The most simple statement is that SAE-AS8879 has replaced MIL-S-8879.
SAE AS8879 serves the simultaneous requirements of FAA & DoD.
Basically stated, in most cases, if the drawing states MIL-S-8879
you are now to refer to SAE-AS8879 for current specification details.
There are some exceptions.
The Johnson Gage Company has developed a Use
Guide for MIL-S-8879
MIL-S-8879 Notice 3 dated 24 February
2003 states that Military Specification: MIL-S-8879C General Specification
for Screw Threads with Controlled Radius Root and Increased Minor
Diameter dated 25 July 1991, and Amendment 1 dated 2 September 1992
are hereby canceled. The MIL-S-8879 is superseded by the Society
of Automotive Engineers (SAE) Aerospace Specification number AS8879.
Threads identified as Application Category "Other Thread"
in MIL-S-8879 are identified as "Category 1" threads in
Threads identified as Application Category "Safety Critical
Thread" in MIL-S-8879 are identified as "Category 2"
threads in AS8879.
The FAA Advisory Circular AC No.:21-41A
dated 6 June 2003 informs, clarifies and guides the civil aviation
industry on using the SAE AS8879 instead of the MIL-S-8879.
The DoD Adoption Notice for SAE-AS8879
states that SAE-AS8879, "Screw Threads - UNJ Profile, Inch
Controlled Radius Root with Increased Minor Diameter", was
adopted on 14-MAY-03 for use by the Department of Defense (DoD).
Copies of SAE-AS8879 may be purchased from:
Society of Automotive Engineers Inc
400 Commonwealth Dr
Warrendale, PA 15096-0001
Tel: 877-606-7323 (Inside USA & Canada)
Tel: 724-776-4970 (Outside USA)
Order a copy online: http://www.sae.org/
For a summary of the standard as it relates to thread gages see
UNJ vs. UN.