Service Bulletins vs. Airworthiness Directives: What’s Actually Mandatory

Few things generate more confusion — and more heated forum threads — than the word “mandatory” stamped across the top of a manufacturer’s service bulletin. An owner reads it, assumes they’re legally on the hook, and starts bracing for a five-figure repair bill. Then their A&P shrugs and says, “You don’t actually have to do that one.”

Both the bulletin and the mechanic can be right at the same time. The key is understanding the difference between what a manufacturer recommends and what the FAA requires — because under Part 91, those are two very different levels of obligation.

Two documents, two very different sources of authority

Airworthiness Directives and service bulletins can look similar on paper. They both describe a problem with an aircraft, engine, propeller, or appliance, and they both tell you what to do about it. The difference is who issues them and what legal weight that carries.

Airworthiness Directive (AD)

• Issued by the FAA
• Legally enforceable under 14 CFR Part 39
• Mandatory for Part 91 — compliance is required for the aircraft to be airworthy
• Non-compliance can ground the aircraft
• Published in the FAA’s AD database (the authoritative source)

Service Bulletin (SB)

• Issued by the manufacturer
• Advisory in nature for Part 91 operators
• Not a federal regulation — even when labeled “mandatory”
• May still matter for safety, warranty, and resale
• Sometimes the basis for a later AD

An Airworthiness Directive is the FAA’s regulatory instrument for correcting an unsafe condition. It’s a rule with the force of federal law, issued under 14 CFR Part 39. Under 14 CFR 91.403, the owner or operator is responsible for maintaining the aircraft in an airworthy condition, and that responsibility explicitly includes complying with applicable ADs. Miss one that’s due, and your aircraft is not legally airworthy — full stop. (For a deeper look at how ADs work, see our guide to understanding Airworthiness Directives and staying compliant.)

A service bulletin is a document created by the manufacturer of your airframe, engine, propeller, or a component. It communicates a recommended inspection, modification, part replacement, or procedure change. Manufacturers issue them for good reasons — they see failure data across the fleet that individual owners never will. But a service bulletin is a manufacturer’s recommendation, not an FAA regulation.

The “mandatory service bulletin” trap

Here’s where owners get tripped up. Manufacturers sometimes label a bulletin “Mandatory” or issue a “Mandatory Service Bulletin” (MSB). It sounds unambiguous. But for aircraft operated under Part 91, that manufacturer’s label does not, by itself, create a legal obligation. The manufacturer can call it mandatory; only the FAA can make it mandatory for a Part 91 operator.

This is a real point of divergence between the general aviation world and the commercial world. Operators flying under Part 121 (airlines) or Part 135 (charter) often are bound by manufacturer service documents, because their FAA-approved maintenance programs commit them to it. But this blog is about Part 91 owner-operators — and in that world, the default for a standalone service bulletin is “recommended,” not “required.”

When a service bulletin does become mandatory

“Advisory by default” is not the same as “always optional.” There are specific, important situations where the content of a service bulletin becomes legally binding on a Part 91 aircraft. This is exactly the kind of thing worth confirming with your A&P or IA, because the details are aircraft-specific:

1. When an AD references it

The most common path: the FAA issues an Airworthiness Directive that adopts a service bulletin by reference. When that happens, the SB’s instructions become the compliance method for the AD — and now they carry the full legal weight of Part 39. Service bulletins are frequently the origin story of an AD; the manufacturer identifies a problem and publishes a bulletin, and if the FAA determines the condition is unsafe across the fleet, an AD follows. Once that AD exists, “optional” is off the table.

2. When it’s part of the airworthiness limitations

Some maintenance actions live in the Airworthiness Limitations Section of a manufacturer’s Instructions for Continued Airworthiness, or in the type certificate documentation. Items in that category are regulatory because the rules require you to comply with the airworthiness limitations, regardless of whether the same information also appears in a service bulletin. Your mechanic can help you identify whether a given item falls here.

3. When you’ve committed to it another way

Certain aftermarket approvals, such as some STCs (Supplemental Type Certificates), can carry their own continued-airworthiness requirements. If your aircraft has modifications like that, the associated instructions may pull specific service actions into your required maintenance. Again: aircraft-specific, and worth a conversation with the mechanic who knows your airplane.

How it plays out: Your engine manufacturer issues a mandatory service bulletin calling for inspection of a particular component at a set interval. You read “mandatory” and assume you’re grounded until it’s done.

Your IA checks: there’s no AD referencing this bulletin, and the item isn’t in your airworthiness limitations. Legally, for your Part 91 operation, it’s a recommendation — you are not grounded.

Six months later, the FAA issues an AD that adopts that same bulletin. Now it’s mandatory, with a compliance deadline. The action didn’t change; the legal status did. That’s exactly the transition an owner needs to catch.

“Not required” doesn’t mean “not worth doing”

It’s a mistake to treat “advisory” as a synonym for “ignore it.” A manufacturer issues a service bulletin because they’ve seen something in the field — a part that wears faster than expected, a failure mode that shows up at certain hours, a fix that heads off an expensive problem. That’s real safety information, produced by the people who designed your aircraft.

There are also practical, non-regulatory reasons a Part 91 owner might choose to comply with a service bulletin:

Safety. The bulletin often reflects fleet-wide failure data you’d never see on your own. When the manufacturer flags something, it’s usually worth taking seriously even if the FAA hasn’t weighed in.

Resale value. A prospective buyer’s mechanic will review your logbooks during a pre-buy. A record of complied-with service bulletins signals a well-cared-for aircraft; a stack of ignored MSBs can become a negotiating point against you.

Warranty and goodwill. For newer aircraft or overhauled engines still under warranty, ignoring a manufacturer’s service bulletin can complicate a future warranty claim.

The right frame isn’t “do I legally have to?” alone. It’s a two-part question: Is this legally required? and, separately, Is this a good idea for my aircraft and how I use it? Sometimes the answer to the first is no and the second is a clear yes. That’s a judgment call to make with your mechanic — not something to dismiss because a label did or didn’t carry legal force.

Why tracking both matters

From a recordkeeping standpoint, ADs and service bulletins call for different handling — and lumping them together is how owners get into trouble in both directions. Track ADs too loosely and you risk flying unairworthy. Treat every “mandatory” SB as a legal grounding item and you may spend money and downtime on work that Part 91 never required.

A well-organized owner keeps a clear picture of three things at once: which ADs apply to their specific airframe, engine, and propeller and whether each is complied with; which service bulletins the manufacturers have issued for their aircraft; and — critically — whether any of those bulletins have since been pulled into an AD or an airworthiness limitation. That last category is the one that quietly changes status over time, and it’s the one most likely to catch an owner off guard at annual.

The bottom line

An Airworthiness Directive is a federal rule you must follow to keep your aircraft airworthy. A service bulletin — even one stamped “mandatory” — is a manufacturer’s recommendation that is advisory for Part 91 operators, right up until an AD, an airworthiness limitation, or another regulatory hook makes it binding.

Knowing which is which does two things for you. It keeps you legal, because you never miss the item that actually grounds the airplane. And it keeps you from overspending on work you were never required to do. Both of those come down to the same habit: track your ADs and service bulletins carefully, keep an eye on whether any bulletin has crossed over into regulatory territory, and lean on your mechanic for the aircraft-specific calls.