Longeron crack repair 1

[Emuman]

I have an 0.25" edge crack with it's root at the 1/8" rivet in a hat section longeron. The longeron is in the tail boom of a Bell 212. I have convinced myself that a doubler with a single row of rivets down the length of the flange is adequate because the load is distributed only along the length of the longeron. Is there a standard repair to a longeron?
I am hesitant to start putting rivets into the web to accommodate an L shaped doubler to replace missing structure if there is no perpendicular load.
Suggestions are welcomed.
Sorry I can’t draw a picture, but I tried my best with the keyboard.
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[rerig]

The hat section is just that, and not a flat doubler, because it is required to withstand many different loads.
If there is no SRM Repair, I would recommend an "L" doubler with the lip nested in the Hat Section radius.
Don't forget to run the numbers for material thickness, size & fastener size.

 

[alexeu]

Is the crack in the direction of the longeron or perpendicular to the longeron direction? Is the crack at the top or the horizontal flanges of the hat section?

By your description above, I would recommend nesting a blueprint splice hat section long enough to distribute the load back into the stringer. It does two things, basically, reduce the axial stress due to additional area and provide a secondary load path due to the crack.

If space is a concern, rerig has a very good proposal with the repair angle in the shape of an L.

It is possible that the hat section profile is used for compression loads. Therefore, an angle or hat section repair would be ideal if you don't know the loads.

AEU

 

[Sparweb]

I am familiar with this type of helicopter, so I'll tell you where I would start:

There is an SRM for the helicopter. I think I've seen these longerons treated there. There are also a series of Technical Bulletins published by Bell regarding these longerons that stretch back to the 204B days. Reading them will increase your understanding of the problems associated with these longerons.

Where is the crack, lengthwise on the tail boom? If you are close to the barrel nut fittings, these 1/8" rivets should be considered "heavily loaded".
Which longeron is sporting the crack? Different longerons are loaded differently by the Tail Rotor and the weight of the boom, plus elevator loads. Bear this in mind.
Does the crack stop at the rivet? If so, you've only lost 0.25"*t of material. You may well get away with a flat strap over the flange of the hat section. Using an "L" section isn't necessarily better - this would require blind rivets, and my experience is that these just shake apart on helis. You're dealing with a fatigue problem - blind rivets are rarely much help in these situations.


Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[rerig]

Clarification about the "L" Doubler.
The "L" bend is for strength. There are no fasteners through the Standing Leg of the "L".

Unless the SRM allows a Flat Strap, most DER's won't allow one. Our DER, who is the Eng. Mgr. for a French Helicopter Co., will not allow a flat strap repair. The Doubler for a hat section flange must have a "stiffening leg". So we put a non-fastened, 1/4" break formed lip on the doubler.

 

[Emuman]

SparWeb thanks: The service bulletin, which deals with the longeron cracks, only applies to cracks in the actual splice. Unfortunately this crack is the next rivet forward of the splice. The longeron is the upper starboard and looks to be the most loaded given the moment arm of the tail rotor.
The crack emanates outward from the rivet hole and has split the outside flange.
The aforementioned service bulletin is installed on the aircraft, but clearly isn't enough to do the job. The repair I am considering now is a tapered doubler to better distribute the stresses from the splice down the longeron. Hopefully this takes care of the cracking and lowers the stress levels to help fatigue issues.

 

[Sparweb]

Both upper longerons have problems, but I remember working out once that the left one gets it worse than the right one. You might want to check the left one extra carefully, too, just in case.

Anyhow, the crack is just forward of the splice. I can think of a host of engineers that wouldn't have expected the crack to start there. While you have the splice apart, you could also splice in a new piece of longeron further forward to eliminate the cracked part. Were the old splices bonded? I can't recall off-hand if they were supposed to be.

If you think buckling has played a role in this failure, perhaps the "L" will be helpful, too.

Steven Fahey, CET
"Simplicate, and add more lightness" - Bill Stout

 

[CRACKKILZ]

Why not trim out the crack and just remake the splice, same as original, but extend it forward of the crack to pick up good material?

In any case, strap repairs are generally not considered ideal for compression type loading. In order to size your strap for inter-rivet buckling, usually it ends up being so thick that a bad fatigue detail is created.