If the SRM does not list an approved substitution, you need to get an FAA DER to approve it.  Some of us have some data, but I have not found a good website that lists all the propriatary data.  Some of it you can find in MIL-HNBK-5 in chapter 8.

Micah

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

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Great post Wil.  What do you say about fastener substitution for secondary structure?  I would be very concerned about replacing a 1.25" diameter bolt, but what about substituting a CherryMax in place of s solid rivet on a thrust reverser (so there is no ingestion hazzard)?  If I can show that the CherryMax can carry the same or better shear and tension load and is of the same material, and therefore has the same corrosion profile (after considering coatings), Do I still need to go to a DER?  I don't want to skirt the system, but I also don't want to spend money I don't have to.  How about non-structural parts?

-Kirby

Kirby Wilkerson

Remember, first define the problem, then solve it.

I'm a "little Airplane" guy. I also have a little side business doing pre purchase inspections & maintenance record reviews for these "little (GA) aircraft".

I will use the Cessna Single engine models for an example.

A common assue on these things, and many of them are fifty years old now, is that the seat tracks crack, and wear where the seat locking pins drop into them. There is an airworthiness directive requiring repetitive inspection, and replacement they fail that inspection. I can't tell you how many of these things I have seen using cherrymax rivets for replacement. I can understand why, as they are a pain to shoot in the field.

Problem is, the seat tracks are used to stiffen the structure longitudinally, and are considered Primary structure. Cessna will not approve the use of the blind rivets for this installation. Look at a package of cherrymax rivets these days, and Textron will tell you the same thing. Hard to break the news to a aircraft owner, that his aircraft "does not meet the requirements of the type certificate", after operating it for 20 years, in this configuration.

Quote: "Old Leroy (his mechanic) was buckin' rivets 'fore you was shittin' green, Mr Inspector!!"

Don't matter to me, I get paid either way.

Not saying you can't get a DER approval, drawings, and an 8130; and seek field approval, but these days it's unlikely. And this is on part 23 aircraft.

Kirbywan & thruthefence

1.    Every organization has a parts substitution protocol. Here is the simplified protocol I work-to for permanent substitute hardware [SUB].

The SUB must be fit-form-function interchangeable for critical dimensions. Minor variations in size, that do NOT appreciably affect installation [excessive protrusion, weight, clash with other parts, etc] is usually permissible. The SUB must be equivalent or superior to the original for strength, stiffness, fatigue strength, shock/vibration resistance, environmental resistance/isolation [corrosion, chemicals, etc], thermal exposure and materials compatibility [corrosion, galling, etc] with mating hardware/components, etc. The SUB must be "sanity/sniff-checked" by at least one other experienced engineer.

NOTE: criteria for temporary SUBs may be relaxed no critical issues are noted for the short-term [limited]  operational environment.

2.    Regarding Blind fasteners (blind rivets [BRs] and blind bolts [BBs]).

I have worked extensively on attack and fighter type aircraft and helos: blind fasteners are essential in these vehicles in primary and secondary structure. Tight clearances and limited accessibility make installing 1 and 2-piece solid fasteners impractical in many areas. In some cases, structures that is built from the center-outward may have limited/no accessibility for repair fastening [one-side access only] without [impractical] major teardown... which could be "sporty" for experienced mechanics, and may be close to impossible without special tools/equipment. In these cases I choose blind fasteners that are way-better than what was originally installed.

Blind SUBs for solid aluminum rivets MUST be have a YIELD strength equal to the ULTIMATE shear and tension strength of the solid. IF necessary, consider installing monel or A286 BR s instead of aluminum BRs with a steel or CRES pin. The fastener MUST be oversized to ensure the hole is clean/tight for the blind-sub [and damaged material at hole-wall removed].

IF tension is a critical consideration, then a Monel A286 or Inconel blind tension head rivet is required.

IF the stack-up depth and critical tension warrant it, use a steel, CRES, Titanium or Inconel tension BLIND BOLT. Most BBs in the smaller diameters have slightly larger shank diameters that force the hole to be drilled/reamed-out resulting in a tight fit.  

NOTE. Blind flush rivets now come in nominal diameters with flush shear heads [IE: NAS1097] or flush tension heads [~MS20426]. Oversized Blind rivets typically have a head closer in size to a MS20426 mounted to the OS shank which produces a shallower flush-head install... but not as shallow as NAS1097.

NOTE. Blind bolt heads and formed tails are big! There are few "shallow shear head" blind bolts on the market... and they can be hard to get [nominal or OS diameters]. Also: flush tension head blind-bolts have really deep heads [MS24693, NAS517].

CAUTION: backside [hidden-side, tail end] clearance and bearing strength must be carefully considered before installing blind rivets or bolts. The fastener head must be fully seated before "pulling-to-set" the fastener tail. In-general, the larger the hole diameter, the more back-side clearance with abutting components is required. Also, the backside material bearing strength must be able to withstand the bulbed-tail forming forces [pressure] as the fastener tail is deformed to final shape. For monel, A286, steel, titanium and inconel fastener materials, these crushing pressures can damage thin/low bearing strength materials.

WARNING. IF structure is countersunk or dimpled for a flush head, the OS blind flush fastener head [blind-rivet or blind-bolt] should be allowed to ride above flush: DO NOT deepen the csk/dimple to accommodate a deeper fastener head: allow it to rive above flush and fillet/fair-around it with sealant for aero-smoothness. If countersink is deepened to accommodate the deeper heads, then the csk may become knife-edged and result in a very poor structural installation and create future problems IF/WHEN the fasteners are replaced with OS nominal rivets [csks too-deep, costly structure replacement needed].

>>>Believe me when I say this: most young structural repair technicians will want to deepen countersinks to accommodate the deeper heads of blind-bolts [and or screws] "cause it looks right"... and bearing/tension capability go right into the toilet.

NOTE.
BR shanks typically swell slightly and will fill holes, some what: drill/ream the holes to the low end of the recommended size-range specified by tech data or the manufacturer.

BB [Huck-style and JO-Bolt style] shanks usually DO NOT swell in the hole. A good fit will result if you drill/ream the holes to the low end of the recommended size-range specified by tech data or the manufacturer. HOWEVER... I have had BBs successfully installed in transition fit holes using wet-sealant or primer as a lubricant. The resultant fit is better for fatigue/vibration resistance. IF the BB hole is cold-worked and reamed to transition fit size, then even better fatigue and vibration performance is possible.  Transition fit is [also called "net fit"] occurs when the fastener fits in the hole that is the nominal shank diameter +/-0.0010. Example: 0.2485—0.2495 fastener Dia is installed in a hole REAMED to 0.2490+/-0.0010 Dia.... resulting in a close-clearance fit or a light interference fit.

Hope this makes sense... gotta go back to work.
 

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

If you read 8900.1 Volume 4 Chapter 9, you will find that unless the manufacturere specifically approves the substitution of solid rivets with blind fasteners, this will require DER approval on an 8110-1. This came from a helicopter crash in NY back in the '80s or early '90s where an improper repair was made on the tail boom and it failed shortly after takeoff. It crashed into the East River and resulted in several fatalities.

Substitution of fasteners should never be taken lightly and there is no place in aviation maintenance for taking shortcuts.  

dgapilot... concur

In the early 1970's my dad built a Thorp T-18 with John Thorp's help and his shop tools/templates in So Cal [Burbank, '67--71]. Dad got to know/trust JT in every respect as a true/knowledgeable aero-designer... and didn't deviate from the plans without John's blessings.

Summer of 1974 we flew our T-18 N455DT to OSH for the EAA fly-in. I hopped rides in all sorts of aircraft, including other T-18s. One in particular had a control wheel instead of a stick... and had terrible control coordination forces: very light pitch forces, very heavy roll forces. Also the owner flew it waaaay too slow in the pattern: my dad was a WWII fighter pilot who respected highly loaded wings and gave ours a few extra knots close to the ground. I asked the owner about his control-wheel mod and he proudly answered that was NOT his only mod... others included flap-settings and some for weight reduction "a few pounds out here-there". I crawled out of his aircraft "good-riddance" relieved.

The next morning [late] on our way to OSH, we saw a column of smoke rising from the vicinity of the airfield. We heard that a T-18 had lost a wing in the pattern... pilot and a young girl were killed... the one "with a control wheel mod". Dad and I were badly shaken-up and miss trusted OUR acft all-the-way  back to SoCal.

Dad contacted John Thorp as soon as he could. I think JT happened to be at OSH at the same time and worked with the FAA/NTSB to find-out what had happened. In a nutshell, the owner/builder had substituted aluminum AN3--AN6 [DD] bolts for the mandatory steel AN3--AN6 [-] bolts (or tension-head Hi-Shear collared lock-bolts), all-over the aircraft. That was his "weight savings" mod. Unbelievable. That's when I realized how deadly serious my chosen profession actually was: simple miscalculations kill.

BTW... JT designed the T-18 to be assembled with a variety of flush and protruding head rivets: solid aluminum; aluminum Cherry-Lock Blinds; or monel USM Pop-Rivets [yes... pop-rivets... my dad's bird had hundreds]... except for certain critical designated areas [spars, fittings, etc] where "solid rivets" or steel bolts [or lock-bolts] were mandatory.

BTW... blind fasteners are no better or worse than solid fasteners if the installation is properly designed and properly [professionally] installed. Problem is, a LOT of engineers go shallow on details and critical thinking-planning... and then,  when techs try to do the execute installation, THEY have to assume what the engineer meant (which most will try to do).

I learned a brilliant/elegant lesson from a senior fastener engineer who stated the following:

"You deserve to get what You ask for. However if You don't ask for something, then DON'T expect to get it."

I have rarely been accused of being "shy" regarding written guidance...

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.