Taper lock / taper pin fastener in cast metals

[FREDERICK E MAUPERTUIS]

Regarding taper lok / taper pin type fasteners in aerospace applications;
Can anyone confirm it would not be prudent to use such fasteners thru investment castings made of aluminum and or cast iron materials loaded predominantly in shear.
In particular, tubular shapes of relatively thin wall thicknesses. Effecting a single precision tapered / reamed hole in such material and specific configuration, could not / would not maintain the controlled interference fit expected / required over time. In this case, stress corrosion fatigue and eventual fracture of an investment cast aluminum flight yoke integral stub boss secured to a cres steel tube via a single taper pin and nut. Cres tube slip fits inside the flight control yoke stub boss. The fractured off stub exhibits an axial stress corrosion crack (grain dark gray to black in color) emanating from the I.D. of the boss
on one side of the radial fracture terminated by overload brittle fracture ( grain light gray color) on the other side.
Dissimilar metal corrosion
is also suspected as the taper pin and it's nut remained attached to the cres tube and exhibited loss of cad plating thru out most of its mating surfaces. Additionally, it seems re-torquing the taper pin nut over many years so as to maintain interference fit and thereby eliminate cumulative "play" would tend to expand the stub end cast metal beyond its Fcy property. I could not find any guidance / standards in any publications so far, including Boeing's taper shank fasteners in aluminum structures, documemt# 6m54-153, McDonnell Douglas process standards (DPS), Michael Niu's publications, NASA. Thank you.

 

[MintJulep]

Seems like you've answered your own question.

I could not find any guidance / standards in any publications

If there's no guidance on how to do something maybe that's because you shouldn't be doing that something.

 

[FREDERICK E MAUPERTUIS]

That was the design implemented by Mooney aircraft then (circa 45+ years ago).
This may not be best design / practice today, thereby the question.
In this case, the cast Al yoke fractured off in the pilot's hands but cannot confirm whilst in flight or on the ground.

 

[SWComposites]

Is this related to a lawsuit? What is your role?

Sounds like a poor design made worse by poor maintenance.

 

[FREDERICK E MAUPERTUIS]

Attached pictures document the condition.

 

[MintJulep]

This?

https://mooneyspace.com/topic/38756-unusual-m20j-incident-yoke-comes-off-in-flight/

https://www.faasafety.gov/files/notices/2024/Sep/2024-09-25_Mooney_Control_Wheel.pdf

 

[FREDERICK E MAUPERTUIS]

At this juncture, i am not aware of any lawsuit(s) so far.

role: safety of flight.

Please see attachment in my previous.

 

[FREDERICK E MAUPERTUIS]

Appears to be headless taper pin AN386-x-xx (B&S taper #1)

 

[SWComposites]

Seems like a casting issue, not a fastener issue.

 

[FREDERICK E MAUPERTUIS]

Do you maybe suspect a bad heat treat?
Agree, but in this case, it is the combination thereof that makes it suspect particularly over time. This pin is headless and thereby the once controlled interference fit durability cannot/will not be maintained. As relative wear/play develops between yoke/tube join, owner(s)/operator(s) and or maintenance entities keep re-torquing the securing nut so as to attempt to restore the expected interference (without limit, eg; headless pin) until in this case the softer material (yoke) yields; Fcy: 16ksi, assuming m/f 356T6 cast Al, ref MIL-HBDK 5 / MMPDS hdbk, VOL 1, page 4-367, para. 3.9.4, titled 356 alloy.

It is note worthy AN386 is today inactive for new design and available as replacement only, ref. AN386 spec, Rev3, notice 2, dated 16JUN97.

As a proposed solution, a straight shank, shear head HI-LOK or shear head TAPER-LOK would be i think a more judicious fastener selection.

But again, what is the guidance / standard(s) / best practices for HI-LOK and or taper shank fasteners application in or thru relatively thin cast metals?

 

[SWComposites]

Unless a solid insert is installed in the tube, there is no way to get fastener clamp-up. And without that the joint will wear and loosen over time. Leading to poor fatigue performance.

I wouldn’t sign off on that design with any fastener. Needs a complete redesign IMO.

I don’t think you are going to find any guidance standards.

 

[MintJulep]

Is this a set screw?

That's a crap design. I agree with SWComposites that a complete redesign is the best option.

Something like this hollow shaft expanding collet might be part of the solution, but I imagine it would be hard to certify friction only. Somehow need to work in a metal-to-metal load path.

 

[3DDave]

I think a steel collar that is interference fit around the aluminum would suffice. I've done a similar repair to a number of plastic knobs that have failed in similar ways. Sometimes close fit brass tube; sometimes just winding a few dozen wraps of thread and some adhesive to provide a higher tensile strength support.
---

Looks like a setscrew for certain. Images at

https://baspartsales.com/917010-507-mooney-m20g-control-wheel-rh/

Putting a collar on that with a custom nut to grab the taper would be an easy repair. Add a clearance hole for access to the setscrew. A few bucks more to round the corners of the collar instead of squaring it off or chamfering would keep with the rest of the look; paint to match.

 

[FREDERICK E MAUPERTUIS]

Gentlemen, thank you both for your feedback.

Mint Julep; it does indeed appear to be a set screw that may induce additional radial stresses in the cast aluminum element of this assembly, leading to eventual fatigue, although i cannot confirm.
Let alone certifying friction only, an expanding friction collet design is certainly an option, altho this design would introduce fabrication constraints as well as costs which is frowned upon by most general aviation "bug smasher" owners ( most do not belong owning an aircraft, IMO).

SW composites; understand, i think, you mean an interference fit (+.xxxx" -0" from nominal tube ID) or slip fit (metal: steel or Al) plug that would nest inside the tube I.D over x" lenght, drilled and reamed straight (HI-LOK) or tapered (TAPER-LOK or AN386-x-xxx) thru cast yoke machined stub wall / CRES tube wall / plug / CRES tube wall / yoke stub wall as a stack up, provided acceptable E.D thru out, so as to effect a lasting clamp up / constant controlled interference fit over a longer if not "infinite" time period.

Your suggestion, is by far the most economical solution so far. Please confirm my understanding above when you can. Thank you.

brgds maupertuis

 

[FREDERICK E MAUPERTUIS]

3DDAVE;

/A/ winding an adhesive saturated thread of any sort/material as a repair in this application is a non starter from an aircraft engineering and or FAA certification/approval stand point. In particular, flight control systems !
However, i have used "close fit" brass tubing in repairing damaged knob(s) on flight simulator consoles and or electronic test instruments/equipment, at least temporarily and provided not flight rated.

/B/ The cast aluminum yoke, integral stub would require machining of its O.D. not to include significant set up, so as to provide the pressed or shrunk fit interference expected thru out the faying surfaces, resulting in the stub thinner wall thickness and potentially having to further accommodate the flaring out or smoothing radius baked on finishes. This would/will result in a loss of rated strenght and therefore no longer within its approved design characteristics, particularly if beyond its manufacturing drawing tolerances. A pressed on steel collar onto the cast aluminum stub would create a dissimilar metal condition or galvanic couple that would lead to eventual and significant corrosion unless the steel collar is CAD plated thru out and polysulfide (eg: PR1422)sealant sealed on assembly. Additionally,"custom" fabricated/manufactured hardware such as nut, bolt, screw, pin, washer, solid and blind rivet, etc are not acceptable and forbidden on type certificated aerospace vehicles and would not be wise on experimentals as well. The taper pin holes thru the stack up including collar if such, would have to be taper reamed further so as to allow for the required AN320-xxx or AN364-xxx nut or MS, NAS, AS equivalent hardware and taper pin minimum rolled thread protrusion, ref. AN386 spec sheet.

 

[3DDave]

OK. Do what you want and it will still break.

There is a fundamental design problem that won't be addressed with some other pin.

 

[3DDave]

It appears not only the yoke might be a problem.

 

[3DDave]

See

https://mooneyspace.com/topic/49586-airworthiness-concern-letter-9242024-on-legacy-control-yoke/

for a more complete discussion than the OP has suggested on the particular matter of this problem.

On the topic of a replacement yoke:

Somebody doing the paperwork. Why don't you start it? You might get some help from ACS if they can sell a bunch of yokes. I can't see the FAA having an issue with this STC. The engineering would be easy.

There would need to be a PMA also.

You may be able to do it without an STC if you can get a PMA for an equivalent for the Mooney part number. I would call the nice lady at the MIDO and ask her what she thinks.

 

[3DDave]

AD 77-17-04 appears to be a possible cause for the problem as the tapered pin and set screw get messed with every time.

Good job FAA. Don't require a fix for a fundamental design problem, just spend time and money hoping that you catch the failure before it becomes a problem.

See

https://mooneyspace.com/topic/40173-updating-the-yokes/

for more about the AD

 

[MintJulep]

See photo 5 for the evilness of the setscrew.

https://www.ebay.com/itm/2353775510...eXyEAFA19q5efs2kzcIPKj4p6W3v8hWgaAq9kEALw_wcB