The 100-Hour That Is Not Due and the Annual That Is
A flying club's Cessna 172N is used for instruction and rental, carrying persons for compensation. The last annual inspection was signed 11 months and 3 weeks ago at 6,890 hours; the airplane now shows 6,952 hours. The last 100-hour inspection was signed 8 months ago at 6,912 hours. The club president wants to skip the annual for now, arguing the airplane is 'only 40 hours into the 100-hour' and that he will bring it in next month when business is slow. He also asks whether he can move the airplane to a progressive inspection program he wrote himself, splitting the airframe into four quarterly segments, and whether that will eliminate the 100-hour inspections entirely.
1. Can the airplane legally be flown next month on the strength of the recent 100-hour inspection?
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No. Section 91.409(a)(1) requires the annual inspection within the preceding 12 calendar months. That clock is measured in calendar months, not in 365 days and certainly not in hours; it runs from the date of the last annual to the corresponding date 12 calendar months later, and the airplane is due on that date - not 'next month.'
The 100-hour requirement in Section 91.409(a)(2) is a separate clock that runs on hours in service for aircraft carrying persons for hire. Being 40 hours into the 100-hour does nothing for the annual. Both requirements apply, and the annual is the binding constraint here. Advise the club in writing; if they operate past the due date, that is their exposure, but you should not create a record that suggests the airplane was compliant.
2. The club brings the airplane in today. Will an inspection performed now satisfy both the annual and the 100-hour requirement?
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Yes, if you actually perform the full scope. Part 43, Appendix D prescribes the same scope and detail for annual and 100-hour inspections; the differences are who may perform them and the certification language. An inspection performed to the complete Appendix D scope, with the Section 43.11(a) annual certification in the record, satisfies the annual requirement.
The trap is the sign-off. If you sign only as a '100-hour inspection,' the annual remains overdue and the airplane is unairworthy. The clean practice is one entry stating both certifications, with the aircraft total time, so the record is unambiguous.
Also brief the club that an annual does not reset the 100-hour clock. If the airplane reaches 100 hours in service before the next annual is due, another 100-hour inspection is required at that point.
3. Evaluate the proposed four-segment progressive inspection program.
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A progressive program is a legitimate route under Section 91.409(d), but it is not something an owner writes and an IA starts using. The program must be submitted to and approved by the FAA, must cover the entire aircraft including engines, propellers, appliances, and every Appendix D item across the cycle, must be completable within 12 calendar months, must specify the intervals at which each segment is due, and must include a checklist and procedures for each segment.
Until the approval is in hand, the annual and 100-hour rules apply exactly as written. Do not begin performing 'quarterly segments' against an unapproved document and do not sign anything that suggests the airplane is on a progressive program.
Practical points to raise: who is designated to sign each segment, how segment completion is tracked so no item is skipped, and how the club's rental schedule will accommodate segment downtime. If the program is later approved, the intervals in the approved document govern for that aircraft.