Two aeroplanes sit on the same ramp. Same type, same year, same engine, same avionics. One belongs to a doctor who flies it to see his grandchildren. The other belongs to a flight school and has a hundred hours put on it every eleven weeks by people learning to land.

The first one gets inspected once a year. The second gets inspected once a year and then again every hundred hours. And here is the part almost nobody knows: the two inspections are the same inspection. The same federal regulation. The same list of items. The same checklist. The only thing that changes between an annual and a 100-hour is who is allowed to sign the logbook at the end of it.

That single signature is worth a separate federal certificate, takes five years of mechanic experience to qualify for, and has to be renewed in March of odd-numbered years. It is called an Inspection Authorization, and it is the most consequential piece of paper in general aviation maintenance that nobody outside the hangar has ever heard of.

The two inspections are the same inspection

Part 43 Appendix D is titled, with the FAA’s usual indifference to prose, “Scope and Detail of Items (as Applicable to the Particular Aircraft) To Be Included in Annual and 100-Hour Inspections”. One appendix. Both inspections. There is no separate list for the annual and no shorter list for the 100-hour.

It opens with the single most expensive sentence in light-aircraft ownership. Before the inspection begins, the person doing it must remove or open all necessary inspection plates, access doors, fairing and cowling, and thoroughly clean the aircraft and the engine. Everything after that is looking. The cost is in the taking apart and the putting back together.

Then come nine groups: fuselage and hull, cabin and cockpit, engine and nacelle, landing gear, wing and centre section, empennage, propeller, radio, and a final paragraph covering every installed item not otherwise listed. Thirty-nine numbered items plus three catch-alls, and the catch-alls do real work — the empennage paragraph alone asks for poor general condition, skin deterioration, distortion, evidence of failure, insecure attachment, improper installation and improper operation.

Two further requirements sit in 43.15 and are routinely missed by owners reading about this online. The first is that a written checklist is mandatory. It may be your own design, the manufacturer’s, or one from anywhere else, but it must cover the scope and detail of Appendix D. The second is that on a piston aeroplane the person approving the return to service has to run the engine first, before signing, and check power output at static and idle RPM, the magnetos, fuel and oil pressure, and cylinder and oil temperature. The inspection does not end with a torch and a mirror. It ends with the engine running.

So what is different? The signature

An A&P mechanic — one certificate, two ratings, airframe and powerplant — may do a great deal. Section 65.85 says a mechanic with an airframe rating may perform the 100-hour inspection required by Part 91 and approve the airframe for return to service. Section 65.87 says exactly the same thing for the powerplant and the propeller. Between them, your A&P can sign off a 100-hour on the whole aeroplane.

Now read what the holder of an Inspection Authorization may do, and notice what is in this list and in no other.

That is the whole mechanism. The annual is not harder than the 100-hour, it is not longer than the 100-hour, and it does not look at more of the aeroplane. It simply requires a signature that a plain A&P certificate does not carry. The same section also hands the IA the other restricted privilege: approving an aircraft for return to service after a major repair or major alteration.

There is one consequence worth getting right, because owners get it backwards. An annual counts as a 100-hour: 91.409(b) accepts “an annual or 100-hour inspection”. A 100-hour does no count as an annual — unless it was performed by somebody authorised to do annuals and was actually entered in the records as an “annual”. The regulation says so in as many words. The logbook entry is not paperwork after the fact; it is the thing that determines what the inspection legally was.

A general-aviation owner walking through what an annual actually involves, and what it costs.

Who actually needs a 100-hour, and who only thinks they do

This is where most of the confusion lives. The 100-hour inspection is not triggered by flying a lot. It is triggered by money. Section 91.409(b) names exactly two situations: carrying any person other than a crewmember for hire, and giving flight instruction for hire in an aircraft that the instructor provides.

Read that second clause again, because it is the one that catches people. The aeroplane has to be provided by the person giving the instruction. A freelance instructor teaching you in your own aeroplane does not put it on a 100-hour cycle, however much you pay them, because they are not providing the aircraft. A flight school renting you its Cherokee with an instructor in the right seat is providing it, and that aeroplane needs both inspections.

The annual clock is calendar months, and it is generous in a way people underuse. Twelve calendar months means the end of the twelfth month, so an annual signed on 14 March 2026 is good until 31 March 2027 — you get the rest of March for free. It runs whether or not the aeroplane is flown. A hangar queen that did not turn a propeller all year is out of annual on the same day as one that flew three hundred hours.

The 10-hour extension on the 100-hour is the other thing worth reading carefully. It is not a buffer. 91.409(b) allows the limit to be exceeded by not more than ten hours while en route to reach a place where the inspection can be done, and then requires the excess to be included in computing the next hundred hours. Borrow eight hours this cycle and the next one is due at ninety-two. There is no equivalent relief for the annual at all.

An owner-assisted annual from the owner’s side. Under 43.3, an owner may do a surprising amount of the work under supervision, which is where the savings come from.

Earning the signature: five years, a written test, and March

Section 65.91 sets the bar for an Inspection Authorization, and it is not a weekend course. The applicant must hold a mechanic certificate with both the airframe and powerplant ratings, each currently effective and each in effect for a total of at least three years. They must have been actively engaged in maintaining certificated aircraft for at least the two years immediately before applying. They must have a fixed base of operations where they can be reached in person or by telephone during a normal working week. They must have the equipment, facilities and inspection data to do the job properly. And they must pass a written test. Fail it, and you may not reapply for ninety days.

Keeping it is its own discipline, and the calendar is oddly specific. Under 65.93, the holder presents evidence during the month of March of each odd-numbered year that they still meet those requirements, for a renewed two-year period. On top of that they must show, in each of the two years separately, one of five activities: at least one annual inspection for every ninety days they held the authority; or at least two major repairs or alterations for every ninety days; or a progressive inspection performed or supervised and approved; or an eight-hour refresher course; or an oral test from an FAA inspector.

Miss the first-year deadline of 31 March and the privileges stop that day. They come back only after passing the oral test — which then also counts as having met the requirement. It is strict, and the one time somebody formally asked the FAA to loosen it, the agency said no.

That rule did change one thing for the better, and it is new enough that most reference material has not caught up. The same 2024 final rule that removed the expiration date from the flight instructor certificate added a brand-new 65.93(d), letting somebody whose Inspection Authorization expired while they were serving outside the United States in support of US armed forces operations renew it under the newly codified relief in 61.40. In the FAA’s own words, 65.93 did not previously allow that relief at all. The rule took effect on 1 December 2024.

The IA presented from the mechanic’s side, as a career step rather than a regulatory hurdle.

The third option nobody uses

There is a way out of the annual-and-100-hour rhythm entirely, and 91.409(d) has had it on the books for decades. A progressive inspection breaks the aeroplane into pieces and inspects them on a rolling schedule, so that the whole aircraft is covered within each twelve calendar months but is never down for a single long stoppage.

The price of admission is why almost no private owner takes it. You submit a written request to the responsible Flight Standards office. You must provide an IA, a certificated airframe repair station or the manufacturer to supervise or conduct it. You need a current inspection procedures manual, readable by pilots and mechanics alike, containing the continuity of inspection responsibility, the reporting, the record keeping, an inspection schedule in hours or days, sample routine and detailed forms, and sample reports. You need enough housing and equipment for the necessary disassembly. And you need current technical information for the aircraft.

That is a maintenance department, not a hangar. Which is exactly why progressive inspections live at flight schools, charter operators and corporate flight departments, and why the doctor with the Cherokee will never see one. If the programme is discontinued, the owner must tell the FAA in writing immediately, and the annual then falls due within twelve calendar months of the last complete inspection.

Why the split exists at all

Strip away the paragraph numbers and the logic is simple. The FAA wanted every certificated aeroplane looked at once a year by somebody with a demonstrated, separately tested, continuously renewed standard of judgement. It also wanted aircraft that carry paying passengers or paying students — the ones flying the most hours in front of the least experienced hands — looked at far more often, and for that it was willing to accept any competent A&P.

Frequency gets the ordinary mechanic. Finality gets the IA. The annual is the yearly moment when one person with a particular certificate puts their name against the whole aeroplane, and the regulation is built so that moment cannot be delegated downwards.

It is worth knowing which one you are paying for. When a shop quotes you for “the annual”, the labour is Appendix D and the opening up that Appendix D demands — the same work a 100-hour would involve. What you are also buying, and what you cannot buy anywhere else, is the signature of somebody the FAA has spent five years and a written test satisfying itself about.

Sources: 14 CFR 91.409, 43.3, 43.7, 43.15 and Part 43 Appendix D; 14 CFR 65.81, 65.85, 65.87, 65.91, 65.93 and 65.95, all read in the eCFR as in force 1 October 2026; FAA final rule “Removal of Expiration Date on a Flight Instructor Certificate”, Docket FAA-2023-0825, Amdt. 65-65, 89 FR 80020, 1 October 2024.