The AD targets three discrete groups in practical effect: aircraft owners/operators that use Lycoming piston engines listed in the applicability, maintenance organisations or maintenance personnel who perform the inspection and any parts-replacement work, and aircraft records custodians who must track compliance against the specified intervals.

Owners and operators: The AD applies to any aircraft whose engine is a Lycoming piston engine fitted with a thermostatic oil cooler bypass valve of the specified part numbers and within the serial-number ranges stated. Operators must determine whether their engine contains: P/N 53E19600 with thermostat valve serial numbers less than 53788, or P/N 75944, P/N LW-13230, or P/N 53E19980 with thermostat valve serial numbers less than 159. That identification task typically requires inspection of engine or part markings and review of maintenance records. If the engine is within the applicability, the operator must ensure the inspection is carried out within the required initial period and at the prescribed intervals.

Maintenance organisations and certificated engineers: The AD requires the physical inspection be carried out in accordance with Lycoming MSB No. 518. That leaves the detailed inspection steps, acceptance criteria and any pass/fail thresholds to the MSB. The maintenance personnel who perform the work must obtain and follow the latest MSB 518 because the AD expressly requires compliance "as in force from time to time". If a replacement is chosen as terminating action, maintenance personnel must use the part numbers and procedures set out in Lycoming Service Instruction No. 1565A or later.

Record-keepers and continuing airworthiness managers: The AD creates a time-in-service inspection rhythm. For dual magneto engines the rhythm is initial action within 25 hours after the AD commencement date and subsequent inspections every 325 hours TIS or less. For non-dual magneto engines the initial action is within 25 hours TIS after 25 May 1995 and repeats at intervals not to exceed 625 hours TIS. Those intervals must be recorded and tracked in maintenance logs to demonstrate compliance. The AD recognises prior compliance if done per a previous issue of the Directive.

Parts suppliers and overhaul stations: The AD identifies a terminating-path by replacement with a superseding thermostatic bypass valve part number listed in Lycoming Service Instruction No. 1565A, or later. Suppliers need to know which part numbers qualify as "serviceable superseding" parts under that instruction. Overhaul stations that stock or fit such parts will be part of operators' options for meeting the terminating action.

Regulatory and certification staff: The AD was issued by the Civil Aviation Safety Authority delegate. The AD has a commencement date of 7 May 2021 and was signed on 22 April 2021. The AD updates applicability and requirements relative to previous amendments; Amendment 5 and Amendment 6 are mentioned. The AD indicates the regulator is using the MSB and Service Instruction as the technical means for compliance.

Duties

• Identification duty: Operators must identify whether their engine includes a thermostatic oil cooler bypass valve within the applicable part numbers and serial number thresholds specified in the AD. The AD lists the applicable parts and serial-number cut-offs explicitly in the Applicability section.

• Inspection duty: Where applicability is met, operators must carry out a physical inspection of the thermostatic bypass valve "in accordance with the procedure detailed in Lycoming Mandatory Service Bulletin (MSB) No. 518, as in force from time to time" (Requirement). That inspection is the central operational duty imposed by the AD.

• Timing duty: For dual magneto engines the inspection must be done within 25 hours time in service after the commencement date of the AD and thereafter at intervals not exceeding 325 hours TIS. For non-dual magneto engines the inspection must be done within 25 hours TIS after 25 May 1995 and thereafter at intervals not exceeding 625 hours TIS. The AD also states that inspections previously carried out in accordance with a prior issue of the Directive satisfy the initial requirement.

• Recordkeeping and follow-up: The AD implies maintenance records must demonstrate compliance with the inspection cycles and any terminating action taken. Operators will need to document inspections carried out in accordance with MSB 518 and any replacement performed under Service Instruction No. 1565A.

• Terminating action duty option: Operators may choose to replace the thermostatic oil cooler bypass valve with a serviceable superseding valve part number identified in Lycoming Service Instruction No. 1565A or later. Performing that replacement constitutes terminating action for the repeat inspection requirement of the AD.

Rights and limits

• Use of latest technical standard: The AD authorises compliance by following MSB 518 "as in force from time to time", so operators have the right to comply with the latest version of the MSB rather than a fixed older text. That also places on operators an obligation to use the most recent MSB.

• Recognition of prior compliance: The AD recognises that inspections previously accomplished in accordance with an earlier issue of this Directive satisfy the requirement to the extent they were performed in accordance with that prior issue. The clause "Unless previously accomplished in accordance with a previous issue of this Directive" limits the need to repeat inspections already done.

• Termination by specified replacement: Replacing the bypass valve with a serviceable superseding valve listed in Service Instruction No. 1565A or later is explicitly accepted as terminating the repeat inspection requirement. That gives operators an alternative compliance path.

Matters not specified in the AD

• The AD delegates procedural detail to Lycoming MSB No. 518 and to Service Instruction No. 1565A. The AD does not itself provide the step-by-step inspection method or the criteria for a failed inspection; those are in the referenced documents. The AD does not specify enforcement penalties within the text provided.

• The AD does not set out who is authorised to carry out the inspection; the AD presumes compliance will be undertaken by appropriately qualified maintenance personnel following MSB 518.

The AD text provided does not include a penalty schedule, explicit sanctions, or enforcement mechanisms within the document itself. The AD sets mandatory inspections and intervals but does not state fines, licence suspensions or other penalties in the text supplied.

What the AD does show about enforcement mechanics is procedural rather than punitive. The AD:

• Specifies a commencement date: "This AD commences on 7 May 2021", which fixes the compliance timing framework.

• Establishes mandatory inspection and replacement options: it requires inspection per Lycoming MSB No. 518 and allows terminating action by replacement under Lycoming Service Instruction No. 1565A.

The absence of penalties in the AD text means that, within the terms of this document, the AD functions as a mandatory regulatory instrument that prescribes required maintenance actions and schedules. Any enforcement action, sanction or penalty for failure to comply is not expressed in the AD text provided. Regulatory enforcement of ADs commonly rests with relevant aviation safety law and the regulator's enforcement policy; however, that enforcement framework is not reproduced in the text supplied here. The AD does identify the regulator's delegate who issued it, Alan Silva, and gives the issuance date (22 April 2021), which may matter for proving the regulator acted within delegated powers, but the AD does not itself set out the mechanisms for detecting non-compliance, imposing penalties, or processing appeals.

Practically, operators should treat the AD as a compulsory continuing airworthiness requirement: the AD prescribes specific actions and intervals and declares a terminating action. Operators will need to document compliance in maintenance records and to maintain evidentiary proof of inspections and any replacement conducted under the specified Service Instruction. The AD recognises prior compliance where carried out under a previous issue of the Directive; this affords operators a record-based defence against allegations of non-compliance where prior inspections were performed and recorded accordingly.

This AD references and relies on manufacturer technical documents rather than reproducing procedural detail in the body of the directive. The AD incorporates Lycoming Mandatory Service Bulletin No. 518 by reference and requires compliance "as in force from time to time". It also ties terminating action to Lycoming Service Instruction No. 1565A or later. That creates an interaction between regulatory compulsion and manufacturer-issued technical standards: compliance with the AD requires following those external documents.

Specific interactions evident from the AD text:

• The AD makes mandatory an inspection procedure that is defined in Lycoming MSB No. 518. The AD does not include the step-by-step procedure; it relies on the manufacturer's technical documentation to define how a compliant inspection is to be carried out. The "as in force from time to time" formulation means that the operative technical standard is the current MSB, not a static snapshot of the document included in the AD. Therefore, the AD interacts with any processes or legal expectations that govern the use of external documents incorporated by reference.

• The AD provides a terminating action tied to Lycoming Service Instruction No. 1565A, or subsequent versions. That directs affected parties to a service instruction which defines the acceptable replacement part numbers and presumably the installation procedure. The AD thereby creates a legal dependency between regulatory requirements and manufacturer service instructions.

• The AD recognises compliance previously accomplished "in accordance with a previous issue of this Directive". That creates an interaction between successive regulatory instruments: prior amendments and issues of the same AD remain relevant to establishing compliance history.

• The AD does not specify how records should be maintained, who may perform the inspections, or how failed inspections are to be remedied beyond replacement. Those operational matters may be governed by other regulatory instruments and by standard maintenance and airworthiness regulations, but such instruments are not quoted or cited in the AD text provided.

• The AD includes a commencement date and amendment history. The AD’s application to an operator will be measured against that commencement date and against previous AD issues as recognised by the text.

Because the AD binds compliance to manufacturer documents "as in force from time to time", any changes Lycoming makes to MSB No. 518 or to Service Instruction No. 1565A after the AD's issuance will become relevant to complying with the AD. The AD thus interacts dynamically with the manufacturer's ongoing service bulletin and service instruction regimes. The AD itself does not set out procedures for contesting manufacturer changes, nor does it specify which version of an MSB or Service Instruction is authoritative beyond the "as in force from time to time" language.

The AD carries a short internal amendment history within the Background block. The sequence and contents disclosed in the text are the following:

• The original issue of Lycoming Mandatory Service Bulletin No. 518 included both an inspection for security of the crimp nut and a dimensional check of the valve crimp nut. The Background states: "The original issue of MSB No. 518 included a dimension check of the valve crimp nut in addition to a requirement to inspect the crimp nut for security on the shaft."

• Following in-service experience, Lycoming amended MSB 518 to relax the crimp nut dimension limits. The Background says the service bulletin "was amended to increase the crimp nut dimension limits."

• Amendment 5 of this Airworthiness Directive required compliance with Lycoming MSB 518B, which removed the crimp nut dimensional check entirely. The Background notes: "Amendment 5 of this AD was issued to require compliance with Lycoming MSB 518B which removed the crimp nut dimensional check entirely."

• Amendment 6 updates the applicability and requirement statements to reflect the most recent version of the requirement document while retaining the time-in-service compliance thresholds. The Background states: "Amendment 6 updates the applicability and requirement statements to reflect the most recent version of the requirement document whilst retaining compliance thresholds on a time-in-service basis."

Administrative metadata and dates in the AD:

• The AD document bears the label "AD/LYC/78 Amdt 6" and the subject "Thermostatic Oil Cooler Bypass Valve". It indicates "9/2021" on the header area.

• The AD text records that the AD commences on 7 May 2021.

• The AD is signed by Alan Silva, Delegate of the Civil Aviation Safety Authority, with a signature date of 22 April 2021.

Notes within the AD:

• Note 1 explains that the words "as in force from time to time" require compliance with the most recent version of the referenced document. This is explicit textual guidance within the AD regarding how to treat incorporated documents.

• Note 2 states that replacement of the thermostatic oil cooler bypass valve with a serviceable superseding part number identified in Lycoming Service Instruction No. 1565A, or subsequent, constitutes terminating action for the repeat inspection requirements of this AD. That note is part of the compliance section and clarifies the terminating-path mechanism.

The AD thus records an evolution from an initial MSB with a dimensional tolerance check, through an increase of the dimensional limits, to a later removal of that dimension check in MSB 518B, and now to an AD amendment that aligns the AD's applicability and requirement statements with the then-current MSB while preserving inspection intervals.

The AD text provided does not refer to any litigation, court decisions, formal challenges, appeals, or judicial outcomes concerning AD/LYC/78 or its amendments. There are no cases named in the document, and there is no indication within the AD text of judicial review, disputes with the manufacturer, or litigation around compliance, enforcement, or scope.

Because the source contains no litigation record, the AD as presented stands as an administrative regulatory instrument with no litigation history disclosed in the text. Any legal challenges, interpretations by courts, or administrative appeals that may have occurred are not mentioned in the AD document provided. Therefore, from the source alone, there are no recorded cases, no reported litigation outcomes, and no court-authoritative interpretations of the AD's provisions within the supplied text.

Practitioners should note that the absence of litigation references in the AD does not establish that none exists; it merely reflects that the AD text does not record any litigation history. If a party requires knowledge of any judicial or administrative challenges, those would need to be identified in external records beyond the content of this AD.

Several practical issues arise directly from the AD text that can create implementation risk, compliance traps, or record-keeping difficulties if not addressed deliberately.

1. "As in force from time to time" imports the latest MSB: The Requirement clause mandates inspection "in accordance with the procedure detailed in Lycoming Mandatory Service Bulletin (MSB) No. 518, as in force from time to time." Note 1 clarifies that this phrase requires compliance with the most recent version of the referenced document. The practical consequence is that the procedural standard may change over time and that compliance must follow the latest MSB. Operators must therefore obtain and use the current MSB version at the time of each inspection. Failure to use the current MSB could be treated as non-compliance even if an older MSB version was followed.

2. Serial-number thresholds matter: The Applicability section specifies part numbers and serial-number limits: e.g., P/N 53E19600 with serials less than 53788, or P/N 75944, P/N LW-13230, and P/N 53E19980 with serials less than 159. Misreading or overlooking serial-number cut-offs can lead to incorrectly assuming an engine is in or out of applicability. Operators must verify part numbers and serial numbers carefully.

3. Different compliance rhythms depending on magneto configuration: The AD prescribes different initial dates and repeat intervals for dual magneto versus non-dual magneto engines. For dual magneto engines the initial inspection is keyed to the AD commencement date and repeats every 325 hours TIS. For non-dual magneto engines the initial inspection is keyed to 25 May 1995 and repeats every 625 hours TIS. The AD does not explain why the start dates differ, so operators need to ensure correct categorisation of their engines and correct interval scheduling.

4. Recognition of prior compliance is limited to "previous issue" compliance: The Compliance text begins with "Unless previously accomplished in accordance with a previous issue of this Directive", which means prior actions count only if done under an earlier issue of the same Directive. Actions taken under other documents or informal inspections may not be recognised.

5. Terminating action requires a specific service instruction: Replacement that terminates the inspection cycle must be with a "serviceable superseding thermostatic oil cooler bypass valve part number identified in Lycoming Service Instruction No. 1565A, or subsequent." The AD does not list those terminating part numbers in-line. Operators must obtain Service Instruction No. 1565A (or later) to identify qualifying parts. That introduces dependence on an external document and potential ambiguity unless the Service Instruction is consulted.

6. The AD does not reproduce inspection criteria or steps: The AD delegates inspection procedure detail to MSB 518. The AD text does not record acceptance criteria, tolerances or remedial actions for a failed inspection. Without obtaining MSB 518, maintenance personnel will not have the necessary procedural detail to carry out or adjudicate an inspection.

7. Unusual historical compliance anchor for non-dual magneto engines: The initial compliance deadline for non-dual magneto engines is within 25 hours TIS after 25 May 1995, an historic date. That may be a relic of prior ADs; operators must interpret whether engines that have flown since that date must still adhere to the stated initial compliance or whether the clause functions practically as a repeat-interval specification. The AD text treats prior accomplishment in previous AD issues as satisfying the requirement.

8. The AD does not specify enforcement actions or remedial steps post-failure: The AD mandates inspection but does not include statements about the immediate steps to take if the crimp nut is found insecure or out of tolerance. The remedial actions will need to be drawn from MSB 518 and Service Instruction No. 1565A.

9. Amendment chaining: The Background references changes across MSB issues and AD amendments (original MSB, MSB 518B, Amendment 5, Amendment 6). Operators must track not only the current MSB but also which AD amendment implemented which change, because the AD recognises previous compliance "in accordance with a previous issue of this Directive."

10. Documentation burden: Because the AD requires timed repeat inspections and recognizes specific prior compliance only when done under previous Directive issues, an operator’s maintenance records are the primary evidence of compliance. Poorly maintained records could create exposure to an enforcement action even if inspections were physically carried out.

The AD prescribes an operational compliance path that requires coordination between identification, inspection per the manufacturer's MSB, timing discipline, and an optional terminating replacement. The following steps, grounded in the AD text, set out a practical compliance checklist.

1. Determine applicability

• Confirm whether the engine in the aircraft has a thermostatic oil cooler bypass valve with one of the specified part numbers: P/N 53E19600, P/N 75944, P/N LW-13230, or P/N 53E19980.

• For each relevant part number, verify the valve serial number and confirm whether it is below the threshold specified: serial numbers less than 53788 for P/N 53E19600, and serial numbers less than 159 for P/N 75944, P/N LW-13230, and P/N 53E19980. The AD’s Applicability section contains these precise serial-number cut-offs. If the valve is not within these part-number/serial-number bounds, the AD does not apply.

2. Identify whether the engine is dual magneto or non-dual magneto

• The compliance intervals differ by magneto configuration. Confirm the engine configuration from the engine maintenance log or engine data plate. The AD sets different timing for dual magneto engines versus non-dual magneto engines.

3. Obtain the current Lycoming MSB No. 518

• The Requirement mandates "in accordance with the procedure detailed in Lycoming Mandatory Service Bulletin (MSB) No. 518, as in force from time to time." Note 1 clarifies that this phrase requires compliance with the most recent version of the MSB. Obtain the current MSB 518 from Lycoming or an authorised distributor before carrying out the inspection.

4. Plan and perform the inspection within the required time windows

• For dual magneto engines: carry out the physical inspection within 25 hours time in service after the commencement date of this AD, and then repeat at intervals not to exceed 325 hours time in service. The AD’s Compliance clause gives these clear times.

• For non-dual magneto engines: carry out the physical inspection within 25 hours time in service after 25 May 1995 and then repeat at intervals not to exceed 625 hours time in service. Note that the initial deadline for non-dual magneto engines is an historic date specified by the AD.

• If the inspection has already been accomplished in accordance with a previous issue of this Directive, the AD recognises that prior work as satisfying the initial requirement.

5. Follow the procedural steps and acceptance criteria in MSB 518

• MSB 518 contains the detailed inspection procedure, criteria for security of the crimp nut on the valve shaft, and any specified measurement or pass/fail thresholds. The AD does not restate those steps, so follow MSB 518 in full.

6. If replacement is chosen as terminating action, obtain and use Service Instruction No. 1565A or later

• Note 2 in the AD’s Compliance section states that replacement with a serviceable superseding thermostatic oil cooler bypass valve part number identified in Lycoming Service Instruction No. 1565A, or subsequent, constitutes terminating action for repeat inspections. Obtain the Service Instruction to identify qualifying part numbers and any installation procedures. Fit a qualifying replacement as required.

7. Record compliance and maintain evidence

• Document each inspection and any replacement in the aircraft’s maintenance log and continuing airworthiness records. Include the MSB revision used, the date and hours TIS at inspection, the serial number of the part inspected or replaced, and the name and signature of the certifying maintenance person or organisation.

• Where a terminating replacement is fitted, record the part number and serial number of the new valve and the Service Instruction version that identifies the part as a serviceable superseding valve.

8. Track repeat intervals

• Implement a tracking system for the repeat inspection intervals: 325 hours TIS maximum between inspections for dual magneto engines, 625 hours TIS maximum for non-dual magneto engines. Scheduling systems should reference the MSB version in force at the time of each inspection.

9. If an inspection reveals non-compliance, follow MSB and Service Instruction remedial steps

• The AD does not set out remedial actions for a failed inspection. If MSB 518 or Service Instruction 1565A specify corrective steps, adopt those immediately. If the MSB instructs replacement or other action, do not return the engine to service until corrective work is recorded and the engine meets the MSB criteria.

10. Be aware of amendment history and prior compliance

• The Background notes the MSB evolved (dimensions first required, then limits increased, then dimension check removed in MSB 518B) and that Amendment 6 aligns the AD with the latest MSB. If earlier inspections were carried out under a previous issue of the Directive, preserve those records since the AD recognises prior compliance "in accordance with a previous issue of this Directive."

11. Maintain access to Lycoming documentation and clarify uncertainties with the regulator or manufacturer

• Because compliance depends on manufacturer documents "as in force from time to time", maintain access to Lycoming’s MSBs and Service Instructions. If there is uncertainty about which version applies to a particular inspection date or about the status of a part number as "serviceable superseding", consult Lycoming or the regulator for clarification.

By following these steps, which track the obligations and options set out in the AD, an operator will align maintenance practice with the AD’s requirements. The AD’s commencement date is 7 May 2021 and it was issued by the Civil Aviation Safety Authority delegate on 22 April 2021, so schedule compliance actions with those dates in mind.