AD 72-21-06
Applicability
| Type | Manufacturer | Model | Details |
|---|---|---|---|
| engine | Lycoming Engines | IGSO-540-A1A | AVCO LYCOMING Model IGSO-540-A and IGSO-540-B Series Engines |
| engine | Lycoming Engines | IGSO-540-A1C | AVCO LYCOMING Model IGSO-540-A and IGSO-540-B Series Engines |
| engine | Lycoming Engines | IGSO-540-A1D | AVCO LYCOMING Model IGSO-540-A and IGSO-540-B Series Engines |
| engine | Lycoming Engines | IGSO-540-A1E | AVCO LYCOMING Model IGSO-540-A and IGSO-540-B Series Engines |
| engine | Lycoming Engines | IGSO-540-A1F | AVCO LYCOMING Model IGSO-540-A and IGSO-540-B Series Engines |
| engine | Lycoming Engines | IGSO-540-B1A | AVCO LYCOMING Model IGSO-540-A and IGSO-540-B Series Engines |
| engine | Lycoming Engines | IGSO-540-B1C | AVCO LYCOMING Model IGSO-540-A and IGSO-540-B Series Engines |
Unsafe Condition
Shifting and possible failures of the crankcase bearings due to metal contaminants in the lubrication system.
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Required Actions
Inspect the lubrication system for metal contaminants within 10 hours after the effective date and every 25 hours thereafter. Engines with metal contaminants or exceeding 500 hours must comply with (d) or (f). Engines with 1200+ hours must comply with (f) within 50 hours. Remove cylinder and examine bearing position per Service Bulletin 327B or repair crankcase per Service Instructions 1112C with straight dowels.
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Compliance Time
Effective October 17, 1972. Actions required within specified hours in service: inspections within 10 hours after effective date and every 25 hours thereafter; compliance with (d) or (f) within 25 or 50 hours as specified.
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Affected Aircraft
AVCO LyCOMING IGSO-540-A series engines except S/N 2537-50 and subsequent; IGSO-540-B series except S/N 2541-50 and subsequent; and specific serial numbers overhauled after 26 January 1970.
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Source: Official FAA Source ↗
Retrieved: Apr 8, 2026
Rights: U.S. Government Public Domain
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