Attention:
Owners, operators and maintainers of Lycoming opposed cylinder engines.
| File Classification No.: | Z 5000-35 |
|---|---|
| RDIMS No.: | 21301123 |
| Document No.: | CASA 2026-07 |
| Issue No.: | 01 |
| Effective Date: | 2026-07-16 |
Purpose:
The purpose of this Civil Aviation Safety Alert is to inform owners, operators and maintainers of Lycoming opposed cylinder engines of the potential of an engine failure due to improper application of sealant to crankcase parting surfaces during engine assembly.
Background:
Transport Canada (TC) has been advised by the Transportation Safety Board of Canada (TSB) of a potential hazard concerning the application of crankcase parting-surface sealant. The hazard was identified during the investigation of a fatal occurrence involving a Lycoming O-320-E2G engine.
A post‑occurrence teardown of the engine revealed that sealant had been applied outside the permissible regions specified in Lycoming’s instructions for continued airworthiness. Specifically, excess sealant was observed on crankcase mating surfaces beyond the approved application boundaries. Additional sealant residue was also noted on the through‑bolt surfaces, as well as on the bearing journal interfaces. The presence of sealant in these areas is inconsistent with Lycoming’s assembly instructions.
The TSB has not conclusively established that improper application of crankcase parting-surface sealant was the direct cause of the engine failure. However, sealant was identified in non-permissible areas during post-occurrence teardown and is considered a potential contributing factor.
Incorrect application of crankcase parting‑surface sealant, whether through excess application, contamination of bolt holes, or use of unapproved materials may cause:
- Oil starvation
- Fastener torque loss
- Bearings distortion and internal engine damage
- Crankcase cracking or structural failures
- Persistent oil leaks and reduced reliability
Guidance for sealing crankcase parting surfaces on all Lycoming opposed‑cylinder engines is found in Lycoming Service Instruction (SI) 1125E – Alternate Crankcase Parting Surface Sealants. The service instruction identifies approved sealants and defines proper application methods to ensure reliable sealing and maintain engine airworthiness. The instruction also specifies areas where sealant must not be applied, including any location that interfaces with load‑carrying fasteners, oil galleries, or bearing support structures.
Lycoming SI 1125D does not adequately warn of the risks associated with applying sealant outside of the specified areas. In contrast, guidance issued by other engine manufacturers contains explicit warnings describing the potential consequences of incorrect sealant application, up to and including engine failure.
Figure 1. Lycoming SI 1125E – Alternate Crankcase Parting Surface Sealants
Figure 2. Occurrence Engine Crankcase
Recommended action:
TC recommends that owners and operators of Lycoming opposed-cylinder engines review Lycoming SI 1125E and consult with their maintenance provider to confirm compliance. Any discrepancies or concerns identified through this review should be addressed prior to further engine operation. Maintainers are further advised to ensure that all personnel involved in engine assembly are familiar with the specific application boundaries and restrictions outlined in Lycoming SI 1125E.
Within Canada, in accordance with the reporting requirement of Part V, subpart 21 (521) of the Canadian Aviation Regulation (CAR), owners, operators and maintainers should submit a Service Difficulty Report (SDR) to TC for the discovery of each reportable service difficulty. Outside Canada, report any such occurrence in accordance with local reporting requirements.
Contact office:
For more information concerning this issue, contact a Transport Canada Centre; or contact Kent Goulding, Continuing Airworthiness in Ottawa, by telephone at 1-888-663-3639, or by e-mail at TC.CAWWEBFEEDBACK-retroactionWebMDLN.TC@tc.gc.ca.
Original signed by
Jenny Young
Chief, Continuing Airworthiness
National Aircraft Certification
The Transport Canada Civil Aviation Safety Alert (CASA) is used to convey important safety information and contains recommended action items. The CASA strives to assist the aviation industry's efforts to provide a service with the highest possible degree of safety. The information contained herein is often critical and must be conveyed to the appropriate office in a timely manner. The CASA may be changed or amended should new information become available.