Flush skin repair over obstructed area? 1

[Rob130]

I am a structural mechanic with 30 years military and civilian experience. Presently I teach structural repair courses for the military. I have come upon some conflicting repair designs for two different types of aircraft.

It involves pressurized flush skin repair over an obstructed area.

Assuming a damge cut out 4 inches wide and 2.5 inches high in 2024 T3 .050.

The structural frame is a Z channel 7075 T6 .063 with 1 inch flanges and a 3 inch web section.

The skin cut out is centered on the rivet row.

This repair obviously requires a two piece doubler. One doubler would be joggled to tie into the Z channel flange and rivet row.

The conflict comes on how the second doubler would be attached on the web side of the Z. One SRM requires the second doubler to have a flange bent to tie into the Z web. This makes sense to me because the doubler now bridges the surrounding skin and the support structure of the Z.

The other SRM shows the second doubler flat to the skin and does not tie in to the Z web in any manner. The doubler just butts up to the Z channel. This does not seem satisfatory to me. Wouldn't the skin cut out be unsupported in the 2.5 inch dimension?

I spend alot of time reviewing the post in this forum. I incorporate pertinent (read understandable by mech's) into the course. I feel it is very important for structural mechanics to have a basic understanding of the engineering principles that go into proper structural repairs.

Thanks for your time, Rob130

 

[rerig]

Rob130,

Our Repair Station DER requires a flanged tie-in on all repair doublers in the area of structure. So I require it on all repairs...even SRM repairs not showing it.

We do some C-17 repairs and a good 50% of the repairs to be accomplished are due to poorly designed/installed temporary repairs or removal damage. (maybe a good training topic)

ie. 1) Thrust Reverser Aft Fairing. We are replacing the Fwd Spar because it was damaged by prying the fairing off.
0.080" thick flange bent up to 0.130" out of fair in 6 locations. (OEM gooped sealant all over the fairing on installation. I suppose they couldn't break it loose)

2) Flap TE Wedge (composite) has a Titanium Temporay repair installed with composi-loks. The holes for these fasteners were drilled into the standing leg of the internal ribs. Skin removal and rib repairs to correct a disbond.

Not complaining...it keeps me busy.
20 years Military, 15 Years Commercial

Teach them well. We hire them when the Military is through with them.

Rerig.

 

[debarra]

hi Rob
Perhaps a little drawing could be more efective.
Could you? cut the upper flange of frame, an a portion of web frame to restore the skin integrity.After that you have to manufacture an angle to join frame and skin.

 

[Rob130]

Thanks for the follow up folks. I have scanned the repair drawing to a jpeg file. If you want to take a look at it please email me rjjackson130@aol.com and I will send the file for review.

Thanks in advance, Rob

 

[HSThompson]

Rob,

I received your sketch. I and other fellow DER's reviewed both of the concepts you mentioned. We agree with you and disagree. Essentially both repair methods are not acceptable. In the no joggle case you are leaving the skin patch unsupported exposing it to differential bending along the stiffener and the doubler. In the second, while the patch will be effective it does not accept the full load as the original design. Therefore more load is force around the cut-out which is unsubstantiated for the added load. (Assuming you do not know the original margins).

A more acceptable repair is to cut the stiffener and double over the entire cut-out, then splice back the original stiffener.

HST

 

[jetmaker]

Rob,

Thank you for the sketch. After reviewing the proposed repair, I would say that neither option is a very good one... at least as a permanent repair. My feeling is that the 2 piece double really can not carry the shear load across the cutout and is relying upon the stiffener and the filler piece to transfer loads. The stiffener was not originally designed for these loads, and so you are now significatly altering the loadpath.

HST's method would be better, or even still, assuming the repair doubler is not too thick, using a one piece doubler and then taper-shimming the stiffener at 200:1 to step over the doubler. Of course, that has access limitations, but keeps as much original structure and loadpath.

Regards,
jetmaker