Rivet conundrum: are CherryMax rivets legal for a wingtip repair without specific authorization?

[quietpilot]

I'll preface by saying I have left a message with Mooney Technical Support, so I am not entirely crowdsourcing this information, but curious if anyone here has strong opinions or applicable knowledge.

I'm very close to buying a 1974 F model. On the PPI my mechanic noted the use of blind rivets, of the CherryMax varietal, that were used recently for a wingtip repair (due to hangar rash). His opinion is that in order to use blind rivets for that repair one would need to obtain specific authorization via form 8100, or at least have it documented as a viable part in either an IPC or a DER. 

The seller, who happens to be a Mooney Service Center mechanic, says that CherryMax rivets are perfectly legal because they are "structural rivets" and the wingtip is a "standard repair." 

Something I have been referencing with very limited understanding is AC 43.13-1B page 159...

f. Blind rivets are used under certain conditions when there is access to only one side of the structure. Typically, the locking characteristics of a blind rivet are not as good as a driven rivet. Therefore, blind rivets are usually not used when driven rivets can be installed.

and then...

CAUTION: For metal repairs to the airframe, the use of blind rivets must be specifically authorized by the airframe manufacturer or approved by a representative of the FAA.

Thanks for reading!

[Austintatious]

Is your mechanic truly wise in the way of Mooney?   IDK about an F... but my 2 M20K's have blind rivets holding the wingtips on... no way to put them on otherwise from what I could tell... The holes are filled and painted over.... Furthermore, every single one of my inspection plates on the underside of the wing is held on with blind rivets.

 

Here is a pic of another wingtip, not mine

 

Edit, found a photo of the F wingtip and it has solid rivets... Guess it is time to open the wing or get and authorization.

Edited August 24, 2023 by Austintatious

[EricJ]

My airplane has blind rivets all over the control surfaces and other structural areas from the factory.    I'd look in the IPC and see what is specified, as it may be cherry max or some similar blind rivet.

[quietpilot]

25 minutes ago, Austintatious said:

no way to put them on otherwise from what I could tell...

Edit, found a photo of the F wingtip and it has solid rivets... Guess it is time to open the wing or get and authorization.

oh boy! that is the way I am leaning right now. 

And I wonder if at the time of manufacturing they have more access and are able to use solid rivets more easily? seems likely

[quietpilot]

19 minutes ago, EricJ said:

My airplane has blind rivets all over the control surfaces and other structural areas from the factory.    I'd look in the IPC and see what is specified, as it may be cherry max or some similar blind rivet.

FSDO also pointed me in direction of the IPC. Time to hunt that down and sift through it. Thanks Eric!

[KSMooniac]

There is general guidance that is good and useful, and sometimes there is general guidance that isn't helpful at all.  "Blind" rivets can mean everything from pot-metal pop rivets all the way up to heavy structural steel blind fasteners, so you need to use some discernment here.  Cherrymax rivets are generally considered an acceptable substitute for driven structural aluminum rivets.  They're not used everywhere because they're heavier and more expensive than traditional driven aluminum rivets.  Aluminum pop rivets are not acceptable for structural use, generally, but you'll see them all over Mooneys holding fairings or inspection panels.  

Is your repair in question at the wingtip to wing joint, or did the repair involve metal wing skin?  

FWIW, a much better way to attach a Mooney wing tip is to BOND Click Bond nutplates to the backside of the fiberglass attach flange, and then use countersunk machine screws through the dimpled wing skin to attach them... now they're removable without drilling rivets.

[Hank]

The fiberglass wingtips aren't structural, and were developed for the J. Earlier models like this F and my own C had the cosmetic wingtips added under STC. Check the instructions for how to apply them. 

I think it was a LASAR STC.

[A64Pilot]

I was under the understanding that an actual Cheery Max as in blind rivet with a steel stem and a locking ring to retain the stem was actually stronger than a driven aluminum rivet and therefore acceptable to replace any solid aluminum rivet.

From memory as I’m not a sheet metal man but there are Cheery lock rivets too, that have aluminum stems and therefore don’t have as much strength as a Cheery Max.

Another reason why Cheery max rivets aren’t always used is the are a Bitch (excuse my language please) to remove, it’s getting that lock ring out that’s difficult. I grind the thing out with a cutting disk in a die grinder myself, once it’s out then you can drill the rivet.

‘Now I’m sure if the MM for some reason says only solids are allowed, well then only solids are allowed, but I’ve not seen that.

Me personally if I were doing it and could I’d use nut plates and screws, as was said they aren’t structural and in fact other than looking cool, and I guess giving a place to mount lights they don’t do anything.

[Hector]

Cherrymax rivets are structural rivets and can be suitable replacement for solid rivets.


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[Alan Fox]

Cherry rivets are an acceptable fastener ,  I believe they come from the factory with 3 or 4 on the end rib..