Every new aeroplane, engine and propeller certificated in the United States arrives with a stack of maintenance instructions written by its manufacturer. Thousands of pages of servicing data, troubleshooting trees, torque values and inspection schedules. Most owners assume the whole stack is law.

It is not. In 2011 the FAA’s own Office of the Chief Counsel said in writing that, for small aircraft flown under Part 91, the intervals and replacement times in a manufacturer’s manuals are, as a general proposition, not mandatory. The exceptions are Airworthiness Directives and one clearly marked section of the manual that the FAA itself has approved. It is called the Airworthiness Limitations Section, and it is the part of the maintenance manual that carries the force of a regulation.

Here is who writes the instructions, what goes into them, and why the difference between “approved” and “accepted” decides what an owner must do.

The manufacturer writes it, the FAA reviews it

The legal starting point is 14 CFR 21.50(b). The holder of a design approval, either a type certificate or a supplemental type certificate, for an aircraft, engine or propeller applied for after 28 January 1981, must furnish at least one set of complete Instructions for Continued Airworthiness to the owner on delivery, or when the first standard airworthiness certificate is issued, whichever is later. After that, the instructions and every change to them must be made available to anyone required to comply with them.

So the regulator does not write the maintenance manual. Boeing, Cessna, Pratt & Whitney or the small company that holds an STC for a cabin modification writes it. The rule only says what it must contain, through a set of airworthiness standards: 23.1529 for small aeroplanes, 25.1529 for transport aeroplanes, 27.1529 and 29.1529 for rotorcraft, 33.4 for engines and 35.4 for propellers. FAA Order 8110.54A, the agency’s guidance on ICA, records that these requirements arrived in 1980 and replaced the patchwork of manual standards that came before.

Section 25.1529 sets the standard for transport aircraft in two sentences. The applicant must prepare the instructions in accordance with Appendix H to Part 25, and they must be “acceptable to the Administrator.” They may even be incomplete at type certification, provided there is a programme to finish them before the first aeroplane is delivered or receives its standard certificate of airworthiness.

What Appendix H puts in the box

Appendix H reads like the contents page of every airliner maintenance manual ever printed, because it is the template for them. It requires the manuals to be in English. They must describe the aeroplane and its systems, give servicing information on tank capacities, fluids, pressures, lubrication points and jacking, and provide scheduling information with recommended periods for cleaning, inspecting, adjusting, testing and lubricating each part.

It goes on: troubleshooting information, removal and replacement procedures, diagrams of access panels, details of special inspection techniques such as radiographic and ultrasonic testing, protective treatments after inspection, structural fastener data including torque values, and a list of special tools. The aeroplane’s instructions must also include those for each engine and propeller, or the essential information if the engine or propeller maker does not supply them.

FAA guidance is firm that the result must be specific to the product. Order 8110.54A warns that applicants lean too heavily on general standard-practice documents such as AC 43.13-1, which it describes as acceptable only when there are no manufacturer repair or maintenance instructions, and not acceptable as the “complete set” of ICA.

EASA’s own explainer on Instructions for Continued Airworthiness, the European counterpart of the same concept.

The section that binds

Then comes paragraph H25.4, and the tone changes. The instructions must contain a section titled Airworthiness Limitations, “segregated and clearly distinguishable from the rest of the document.” For a transport aeroplane it must list each mandatory modification time, replacement time, structural inspection interval and procedure approved under the fatigue and damage-tolerance rule, 25.571; the fuel tank system limitations, including critical design configuration control limitations, under 25.981; mandatory replacement times for wiring components; lightning protection limitations; and the certification maintenance requirements.

It must also state a limit of validity, the number of flight cycles or hours up to which the engineering data behind the structural maintenance programme holds good. Until full-scale fatigue testing is complete and the FAA has approved that figure, an aeroplane may not accumulate more than half the cycles reached by the fatigue test article. That is the mechanism behind the long fatigue-test campaigns described in our Boeing 777-9 fatigue test feature. For engines, the approved lives of discs and other life-limited parts sit in the same kind of section.

And the section has to say what it is. Appendix H requires a legible statement in a prominent place, word for word.

Three regulations make that sentence true. Section 21.31(c) lists the Airworthiness Limitations Section as part of the type design itself, alongside the drawings and the structural data. Section 43.16 requires anyone performing a task it specifies to do it in accordance with that section, unless an approved Part 121 or 135 operations specification or an approved inspection programme under 91.409(e) provides otherwise. And 91.403(c) says no person may operate the aircraft unless its mandatory replacement times, inspection intervals and related procedures, or those approved alternatives, have been complied with.

Approved versus accepted

This is the distinction that sorts the binding pages from the rest. Order 8110.54A states that the airworthiness regulations require ICA to be acceptable to the FAA, while certain portions must be approved, such as the Airworthiness Limitations Section. For imported products it spells it out: approval is required for the ALS, certification maintenance requirements may be involved and approved as well, and “the balance of the ICA must be accepted.”

Accepted means the FAA has evaluated the instructions and found that they meet the regulations. It does not turn every recommended interval into a legal requirement. The 2011 interpretation, written in reply to a request asking whether the intervals in a Beech Baron’s manuals were binding, explains why: if manufacturers could make their own intervals mandatory, private companies would be issuing rules without the public notice and comment that the Administrative Procedure Act requires.

That is why, as AOPA’s maintenance column put it in 2021, engine time between overhauls, service bulletins and service letters are advisory rather than mandatory for the average Part 91 operator. An Airworthiness Directive can make a task mandatory, and so can the ALS. Commercial operators are a different case: their maintenance programmes are approved through operations specifications, which is where the MSG-3 and Maintenance Review Board process comes in.

Cessna on its Instructions for Continued Airworthiness documents, from the manufacturer’s side of the process.

Not mandatory is not optional

None of this licenses a mechanic to throw the manual away. Section 43.13(a) requires maintenance to be done with the methods, techniques and practices in the current manufacturer’s maintenance manual or ICA, or other methods acceptable to the FAA. The same 2011 letter closes that door firmly.

Section 43.13(b) adds that the finished work must leave the product at least equal to its original or properly altered condition. In practice the manual remains the default, and an alternative has to be defensible to an inspector who asks why it was used.

So the honest answer to “must you obey the maintenance manual?” has three parts. The manufacturer writes all of it. The FAA accepts most of it as a sound basis for maintenance, and approves one segregated section. That section, with its replacement times, inspection intervals and limit of validity, is the part that grounds an aeroplane when it is ignored.

Sources: eCFR (14 CFR 21.31, 21.50, 25.1529, Part 25 Appendix H, 43.13, 43.16, 91.403), FAA Order 8110.54A Instructions for Continued Airworthiness, FAA Office of the Chief Counsel legal interpretation (MacMillan, 2011), AOPA

Header photo: Caholguin109 / Wikimedia Commons, CC BY-SA 4.0