Document
Advisory
U.S. Department
of Transportation Circular
Federal Aviation Administration Subject: REPLACING MIL-S-8879C Date: 6/20/03 AC No: 21-41A WITH SAE AS8879 Initiated by: AIR-110 Change: 1. PURPOSE . This advisory circular (AC) informs, clarifies, and guides the civil aviation industry on using the Society of Automotive Engineers’ (SAE) Aerospace Standard AS8879, Screw Threads – UNJ Profile, Inch - Controlled Radius Root with Increased Minor Diameter , instead of the Department of Defense’s (DoD) MIL-S-8879C, General Specification For Screw Threads, Controlled Radius Root with Increased Minor Diameter .
2. CANCELLATION . This AC cancels AC 21-41, Continued Use Of MIL-S-8879C, General Specification For Screw Threads, Controlled Radius Root With Increased Minor Diameter ; dated February 26, 1999.
3. USING THE CORRECT SPECIFICATION . In general, Federal Aviation Administration (FAA) design and production approval holders should use the latest revision of referenced specifications unless their design data specifically references a previous revision. However, the DoD has specified that users of MIL-S-8879C should instead use AS8879. The following table helps FAA design and production approval holders determine which specification to use, based on what is referenced in their FAA-approved design data: If FAA-approved data specifies: Then design and production approval holders should use: MIL-S-8879 (no revision specified) AS8879 (latest revision) MIL-S-8879 revision C AS8879 (latest revision) MIL-S-8879C Other Thread AS8879 Category 1 (latest revision) MIL-S-8879C Safety Critical Thread AS8879 Category 2 (latest revision) MIL-S-8879 revision B MIL-S-8879B MIL-S-8879 revision A MIL-S-8879A 4. BACKGROUND .
a. The FAA, on February 26, 1999, issued AC 21-41 to address problems stemming from the DoD inactivating MIL-S-8879C (Notice 2; August 12, 1997) without designating a replacement document. The SAE had issued Aerospace Standard AS8879 Revision A (as a possible replacement for MIL-S-8879C, but the DoD did not officially designate it as a replacement. At that time AC 21-41 advised the civil aviation industry to continue to use MIL-S-8879C if they referenced it in their FAA-approved data.
AC 21-41A 6/20/03 b. The FAA Aviation Fastener Audit (FAA Report No. FAA-IR-01-02), conducted in 2000-2001, identified regulatory deficiencies with MIL-S-8879C and noted that the lack of a replacement specification harmed industry. In June 2001, industry stakeholders formed the Screw Thread Conformity Task Force (STC-TF) comprised of representatives from transport and general aviation aircraft and engine manufacturers, fastener manufacturers, FAA, DoD, and thread gage manufacturers. The STC-TF was tasked to develop a new specification that the DoD would designate as replacing MIL-S-8879C, would meet the FAA’s regulatory needs, and would be accepted by the varied users of MIL-S-8879C.
c. The SAE published the STC-TF recommendation in January 2003 as AS8879C, Screw Threads – UNJ Profile, Inch, Controlled Radius Root with Increased Minor Diameter . The DoD accepted AS8879C and issued a Notice of Cancellation (Notice 3; February 24, 2003) which states in part: MIL-S-8879C dated 25 July 1991, and AMENDMENT 1 dated 2 September 1992, are hereby cancelled, and superseded by the Society of Automotive Engineers (SAE) Aerospace Specification AS8879.
5. AVAILABILITY OF RELATED DOCUMENTS .
a. You can buy copies of SAE Document AS8879 from SAE International, 400 Commonwealth Drive, Warrendale, PA 15096-0001; telephone 877-606-7323 (USA and Canada only) 724-776-4970 (outside USA and Canada), fax 724-776-0790. You can also obtain copies through the SAE Internet website at www.sae.org .
b. You can buy copies of the following Title 14 Code of Federal Regulation (CFR) sections from the Superintendent of Documents, P.O. Box 371954, Pittsburgh, PA 15250-7954; telephone 202-512-1800, fax 202-512-2250: § 21.53, Statement of Conformity. § 29.603, Materials.
§ 23.603, Materials and Workmanship. § 31.33(a) and (b), Materials.
§ 25.603, Materials. § 33.15(b), Materials.
§ 27.603, Materials. § 35.17(b), Materials.
NOTE: You can obtain free copies from the Government Printing Office electronic CFR Internet website at www.access.gpo.gov/ecfr/ .
c. You can view a list of all ACs on the FAA’s Regulatory and Guidance Library (RGL) at www.airweb.faa.gov/rgl . On the RGL website, click on “Advisory Circulars.” To request free ACs, contact: U.S. Department of Transportation, Subsequent Distribution Office, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20785. To add your name to FAA’s mailing list for free ACs, contact: U.S. Department of Transportation, Distribution Requirements Section, SVC-121.21, Washington, D.C. 20590.
/s/ David W. Hempe David W. Hempe Manager, Aircraft Engineering Division Aircraft Certification Service