{"id":"F2021L00479","name":"AD/LYC/78 Amdt 6 Thermostatic Oil Cooler Bypass Valve","slug":"ad-lyc-78-amdt-6-thermostatic-oil-cooler-bypass-valve","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":101716,"registerId":"commonwealth-F2021L00479-current","compilationNumber":null,"startDate":"2026-04-03","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"Lycoming Piston Engines","sectionType":"part","heading":"Lycoming Piston Engines","content":"# Lycoming Piston Engines\n\n| AD/LYC/78 Amdt 6 | Thermostatic Oil Cooler Bypass Valve | 9/2021 |\n| ---------------- | ------------------------------------ | ------ |\n\n| Applicability: | All Lycoming engines with thermostatic oil cooler bypass valves:(a) Part number (P/N) 53E19600 (with thermostatic oil cooler bypass valve serial numbers less than 53788),(b) P/N 75944, P/N LW-13230, and P/N 53E19980 (with thermostatic oil cooler bypass valve serial numbers less than 159).                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |\n| -------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |\n| Requirement:   | 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.Note 1: The words “as in force from time to time” require compliance with the most recent version of the referenced document.                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |\n| Compliance:    | Unless previously accomplished in accordance with a previous issue of this Directive:a. For dual magneto engines - within 25 hours’ time in service after the commencement date of this AD and thereafter at intervals not to exceed 325 hours’ time in service.b. For non-dual magneto engines - within 25 hours’ time in service after 25 May 1995 and thereafter at intervals not to exceed 625 hours’ time in service.Note 2: Replacement of the thermostatic oil cooler bypass valve with a serviceable superseding thermostatic oil cooler bypass valve part number identified in Lycoming Service Instruction No. 1565A, or subsequent, constitutes terminating action for the repeat inspection requirements of this AD.                                                                       |\n|                | This AD commences on 7 May 2021.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |\n| Background:    | This Directive requires an inspection of the oil cooler thermostatic bypass valve for security of the crimp nut on the valve shaft as detailed in the requirement document. 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, the service bulletin was amended to increase the crimp nut dimension limits.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 whilst retaining compliance thresholds on a time-in-service basis. |\n\n![A picture containing insect\nDescription automatically generated](image.001.jpeg)\n\nAlan Silva  \nDelegate of the Civil Aviation Safety Authority\n\n22 April 2021","sortOrder":0},{"sectionNumber":"AD/LYC/78 Amdt 6","sectionType":"part","heading":"AD/LYC/78 Amdt 6","content":"## AD/LYC/78 Amdt 6","sortOrder":1},{"sectionNumber":"Thermostatic Oil Cooler Bypass Valve","sectionType":"part","heading":"Thermostatic Oil Cooler Bypass Valve","content":"## Thermostatic Oil Cooler Bypass Valve","sortOrder":2},{"sectionNumber":"9/2021","sectionType":"part","heading":"9/2021","content":"## 9/2021","sortOrder":3}],"analysis":{"flash_summary_failed":{"failed":true,"reason":"Unauthenticated. Configure AI_GATEWAY_API_KEY or use a provider module. Learn more: https://ai-sdk.dev/unauthenticated-ai-gateway","source":"analysis-cron"},"flash_summary":{"complexity_score":4,"scope_assessment":{"changed":true,"description":"Amendment 6 updates the applicability and requirement wording to align the AD with the most recent Lycoming MSB 518 while keeping the existing time‑in‑service compliance thresholds. Earlier amendments removed a crimp‑nut dimensional check; this amendment shifts procedural detail to the current MSB and clarifies which part numbers/serial ranges are captured."},"complexity_factors":["Multiple affected part numbers and serial‑number cutoffs require individual unit identification (Applicability).","Two different recurring inspection intervals depending on engine magneto configuration (Compliance).","Reliance on an external, updateable technical document—MSB No. 518—using the phrase “as in force from time to time”, which shifts procedural detail outside the directive (Requirement).","Terminating action depends on replacement with specific superseding parts identified in a separate Service Instruction (Service Instruction No. 1565A referenced in Compliance).","Background history: earlier dimensional check was removed, so inspectors must follow the current MSB version to know what to inspect (Background).","Practical tracking and recordkeeping burden to ensure initial 25‑hour windows and subsequent hour‑based intervals are met (Compliance)."],"plain_english_summary":"Requires operators of Lycoming piston engines fitted with specific thermostatic oil‑cooler bypass valves to perform a physical inspection using Lycoming Mandatory Service Bulletin No. 518 (\"as in force from time to time\"). Operators must pay for and schedule inspections; initial and recurring intervals differ by engine magneto type and replacing the valve with an approved superseding part stops the repeat inspections (Requirement; Compliance)."},"kimi_summary":{"content_quality":"ok","complexity_score":3,"scope_assessment":{"changed":true,"description":"This started as a simple inspection for a crimp nut dimension and security check. Over multiple amendments (now Amendment 6), it has evolved: the dimensional check was removed entirely (Amendment 5), applicability has been expanded and refined to cover more part/serial number combinations, and the compliance structure has been modified to use 'time in service' rather than calendar dates for recurring inspections. The current version is more targeted but administratively more complex than the original."},"complexity_factors":["Multiple part number and serial number combinations in applicability (4 different P/Ns with varying serial number cutoffs)","Two different compliance schedules based on engine magneto configuration (dual vs non-dual)","Reference to external documents 'as in force from time to time' creates dynamic compliance obligations","Terminating action provision requires cross-referencing another service instruction (SI 1565A)","Amendment history references (original MSB 518, then 518B, now current version) create contextual complexity for operators tracking compliance history"],"plain_english_summary":"This CASA AD mandates physical inspection of the thermostatic oil cooler bypass valve crimp nut on specified Lycoming piston engines to ensure it’s secure, following Lycoming Mandatory Service Bulletin No. 518. It affects owners/operators of engines with the listed valve part/serial numbers and sets recurring inspection intervals (shorter for dual‑magneto engines) until the valve is replaced with an approved newer part. A loose crimp nut can cause valve failure, risking oil overheating or loss of oil pressure in flight."}},"importantCases":[],"_links":{"self":"/api/acts/ad-lyc-78-amdt-6-thermostatic-oil-cooler-bypass-valve","history":"/api/acts/ad-lyc-78-amdt-6-thermostatic-oil-cooler-bypass-valve/history","analysis":"/api/acts/ad-lyc-78-amdt-6-thermostatic-oil-cooler-bypass-valve/analysis","conflicts":"/api/acts/ad-lyc-78-amdt-6-thermostatic-oil-cooler-bypass-valve/conflicts","importantCases":"/api/acts/ad-lyc-78-amdt-6-thermostatic-oil-cooler-bypass-valve/important-cases","documents":"/api/acts/ad-lyc-78-amdt-6-thermostatic-oil-cooler-bypass-valve/documents"}}