100-Hour vs Annual Inspection: A Flight School Compliance Guide

A trainer rolls in at 1,499.4 hours on the Hobbs after the last lesson of the day. The next student is booked at 8 a.m. The dispatcher knows the airplane is "due for something soon" but not whether it is the annual that lapses next week or the 100-hour that the airplane is about to blow past tonight. Those are two different inspections, on two different clocks, signed off by two different people, and confusing them is how a school dispatches an airplane that is no longer legal to fly for hire.

Two Inspections, Two Triggers

The annual and the 100-hour are often spoken of together, but only one of them runs on the calendar. Under 14 CFR 91.409, every aircraft must have an annual inspection within the preceding 12 calendar months before it flies. That applies to a privately owned weekend airplane and a busy trainer alike. Miss the date and the airplane is grounded until an inspector signs it off, no matter how few hours it has flown.

The 100-hour is a different animal. It is triggered by hours, not dates, and it only applies to aircraft that carry persons for hire or that are used to give flight instruction for hire. A flight school's rental and dual fleet lives squarely inside that rule. A privately owned airplane that a CFI uses to instruct only its owner does not, which is a distinction worth knowing when a member offers up their own aircraft for training.

So a school trainer is on both clocks at once. It needs an annual every 12 calendar months and a 100-hour every 100 hours of use. Whichever comes first is the one that grounds it.

Annual Covers the 100-Hour, but Not the Other Way Around

A fresh annual resets the 100-hour count, because an annual is the more thorough inspection and satisfies the 100-hour requirement on the day it is signed. The reverse is not true. A 100-hour inspection never counts as an annual, so an airplane that is current on its 100-hour but past 12 calendar months on its annual is still grounded.

The signoff authority differs too. An annual must be returned to service by a mechanic holding an Inspection Authorization. A 100-hour can be signed by any certificated airframe and powerplant mechanic. For a school that means the annual is a scheduled, plan-ahead event tied to a specific IA's availability, while the 100-hour recurs often enough that it belongs in the routine rhythm of fleet care rather than treated as a surprise.

Counting the 100 Hours: Time in Service, Not Hobbs

The single most common mistake is counting the 100-hour against the Hobbs meter. The regulation counts time in service, defined in 14 CFR 1.1 as the time from the moment an aircraft leaves the surface of the earth until it touches down at the next point of landing. Hobbs time includes taxi, run-up, and every minute the engine idles on the ramp, so it accumulates faster than time in service. A dispatcher tracking only Hobbs reaches the inspection limit late and can overshoot it without realizing.

This is the same two-clock problem that drives billing, and the relationship between the meters is worth understanding before you trust either one for maintenance. Our breakdown of Hobbs versus tach time walks through why the number on the invoice and the number that counts toward the 100-hour are rarely the same, and why a school should state in its maintenance program exactly which reading approximates time in service.

There is one narrow allowance. The 100-hour limit may be exceeded by up to 10 hours, but only while the aircraft is en route to a place where the inspection can be done, and the overflown time is then subtracted from the next interval. It is a ferry provision, not a buffer. An airplane that flies a student lesson at 100.5 hours has flown for hire out of inspection, and "we were close" is not a defense.

The Inspections Hours Do Not Trigger

The 100-hour and annual get the attention, but a trainer carries other recurring requirements that lapse on the calendar and ground the airplane just as hard. The transponder and its altitude reporting must be tested every 24 calendar months. For any aircraft flown IFR, the altimeter and static system carry the same 24-month cycle. The emergency locator transmitter has its own inspection and battery rules. Airworthiness directives are the wild card: some are one-time, but recurring ADs come due on hours, on the calendar, or on cycles, and a single missed AD makes the airplane unairworthy regardless of how fresh the annual is.

None of these fit neatly under "100-hour or annual," which is exactly why they get missed. A complete fleet record tracks each item on its own clock rather than assuming the annual sweeps everything up. The VOR check that instrument students rely on every 30 days is a pilot action logged in the aircraft, not a maintenance inspection, but it belongs in the same status picture so nobody dispatches an IFR lesson against a stale check.

Where Schools Get Caught

The failure modes cluster. An airplane gets dispatched a few tenths past its 100-hour because the limit was tracked on Hobbs. An annual lapses while the airplane sits in the shop for an unrelated squawk and nobody resets the calendar reminder. A recurring AD comes due mid-month and the only record of it is in a binder the dispatcher never opens. In each case the airplane was airworthy yesterday and is not today, and the schedule never noticed the line being crossed.

A squawk can also walk an airplane out of compliance. A discrepancy that turns out to be an AD-driven item, or one that grounds the aircraft until a part arrives, has to move the airplane out of the dispatch pool immediately. That is the same closed-loop discipline covered in our guide to tracking aircraft squawks: a maintenance status that lives only on a sticky note is a status the schedule cannot honor.

Make Dispatch Enforce the Limit

The fix is not a better binder. It is making the inspection status and the booking calendar the same source of truth, so the airplane drops out of available slots the instant it reaches a limit. Set the warning before the legal line, not on it: flag a trainer several hours of time in service before the 100-hour and several weeks before the annual, so there is room to schedule the work without losing revenue days. Any aircraft booking system worth running should refuse to release an airplane whose maintenance record says it is due, the same way it refuses to double-book a tail number.

Done that way, the inspection stops being a thing someone has to remember and becomes a property of the airplane that the schedule reads automatically. A platform that ties maintenance status to dispatch, the way HangarOS does, can hold the 100-hour count in time in service, the annual due date, and every recurring AD in one place, so a dispatcher cannot hand over the keys to an airplane that is out of inspection.

That single change, putting the airworthiness clock where the people booking flights can see it, removes one of the quieter risks in running a flight school. The airplane that gets caught at 99 hours is the one that never becomes a question at the next ramp check.