On 8 April 2026 the Federal Aviation Administration published a document about seat belts. It told owners of general aviation aircraft that the National Transportation Safety Board had notified the agency of multiple accidents in which restraint assemblies had come apart under impact loading, that testing had found one shoulder harness failing at loads below the standard it was built to, and that the FAA had since identified at least fifteen more accidents with full or partial restraint failure. Many of them failed at the same place: the stitched Y-junction where the shoulder straps meet.

Directly under the date, in plain type, sat one line: This is information only. Recommendations aren’t mandatory.

That sentence is not a disclaimer somebody bolted on. It is the whole legal character of the document, stated up front, and it marks the line that runs through every piece of paper in aircraft maintenance. Some of them are orders. Most of them are not. Telling the two apart is the single most useful piece of regulatory literacy an aircraft owner can have, and the aviation industry is extremely bad at explaining it.

The Bulletin That Tells You Nothing Has To Be Done

The restraint bulletin is SAIB 2026-10, subject line “Equipment / Furnishings - Restraint Systems”. The accident that triggered it is identified by its NTSB docket number, CEN12FA601: a 1946-vintage Aeronca 7AC Champion, and an Aero Fabricators four-point restraint, model H-702-300, that separated at the shoulder harness Y-junction threading.

The FAA’s recommendations are specific and genuinely useful. Check that your restraints are labelled to TSO-C114 for torso restraints or TSO-C22g for pelvic belts, or a later revision. Look for a rated strength of 3,000 pounds or more. Replace anything whose label is missing, illegible or absent of those markings. Inspect the Y-junction stitching and the fastener hardware at every preflight and every annual. Photograph and report what you find.

Every one of those is a recommendation. Not one of them is enforceable. An owner who reads SAIB 2026-10, shrugs, and keeps flying a 1970s harness with an unreadable label has broken no regulation at all.

That is not an oversight. It is the design.

What an Airworthiness Directive Actually Is

To understand what a SAIB is, you have to understand what it deliberately is not. The instrument above it is the Airworthiness Directive, and the FAA does not hedge about its status. Part 39 of Title 14 opens by calling airworthiness directives “legally enforceable rules”. They are, in the strict sense, regulations: 14 CFR 39.13 confirms that they are part of the Code of Federal Regulations, “but they are not codified in the annual edition” — which is why you will never find the text of a specific AD by leafing through a printed CFR volume. They are published in full in the Federal Register instead.

The teeth are in 39.7 and 39.9. Anyone who operates a product that does not meet the requirements of an applicable AD is in violation, and the violation is not a one-off: you commit it again on every flight the condition goes unmet. Deviating from an AD is possible, but only through the alternative method of compliance process in 39.19, and only once an FAA manager has approved it. And where an AD and the manufacturer document it was built from disagree, 39.27 settles it: the AD controls.

The trigger for all of that is a two-part test, and it is short enough to quote whole. Under 14 CFR 39.5, the FAA issues an airworthiness directive addressing a product when it finds that “An unsafe condition exists in the product” and that “The condition is likely to exist or develop in other products of the same type design.”

Both halves have to be true. A one-off failure on one airframe, however ugly, is not an AD. A theoretical weakness that has never shown itself as an unsafe condition is not an AD either. The regulation is a gate, and a great deal of real-world airworthiness concern arrives at that gate and does not get through.

Below the AD Line: Where the SAIB Lives

Everything that fails the 39.5 test but still worries somebody at the FAA needs somewhere to go. That is the job FAA Order 8110.100B gives the Special Airworthiness Information Bulletin. The order describes it as an information tool to alert, educate and make recommendations to the aviation community, and says in the same breath that it contains “non-regulatory, non-mandatory information and guidance” for safety issues that do not meet the criteria for AD action under part 39.

The order is not shy about it. One paragraph states flatly that a SAIB “is advisory in nature, and not mandated by regulatory action”. Another says that an SAIB “is a non-mandatory document that is advisory in nature. It only makes recommendations”. A third requires the bulletin itself to say so: the SAIB “must clearly state that, at the time the SAIB is issued, the airworthiness concern is not an unsafe condition that would warrant AD action under 14 CFR part 39.” That is the sentence you find near the top of every one of them, and now you know why it is there.

There is one more clause worth knowing, because it is the one that tells you how seriously the agency takes the distinction. The order says the FAA “is not required to provide justification as to why the airworthiness concern does not meet the criteria of an AD.” It has to tell you the bulletin is not binding. It does not have to tell you why.

The mechanics are mundane by comparison. SAIBs are numbered by calendar year plus a running count, which is why the theft bulletin further down this article is 2026-11 and not something more cryptic. Revisions get an R1, R2 and so on bolted onto the original number. The Engineering Procedures Office assigns the numbers, and an approved bulletin has to be published within two business days.

The Three Things a SAIB Is Forbidden To Do

This is the part almost nobody knows, and it is the most interesting thing in the whole order. A SAIB is not merely non-binding. There are specific jobs it is explicitly barred from taking on.

It may not be used to identify an interim corrective action while an AD is being developed. That rules out the obvious shortcut: the agency has spotted something genuinely unsafe, the rulemaking will take months, so push out a bulletin in the meantime. The order says no. If the concern is an unsafe condition, the order tells engineers not to use a SAIB to convey the required corrective actions at all, including where the concern arrives as mandatory continuing airworthiness information from a foreign authority.

It may not be used to clarify the requirements of an existing AD. If an AD is ambiguous, the order says, revise or supersede the AD. And it may not be used to grant an exemption from one, or an alternative method of compliance; that is what the AMOC process in 39.19 is for.

Taken together those three prohibitions do something unusual for a government document. They stop the advisory instrument being quietly used as a substitute for the regulatory one. A SAIB cannot become a soft AD, because the order that creates SAIBs forbids it.

The Other Pile of Paper: Service Bulletins

The FAA is not the only organisation sending documents to aircraft owners. Manufacturers issue service bulletins, alert service bulletins and service letters, and a good number of them arrive with the word MANDATORY printed across the top in red.

A manufacturer cannot make a regulation. What it can do is write a document, and then persuade the FAA to point at that document inside an AD. Until that happens, the FAA’s own position for private operators is unambiguous. Its fact sheet for aircraft owners, published by Flight Standards as AFS-850 20-04, puts it in one sentence.

The same sheet is careful not to overstate the point. It says compliance “may or may not be required depending on the type of operation and whether or not it is included in an AD”, which is the FAA conceding that how you operate the aircraft changes the answer. It also warns against the obvious misreading: a bulletin marked mandatory, alert or emergency is the manufacturer flagging a significant safety concern, and manufacturers in that position may ask the FAA to issue an AD. And it adds, pointedly, that you should not ignore the ones marked merely recommended or optional either.

The question has been fought over. In 2006 the National Transportation Safety Board took a position that would have made service bulletin compliance effectively compulsory for private operators, and the FAA issued a legal interpretation that went the other way. The aviation lawyer Gregory J. Reigel, writing that September, summarised the outcome as plainly as it can be put: compliance with a service bulletin on a Part 91 aircraft, absent an AD or other regulatory requirement, “is not mandatory simply because the NTSB says it is.”

What a Non-Mandatory Bulletin Looks Like When It Matters

If all of this sounds like a technicality, read SAIB 2026-11, issued on 8 June 2026 under the subject line “Theft of Turbofan Engine Parts before Mutilation”.

Non-airworthy engine parts are supposed to be destroyed, physically and irreversibly, so that they can never find their way back onto an aircraft. In late January 2026, according to the bulletin, a third party impersonated a contracted mutilation provider and intercepted twelve containers of parts on their way to be destroyed. The haul ran to 625 serialised parts, among them critical engine components and life-limited parts, plus 9,740 non-serialised items, across four engine families: the CFM56, the V2500, the PW1100G and the RB211-535E4.

Spain’s civil aviation authority spotted the rerouting and reported the theft to the European Union Aviation Safety Agency on 17 March 2026. Spanish police and the Guardia Civil took the criminal investigation. The FAA bulletin spells out what those parts could do if they were fitted: life-limited parts flown past their limits, and the possibility of an uncontained engine failure.

And the bulletin recommends. It recommends reading the EASA alert, subscribing to the FAA’s unapproved parts notifications, cross-referencing serial numbers against the published lists, tightening supplier evaluation under advisory circular AC 21-29D, and reporting anything suspicious. Under the date, the same line as always: this is information only, recommendations are not mandatory.

It reads oddly at first. Then you reread 39.5 and it makes sense. There is no established unsafe condition in a particular product that is likely to exist or develop in others of the same type design. There is a quantity of parts of unknown location and unknown status. That is a supply-chain problem, not a type-design problem, and the AD machinery has nothing to bite on. So the FAA used the only instrument it had, and said so.

The Corner Where No AD Can Reach

There is one category of aircraft for which the advisory document is not a step down from an AD, because an AD was never on the table. FAA Order 8110.100B addresses aircraft holding special airworthiness certificates that have no type certificate behind them, and the wording is blunt: the FAA will not issue an AD for them. For those aircraft, the order notes, the SAIBs “are still recommended actions and not mandatory.”

That is a larger population than it sounds. Experimental aircraft, amateur-built aircraft, and the civilian ex-military jet fleet all live there. The L-39 Albatros and Hawker Hunter that fly today in civilian hands do so on special airworthiness certificates, maintained to an approved programme rather than to a civil type certificate that never existed. The continuing airworthiness of those aeroplanes rests on operators, type clubs and maintenance organisations choosing to act on information that no regulation obliges them to act on.

How To Read the Stack

So here is the hierarchy, from the top down. An Airworthiness Directive is a rule, and breaking it is a violation on every flight. A manufacturer’s service bulletin is a recommendation that becomes a rule the moment an AD incorporates it, and which may already be binding depending on how the aircraft is operated. A Special Airworthiness Information Bulletin is the FAA telling you something it has decided it cannot order you to act on, and it is required to admit as much in writing.

None of which makes the bottom of the stack worthless. The FAA’s SAIB database is free, searchable by bulletin number or by aircraft make and model, and it is where the agency puts the concerns that have not yet hardened into regulation. Reading it is how you find out what is worrying people before it becomes an order.

The restraint bulletin will not make anyone replace a forty-year-old harness. It simply tells you, in the FAA’s own measured prose, that fifteen-plus accidents have involved restraints coming apart, often at the same stitched junction, and that the agency has looked at that and concluded it cannot compel you to do anything about it.

What you do with that is, legally speaking, entirely up to you. Which is rather the point of knowing the difference.

Sources: FAA Order 8110.100B, Special Airworthiness Information Bulletin (24 July 2013); FAA SAIB 2026-10, Equipment / Furnishings - Restraint Systems (8 April 2026); FAA SAIB 2026-11, Theft of Turbofan Engine Parts before Mutilation (8 June 2026); 14 CFR part 39; FAA Safety Briefing fact sheet AFS-850 20-04, Service Bulletins and the Aircraft Owner; Gregory J. Reigel, Service Bulletins Clarified, Aero Legal Services, September 2006; NTSB docket CEN12FA601.