manuelo...
Hardware interchangeability can be a very legal and "emotional" issue.
1. When an aircraft is designed, tested, certified, etc... the configuration is "frozen" for production. This includes fasteners and common hardware, too. Deviations from this configuration [IE: hardware substitution] should NEVER be taken lightly, since the price for failure can be really high.
2. Often, the company part-number hardware [IE: BACB30**, ST3M**, etc] is physically IDENTICAL to a common NAS***, HL, HT, etc P/N... however inspection, testing, packaging, marking, etc... and documentation criteria are usually more rigorous and costly... and is intended to ensure the part is "fully functional as advertised" when received.
NOTE: in some cases, such as Hi-Loks, there are cross-over-charts that relate Company PNs to the common HL PNs.
3. The reason companies get so crazy with these "simple parts" is very straightforward: "Bogus [unapproved, counterfeit, etc] parts" or under-qualified parts get into the aviation supply stream and can reek havoc. Not only is there a real potential for catastrophic failure, FAA AD/SB and warranty issues can be extremely costly [$$s, man-hours to replace, corporate reputation, etc]... along with the very real legal/moral/ethical issues when a failure occurs and lives are lost or disrupted. See FAA AC21-29C "Detecting and Reporting Suspected Unapproved Parts".
The high cost paid for genuine aerospace quality hardware is the assurance [QA/QC] to ensure that You are getting what You pay for. Bad guys figured out that producing something that looks/feels/smells like the real thing, and has genuine looking paper associated with it, can be a gold mine. Unfortunately, even legitimate mom-and-pop shops try to make critical hardware without a full depth of knowledge can also screw-up the process... even in good faith.
NOTE. Spec hardware is notoriously easy to counterfeit and get into supply channels... while company hardware, coming thru very specific vendor channel is very difficult to counterfeit and get into supply channels.
4. This issue can become very emotional when a "perfectly good substitute" [equivalent or repair-oversize, etc], is identified that is readily available from a reputable source [vendor]... but is out-of-reach due to the technicality of "suitability".... and who has the authority to determine suitability. This is why the DER gets paid the big-bucks! The DER must understand the performance requirements, measure the proposed alternate for theoretical capability and then evaluate the proposed source [parts vendor] for faithful compliance to the standards. In many cases the proposed part looks great on paper... but comes from a questionable source with vague documentation... or worse... NO documentation.
5. NOTES.
5.1 Aircraft companies used to sell surplus aircraft hardware [shop leftovers, shop-floor sweep-up, kit residues, etc] by the pound: acft-homebuilders and small acft maintenance shops used to scoop it up for bargain prices [dirt cheap]. Lockheed-Burbank [in So-Cal] had a great surplus store with ton of hardware for sale for a couple of $s per pound in the 60's and 70's. Then came the stark/legal reality that there was NO traceability any of these parts [other than "it came from Lockheed surplus store"], which left the mechanic who uses this material as the sole individual completely/utterly responsible for the parts. Without the paper trail on-file, this leaves the mechanic wide-open to legal action. Note: even "good hardware" can become unserviceable if it dropped in a shop environment [picking up nicks/dings, contaminates, etc].
5.2 I have a 6.25-inch grip X 1.25-Dia + 0.0156-OS Protruding tension head bolt, I-718 [220-KSI], made for a very special application. New ones cost upwards of $6000. The Bolt even has the protective plastic mesh for the shank and threads and appears to be in pristine condition. However, this one was given to me when the certification documentation was lost: it is only worth scrap value of the I-718... without the documentation.
5.3 Some companies apply a performance factor to non-company [NASX] hardware... before an allowables comparison is run. IE: the projected strength of "NASX bolt" [technically equivalent to "CompanyPNX bolt"] is determined based on the CompanyPNX bolt allowables, reduced by [divided by] a safety factor of Y [Y= 1.15, =1.25, etc]. Crude, but workable, when the allowables data for NASX is not available... but the spec configurations are comparable… and the vendor/source is "reliable". This can disqualify NASX hardware outright, unless a good stress analysis is available which shows adequate margins for the hardware.
5.4 The unique problem encountered with spec fastener hardware [NASX] is that with so many vendors making NASX, each vendor evolves the NASX with a slight themes/variation in production and QA/QC. The performance of EACH vendor becomes another element of the allowables equation. When a large a cross-section of vendors submit enough test data for allowables ... then that can be factored together for minimum performance for all vendors... and NASX data can be published. What a mess.
Now, Gotta go back to work…
Regards, Wil Taylor
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