For 957 Lycoming engines, the paperwork defines the AD's applicability

No — this AD does not ask whether the piston pin in your engine is defective. For an engine inside its applicability list, it asks whether the pin's FAA Form 8130-3 exists and is legible, and if the answer is no, the AD applies by default. AD 2026-17-10, published in the Federal Register on August 28, 2026 and effective September 14, 2026, covers an FAA-estimated 957 Lycoming engines on the U.S. registry. For shops, owners and IAs, the applicability question is documentary before it is mechanical.

The sentence that decides everything

Paragraph (c) of the AD ends with one sentence that has no precedent in our corpus. The AD applies to O-235, O-540 and IO-540 series engines with a serial number in Table 2 or Table 3 of Lycoming Mandatory Service Bulletin 667A, dated March 12, 2026. The pin condition: an installed piston pin P/N LW-13445 with an OEM 8130-3 dated between January 7, 2021 and December 15, 2025. Then the AD states:

"If the production date on the FAA Form 8130-3 cannot be determined, or if the FAA Form 8130-3 is not available, this AD is applicable."

Read it as an engineer would. The regulation does not say "suspect the pin" or "investigate further". It says the absence of the document determines the applicability of the maintenance obligation. The default is fail-closed: no form, no legible date, no exit. One boundary, stated once and never relaxed in what follows: the presumption binds only inside the population paragraph (c) just defined — a listed model, a serial number in Table 2 or Table 3, and an installed P/N LW-13445. An engine outside that population is outside the AD, whatever its paperwork.

The definitions in paragraph (g)(2) close every door around the same logic. An "affected piston pin" is P/N LW-13445 plus one of three conditions: an OEM 8130-3 dated inside the window; an 8130-3 whose production date cannot be determined; or an 8130-3 that is not available. The corollary deserves to be stated explicitly, because it is the only way out: a legible OEM 8130-3 dated outside the January 7, 2021 – December 15, 2025 window is what removes the pin from the definition. Everything else — a copy you cannot date, a form lost in a logbook transfer, a pin installed without its paperwork — leaves the engine inside the AD.

What the FAA knows, and why it wrote the rule this way

The background section of the AD is factual. Lycoming reported metal particulates found in engine oil during oil changes, and two engine failures the FAA describes as non-detectable. The manufacturer's investigation traced the cause to certain thin-wall piston pins "manufactured with microcracks and inclusions in the base metal due to honing and tooling process changes from the piston pin supplier". A process change at a third-party supplier, invisible to normal inspection: that is the unsafe condition.

The manufacturer's own warning, in MSB 667A, is worth quoting in full: "PISTON PIN FAILURE CAN CAUSE AN UNCOMMANDED AND COMPLETE LOSS OF POWER." The FAA's version of the consequence chain adds oil starvation, in-flight shutdown and loss of control of the airplane.

The procedural framing needs one precision, because dates matter here. The FAA bypassed prior notice and comment under the good-cause exception of 5 U.S.C. 553(b) and (d), and justifies what it calls the "immediate adoption" of the rule. That phrase describes the procedure — no NPRM before the final rule — not the effective date. The rule itself is effective September 14, 2026. Comments are still open until October 13, 2026, under Docket FAA-2026-8789, and the FAA may amend the rule in response. Published August 28 is not effective September 14, and neither date is a compliance deadline: the compliance times below run in hours, not days.

The compliance tree, in hours TIS

Every deadline in this AD is expressed in hours time-in-service. Nowhere does the AD say "17 days". What it requires, by branch:

• Oil change and inspection, for all affected engines. For engines in Table 3 of MSB 667A with more than 150 total hours TIS since the affected pins were installed: before further flight. For all other affected engines: within 5 hours TIS after the effective date, unless an oil change was already performed within the last 10 hours TIS. The inspection covers the oil filter, oil pressure screen and oil suction screen, with Lycoming MSB 480F as guidance for identifying metallic solids. The delta over the bulletin is precise, and the AD's own "Differences" section names it: the 5-hour TIS oil change and inspection for the other affected engines is an FAA addition — MSB 667A has no such requirement. The bulletin is not silent on oil, though: for Table 3 engines over 150 hours TIS, it already prescribes the MSB 480 "Oil Servicing, Metallic Solids Identification After Oil Servicing, and Associated Corrective Action" before the next flight.
• Pin replacement if metal particulates are found. Before further flight — the finding criterion is the inspection above: metal particulates in the oil filter, oil pressure screen or oil suction screen.
• Pin replacement when no metal particulates are found, Table 2 engines. At the next maintenance event requiring removal of a cylinder assembly with an affected pin, or at the next engine overhaul not exceeding the time between overhaul in Lycoming Service Instruction 1009BE, whichever occurs first after the effective date.
• Pin replacement when no metal particulates are found, Table 3 engines at 150 hours TIS or less. Within 25 hours TIS after the effective date, and in any case before reaching 150 total hours TIS since the pins were installed.
• Installation prohibition. After September 14, 2026, no affected piston pin may be installed on any engine. MSB 667A adds the mirror instruction on the shelf side: remove and quarantine unused LW-13445 pins in the affected 8130-3 date range.

Special flight permits follow the same logic: they may be issued for the replacement requirement only if no metal particulates were found, and they are not allowed for the oil-inspection requirement.

The records chain behind the AD

An airworthiness directive is law, not advice. Under 14 CFR 39.13, ADs are part of the Code of Federal Regulations, published in the Federal Register as amendments to that section. Under 14 CFR 39.7: "Anyone who operates a product that does not meet the requirements of an applicable airworthiness directive is in violation of this section."

Compliance then lives in records. Under 14 CFR 43.9, the person performing the work makes a maintenance record entry containing a description of the work, the date of completion, and the signature, certificate number and kind of certificate of the person approving the work — the signature constituting approval for return to service only for the work performed. Under 14 CFR 91.417, the owner must keep the current status of applicable ADs, including the method of compliance, and the records "shall be retained and transferred with the aircraft at the time the aircraft is sold." As we detailed in the pre-purchase records audit, the buyer inherits the documentary position, defects included. For an aircraft whose engine falls inside the AD's population — a listed O-235, O-540 or IO-540 model with a serial number in the MSB tables — an undocumented AD 2026-17-10 in the logbook is now a pricing fact at the next sale.

What to do before September 14

You run a shop, manage an aircraft with a Lycoming O-235, O-540 or IO-540, or sign its records. The sequence is documentary first, mechanical second.

1. Match the engine serial number against Tables 2 and 3 of MSB 667A. If the serial number is not listed, the AD does not apply to that engine, whatever the paperwork says. Record the check and its date in the records anyway — a documented negative closes the question for the next buyer, inspector or auditor.
2. Pull the 8130-3 for the installed pins. You are looking for the OEM form for production of the LW-13445 pins, and specifically the production date. MSB 667A locates it in block 13 of the form. Photograph or scan what you find.
3. Apply the three-way test exactly as paragraph (g)(2) writes it. Date inside the window: affected. Date undeterminable: affected. Form unavailable: affected. Only a legible OEM 8130-3 dated outside the window takes the pin out of the definition.
4. Treat "we cannot find the form" as an applicability finding, not a filing problem. That is the doctrinal shift of this AD. The missing document does not suspend judgment while you search; it resolves the question against the pin.
5. Branch into the compliance tree by table and hours. Table 3 above 150 hours TIS since pin installation is a before-further-flight oil change and inspection. Everything else runs on the 5-hour, 25-hour and 150-hour clocks above, and the SI 1009BE overhaul bound for Table 2.
6. Check the shelf, not just the engine. Spare LW-13445 pins with an 8130-3 dated in the window are quarantined per MSB 667A, and after the effective date none can be installed on any engine. A compliant engine with a contaminated spares bin is one rushed repair away from a violation.
7. Write the determination down, whichever way it goes. The 43.9 entry — description, date, signature, certificate number — is what makes compliance provable at the next annual, the next pre-purchase, or the next ramp check. A determination that exists only in a technician's memory has zero regulatory value.
8. If the date window caught you mid-overhaul or mid-sale, escalate early. AMOCs go to the FAA's East Certification Branch under 14 CFR 39.19; questions on the docket go to the contact named in the AD. Comments close October 13, 2026.

The limits of what this proves

Three limits keep this analysis honest.

• Applicability is not guilt. An engine inside the AD's scope is an engine with an obligation, not an engine with a cracked pin. The FAA's reports involve metal particulates and two engine failures; for the rest of the fleet, the AD manages a risk that has not manifested. Absence of a finding on your engine is not proof of absence either — the microcracks are, in the FAA's own words, difficult to detect prior to failure.
• The fleet count is an estimate. The 957-engine figure is the FAA's cost-estimate count of affected engines on the U.S. registry, as published on August 28, 2026. It is not a serial-number census, and the comment docket may move it.
• The rule may change. This is a final rule with an open comment window until October 13, 2026. What is written today is the law from September 14, but AMOCs and amendments are live possibilities, and this article dates its reading to August 31, 2026.

AeroCert's take

AeroCert's take: our corpus has been building to this case. AOG Technics was forged paperwork. GA Telesis was the doctrine for verifying paperwork. The MadeByMike3D UPN was the absence of paperwork as a grounding cause at receiving. AD 2026-17-10 goes one step further: the absence of the document is written into the rule itself as the trigger of the maintenance obligation. Verification must fail closed because the regulation now fails closed. When a certificate's SHA-256 hash is anchored at issuance by an authenticated issuer, the question "does a verifiable 8130-3 exist for this pin, from this source?" has a binary answer in one query — before the logbook is even opened. Two honest limits, the same as ever. A registry proves what is anchored in it: an empty result does not prove the universal absence of a form issued outside the anchoring network, and the fail-closed lookup is only as strong as issuer adoption. And anchoring is forward-looking — it cannot reconstruct a 2021 8130-3 that was never issued or has been lost. For the 8130-3s issued from today onward, it makes the applicability question answerable in seconds instead of weeks. See how AeroCert works, or integrate verification into your own systems with the developer platform.

Sources

Primary sources:

• FAA Airworthiness Directive 2026-17-10, Lycoming Engines — Federal Register, 91 FR 55451, Docket FAA-2026-8789, published August 28, 2026, effective September 14, 2026 (text mirror: govinfo): the applicability sentence quoted from paragraph (c); the "affected piston pin" definition and the three 8130-3 conditions in paragraph (g)(2); the "part eligible for installation" definition in (g)(1); the compliance tree in paragraph (h) — before-further-flight oil change and inspection for Table 3 engines over 150 hours TIS, 5 hours TIS otherwise (10-hour lookback), before-further-flight replacement on metal particulates, the next-cylinder-event / next-overhaul branch for Table 2 per SI 1009BE, the 25-hour / 150-hour branch for Table 3; the installation prohibition in (i); the special-flight-permit limits in (k); the background (metal particulates in oil, two non-detectable engine failures, microcracks and inclusions from supplier honing and tooling process changes); the good-cause justification for "immediate adoption" under 5 U.S.C. 553(b) and (d); comments due October 13, 2026; the FAA estimate of 957 affected engines on the U.S. registry and the per-product cost estimate; the "Differences Between This AD and the Referenced Material" section, under which the 5-hour TIS oil change and inspection (10-hour lookback) is an FAA requirement that Lycoming MSB 667A does not contain.
• Lycoming Mandatory Service Bulletin No. 667A, Piston Pin Replacement, March 12, 2026 (incorporated by reference in the AD): affected models and engine serial numbers (Tables 1-3), the 8130-3 date window January 7, 2021 – December 15, 2025 read in block 13 of the form, the warning "PISTON PIN FAILURE CAN CAUSE AN UNCOMMANDED AND COMPLETE LOSS OF POWER", the removal-and-quarantine instruction for unused pins, the MSB 480 "Oil Servicing, Metallic Solids Identification After Oil Servicing, and Associated Corrective Action" prescribed before the next flight for Table 3 engines over 150 hours TIS, and the SI 1009 overhaul bound.
• 14 CFR 39.13 and 14 CFR 39.7: airworthiness directives are part of the CFR; operating a product that does not meet an applicable AD's requirements is a violation.
• 14 CFR 43.9: content of maintenance record entries — description of the work, completion date, and the signature, certificate number and kind of certificate of the person approving the work.
• 14 CFR 91.417: maintenance records to be retained and transferred with the aircraft at sale, including the current status of applicable airworthiness directives with method of compliance.