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FAA notification on fasteners 3
- Thread starter berkshire
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MikeHalloran
Mechanical
Yeah, just take your airplanes apart and replace the nuts, after you test the nuts in your inventory, and order and test some replacements, and hope you end up with a few good ones.
Mike Halloran
Pembroke Pines, FL, USA
Mike Halloran
Pembroke Pines, FL, USA
GrandpaDave
Aerospace
Better start with all those DC-3s out there in Flying-World.
Must be a bunch of kids running the FAA!![[pipe]](/data/assets/smilies/pipe.gif "[pipe] [pipe]")
Must be a bunch of kids running the FAA!
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- #6
RB
It was something that Eng tips did when the link was posted.
The original SAIB was HQ 14-16 and was applicable as follows
HQ-14-16 - Small Airplane/ All/ All Models
HQ-14-16 - Large Airplane/ All/ All Models
HQ-14-16 - Engine/ All/ All Models
HQ-14-16 - Rotorcraft/ All/ All Models
HQ-14-16 - Small/Large Airplane/ All/ All Models
It was because of the broad coverage of this Special Airworthiness Information Bulletin that I posted it up here, I know that not all of you are privy to the FAA airworthiness reports, and airworthiness directives.
B.E.
You are judged not by what you know, but by what you can do.
It was something that Eng tips did when the link was posted.
The original SAIB was HQ 14-16 and was applicable as follows
HQ-14-16 - Small Airplane/ All/ All Models
HQ-14-16 - Large Airplane/ All/ All Models
HQ-14-16 - Engine/ All/ All Models
HQ-14-16 - Rotorcraft/ All/ All Models
HQ-14-16 - Small/Large Airplane/ All/ All Models
It was because of the broad coverage of this Special Airworthiness Information Bulletin that I posted it up here, I know that not all of you are privy to the FAA airworthiness reports, and airworthiness directives.
B.E.
You are judged not by what you know, but by what you can do.
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One of the fasteners covered by the SAIB are MS21042 locknuts. This is a cad plated alloy steel locknut that is not used for new design much anymore due to the cad plating. But when the use of cad plated fasteners was still acceptable, this was probably the most widely used locknut in the US aerospace industry. MS21042 locknuts are also widely used outside of the aircraft industry for race cars, etc. You can readily purchase what appear to be MS21042 locknuts from commercial fastener suppliers at a relatively low cost, but they often come with no certs or traceability. Due to the locknut's wide usage outside of the aircraft industry it's easy to see how non-conforming parts could occasionally end up installed on an aircraft being repaired/serviced. Due to the incredibly large number of these locknuts in service and inventory, this would appear to present a huge problem.
The SAIB describes the problem as being fracture failures in the locknuts due in part to hydrogen embrittlement. The NASM21042 spec requires hydrogen embrittlement relief after plating, so this should only be a problem with locknuts that were not processed correctly. However, the SAIB also recommends checking the prevailing torque of new locknuts at installation to verify there is not excessive interference in the deformed threads of the locking feature that would cause fracture failure in the locknut body. I have personally seen the condition numerous times where the prevailing torque of a few parts in a lot of brand-new conforming MS21043 locknuts was way above the range prescribed in the procurement spec. The problem is worse with smaller thread sizes from 1/4-28 to 6-32. Everywhere I have worked as a design engineer, it is established practice to verify the prevailing torque of deformed thread locknuts (like MS21042) is within spec at each installation. In my own experience, it is common to discard around 5% of brand-new 1/4-28 or 10-32 MS21043 locknuts for excessive prevailing torque, and with brand-new 6-32 MS21043 locknuts the discard rate for excessive running torque is often 25% or more.
Due to the way large batches of deformed thread locknuts are acceptance tested, and the difficulty in controlling the prevailing torque with small thread diameters, there will always be a certain percentage of these new locknuts that have excessive running torque. The best way for the FAA to deal with this issue would be to make sure that all guidelines for design, manufacturing and maintenance require the prevailing torque range of all self-locking threaded fasteners to be published and measured at each installation.
Interesting topic.
The SAIB describes the problem as being fracture failures in the locknuts due in part to hydrogen embrittlement. The NASM21042 spec requires hydrogen embrittlement relief after plating, so this should only be a problem with locknuts that were not processed correctly. However, the SAIB also recommends checking the prevailing torque of new locknuts at installation to verify there is not excessive interference in the deformed threads of the locking feature that would cause fracture failure in the locknut body. I have personally seen the condition numerous times where the prevailing torque of a few parts in a lot of brand-new conforming MS21043 locknuts was way above the range prescribed in the procurement spec. The problem is worse with smaller thread sizes from 1/4-28 to 6-32. Everywhere I have worked as a design engineer, it is established practice to verify the prevailing torque of deformed thread locknuts (like MS21042) is within spec at each installation. In my own experience, it is common to discard around 5% of brand-new 1/4-28 or 10-32 MS21043 locknuts for excessive prevailing torque, and with brand-new 6-32 MS21043 locknuts the discard rate for excessive running torque is often 25% or more.
Due to the way large batches of deformed thread locknuts are acceptance tested, and the difficulty in controlling the prevailing torque with small thread diameters, there will always be a certain percentage of these new locknuts that have excessive running torque. The best way for the FAA to deal with this issue would be to make sure that all guidelines for design, manufacturing and maintenance require the prevailing torque range of all self-locking threaded fasteners to be published and measured at each installation.
Interesting topic.
Definitely an interesting topic. The specification allows 49 HRC, and when coupled with electroplating, really is not best practice. Even with de-embrittlement heating/baking, there is risk for hydrogen embrittlement. If I were involved in manufacturing or use of these parts, I would make sure there was regular testing such as ASTM F519, F1624, etc.
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- #9
This whole thing will give " Goods inwards" inspection departments at many companies, and repair stations the heeby jeebies for many years to come. ( What if I passed off a bad batch as good? )
B.E.
You are judged not by what you know, but by what you can do.
B.E.
You are judged not by what you know, but by what you can do.
Here are some links to specs that describe the acceptance testing guidelines for metal locknuts like MS21042. Table 1 of MIL-DTL-45913D shows how wide the prevailing torque value of locknuts can vary, either when new or after just a few installation/removal cycles.
I've also attached a table listing the sample sizes for acceptance of large batches of metal locknuts. For example, a batch of up to 10,000 locknuts would only require a sample size of 5 pieces for acceptance.
I've also attached a table listing the sample sizes for acceptance of large batches of metal locknuts. For example, a batch of up to 10,000 locknuts would only require a sample size of 5 pieces for acceptance.
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