the pilot checked the maintenance logs to confirm the annual inspection was current and the 100-hour inspection was not overdue.
Example Sentence 2
The flight school tracked every aircraft's tach time to schedule the next 100-hour inspection on time.