Restricted Category Type Certificate Holder Responsibilities

FAA Aviation Safety SPECIAL AIRWORTHINESS INFORMATION BULLETIN SUBJ: Restricted Category Type Certificate Holder Responsibilities SAIB: HQ-14-16R1 Date: December 10, 2025 This is information only. Recommendations aren’t mandatory.

Introduction This revised Special Airworthiness Information Bulletin (SAIB) informs the aviation industry of potential non-conforming Military Specifications (MS), Army-Navy Standards (AN), and National Aerospace Standards (NAS) fasteners. The SAIB applies to manufacturers, owners, operators, and maintenance personnel of airplanes, rotorcraft, engines, and appliances. This SAIB also asks the aviation industry to report fastener failures and other non-conformities. This bulletin focuses on MS 21042 nuts due to their wide usage in aviation, but is relevant to all other standard fasteners.

The FAA is revising this SAIB to update the email for reporting cracked or deficient MS 21042 nuts and other standard fasteners and also updates the FAA contact information.

At this time, the airworthiness concern is not an unsafe condition and does not warrant an airworthiness directive (AD) action under Title 14 of the Code of Federal Regulations (14 CFR) part 39. However, a significant number of past ADs noted possible unsafe conditions caused by non-conforming fasteners.

Background EASA SIB 2012-06R2 references reports of non-conforming standard fasteners, notably MS 21042 and NAS 1291 nuts and NAS 625 bolts. Various manufacturers have produced sporadic lots of fasteners with non-conformities since 2008. The MS 21042 nuts had the preponderance of these non-conformances. Specifically, the -3 through -6 sizes of MS 21042 nuts cracked in service and were discovered during inspections or were mentioned in accident and incident investigation reports. These nonconforming nuts had evidence of hydrogen embrittlement or other latent manufacturing defects. However, the defects in these nuts are emblematic of potential flaws in other standard hardware, as seen in similarly cracked NAS 1291 nuts and NAS 625 bolts.

The management of many MS, AN and NAS standards migrated from U.S military and government to various civil standards organizations since the mid-1990s. Unlike the military, these civil standards organizations do not certify, monitor compliance or perform surveillance of MS, AN, and NAS fasteners or their respective manufacturers. Conformance with these standards and specifications lies with their respective manufacturers.

Recommendations Take appropriate measures to find and remove any non-conforming MS 21042 nuts from service and stores. Various aircraft, engines, and appliances have MS 21042 nuts throughout their approved designs. The nut’s significance to aviation safety varies according to their installed applications. These applications range from joining structures to connecting mechanical linkages and flight controls. The FAA recommends a visual inspection of nuts installed in safety significant applications at the next opportunity. Look for surface irregularities such as gouges, cracks, etc. Use 10X magnification when possible.

In addition, the FAA recommends that you subject all incoming lots of new self-locking nuts to a torque check per Table 1 below. Inspect for cracks in the wrenching surfaces. Consider other inspection enhancement aids like 10X magnification and dry penetrant of comparable sample sizes for new lots of these nuts.

Table 1 - Torque Procedures

Install the nuts on the appropriate bolts, with spacers as required, and keep them torqued (see relevant torque values below) for 1 week. For statistical reasons, test a quantity of 1% (round up) or 20 nuts of one manufactured batch – whichever is less. After the test period, visually inspect the nuts for cracks on the bearing and wrenching surfaces. Size Dash No. Thread Wrenching Torque Test Value

Wrenching Torque Test Value for steel A286 (NAS1291C) In-lb Nm In-lb Nm -02 .0860-56 UNJC-3B 5 0.6 3 0.4 -04 .1120-40 UNJC-3B 10 1.1 7 0.8 -06 .1380-32 UNJC-3B 20 2.3 15 1.6 -08 .1640-32 UNJC-3B 30 3.4 20 2.4 -3 .1900-32 UNJF-3B 60 6.8 40 4.7 -4 .2500-28 UNJF-3B 150 17 105 12 -5 .3125-24 UNJF-3B 330 37 230 26 -6 .3750-24 UNJF-3B 530 60 370 42 -7 .4375-20 UNJF-3B 825 95 575 65 -8 .5000-20 UNJF-3B 1125 125 780 85 -9 .5625-18 UNJF-3B 1550 175 1075 120 -10 .6250-18 UNJF-3B 2000 225 1390 155 *Use a calibrated torque wrench

Recommend aircraft and appliance manufacturers, aircraft owners, operators, and maintenance personnel review the rigor of their incoming inspection procedures and overall quality control processes to capture non-conforming fasteners before their introduction into the quality system.

Report MS 21042 nuts and other standard fasteners found cracked or deficient during these recommended inspections via email to [email protected] with “Standard Fasteners” in the subject line. Include the hardware part number, manufacturer, and or a description of markings (picture will be helpful), lot number, location on the product, and time since installation. If possible, attach jpg photographs of the fastener makings and its flaws. Under the provisions of the Paperwork Reduction Act (44 U.S.C. 3501 et seq.), the Office of Management and Budget (OMB) has approved the information collection contained in this SAIB, and assigned OMB Control Number 2120-0731.

For Further Information Contact Matthew Bryant, Aviation Safety Engineer, FAA, 26805 East 68th Avenue, Denver, CO 80249; phone: (303) 342-1092; email: [email protected].