Issue 02 - Defective standard aircraft hardware - Self-locking nuts - MS21042 and NAS1291 - Civil Aviation Safety Alerts (CASA) No. 2013-04


Owners, operators, maintainers and manufacturers of aircraft

Issuing Office: National Aircraft Certification Branch
No.: CASA 2013-04
File Classification No.: Z 5000-35
Issue No.: 02
RDIMS No.: 9713897
Effective Date: 2015-05-25


The purpose of this Civil Aviation Safety Alert (CASA) is to advise the aviation community of the potential hazard resulting from the possible failure of MS21042 and NAS1291 series nuts.

The reason for the re-issue of this CASA is to correct an error of identification for lot 18261, mistakenly identified as lot 18621 and to identify an additional affected lot manufactured by Airfasco Industries (Canton, Ohio, USA)

This revision also addresses a recent Service Difficulty Report (SDR) event of a failed NAS1291-7 nut and introduces a recommended torque-test bench check for received and in-stock hardware.


Transport Canada Civil Aviation (TCCA) has been made aware through the Web Service Difficulty Reporting System (WSDRS) of several occurrences for cracked and discrepant MS21042-06, -4 and NAS1291-7 self-locking nuts.

Analysis of MS21042-4 failed nuts, as displayed in figure 1, concluded that in this occurrence the nuts failed due to hydrogen embrittlement which was likely introduced during the manufacturing process.

In another occurrence, a crack originating from the manufacturer’s mark and crimp, used to create the self-locking feature, was found on a MS21042-4 nut, as displayed in figure 2.

These defective MS21042-4 nuts originated from Airfasco lots 17147, 17559 and lot 19988. Similar defects were found in lot 18261 for MS21042-06 nuts.

Figure 1; A typical finding of a cracked MS21042-4 nut due to hydrogen embrittlement.

Figure 2; Top view of an unused MS21042-4 nut found from an AMO’s stock. The cracking was induced by the self-locking crimp.

A recently submitted SDR from a Canadian rotorcraft operator revealed the complete fracture of a NAS1291-7 nut, as displayed in figure 3, at the aft landing gear strut attachment of a R66 helicopter during its’ first 100hr inspection since entering into service.

Analysis of the nut revealed that it failed as a result of a delayed fracture due to hydrogen embrittlement. Robinson Helicopter Company has issued service bulletins R66 SB-12 and R44 SB-88 to address this failure scenario.

Also issued is service letter R66 SL-09, recommending the replacement of specific MS21042L and NAS1291 series nuts with a D210 series nut. Similar service letters were also issued for all R22, R44 models.

Figure 3; NAS1291-7 cracked nut found by a rotorcraft operator.

Airfasco Industries has released Airfasco “Technical Quality Notice Bulletin MS21042/NAS1291 450 F Steel” in reference for thin wall hex and lightweight hex-flange nuts.

FAA SAIB HQ-14-16 and EASA SIB 2012-06R2 are available for additional information concerning this issue of defective hardware.

TCCA concurs with the FAA and EASA which warns of the possibility that similar design nuts of other size or from other lots or other manufacturers could be affected.

Failure of self-locking nuts in certain applications could ultimately lead to a catastrophic event.

At this time, TCCA has issued AD CF-2014-38 to rectify an unsafe condition directly related to the defective hardware issue on all DHC-2 (Beaver) aircraft. TCCA will continue to monitor the Service Difficult Reports to determine if an unsafe condition exists in any type certified aeronautical products.

Additional corrective action may be considered necessary by TCCA as a result of the cracking of the aforementioned nuts.

Recommended action:

Nuts from Airfasco lots 17147 (manufacturing. date 12 Jun. 2009), 17559 (manufacturing date 12 Jun. 2009), 18261 (manufacturing date 23 Oct. 2009) and 19988 (manufacturing date 19 Oct. 2010) should be removed from service, where identification is possible.

Close scrutiny and inspection of all stocked and received MS21042 and NAS1291 nuts should be done prior to installation, regardless of their origin.

Due to the aforementioned rotorcraft nut failure event, TCCA is recommending all Robinson rotorcraft R22, R44 and R66 operators to incorporate service letters R22 SL-64, R44 SL-50 and R66 SL-09 as applicable and service bulletins R66 SB-12 and R44 SB-88.

In addition, TCCA recommends to perform a torque check test on a sample of thin walled self-locking nut lots in inventory or newly received as follows:

The nuts are to be installed on a matching bolt and torqued to the required value for a 1 week time period. A sample of 1% of the lot quantity or 20 nuts per lot, whichever is less, should be tested and inspected for cracks prior to nuts from this lot being used.

Refer to table 1 for torque values and discard all tested nuts.

Size Dash No. Thread Wrenching Torque Test Value
(in lb)
Wrenching Torque Test Value
Wrenching Torque Test Value for steel A286
Wrenching Torque Test Value for steel A286
-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

Table 1; Torque values to be used for the torque check (reference NASM21042):

If the visual inspection of the tested nuts reveals the presence of one or more cracked nuts, the complete lot is to be rejected.


TCCA also recommends all owners, operators, maintainers and manufacturers of aircraft to be vigilant when performing inspections in the areas that may use these types of self locking nuts.

Defective hardware is to be quarantined and all defect information reported to TCCA, in accordance with CAR 521 division IX mandatory Service Difficulty Reporting requirements.

Contact office:

For more information concerning this issue, contact a Transport Canada Centre; or contact Gary Voght Continuing Airworthiness, Ottawa, at 613-952-4357, facsimile 613-996-9178, or e-mail

For the electronic version of this document, please consult the following Web address

For the Director, National Aircraft Certification,


Derek Ferguson
Chief, Continuing Airworthiness

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.