fuselage tubes types and size?

[groundloop]

Hi all
Does any one know if the 7ac share the same fuselage as the l-16(a or b)
I have a copy of a military hand book that list the sizes and i am wanting to know if these are correct for my 46 65 hp champ.
thanks
Chris

[s2d]

chris

since nobody will answer you, I'm going to go out on a limb and say they probably are.I have the same manual and looking at it, I can't see anything that would be different on the basic frame. Would be interesting if someone had all the tube layout for the 7AC so we could compare. I was going to post a question about use of 1025 tubing use in the fuselage, but see from this manual that quite a bit was used.

Does anybody have any documentation from the FAA stating that it is permissible to use 4130 in place of 1025? I know it is done as SOP, but has it ever been specifically approved? Section 4-4 in this manual says to replace damaged tubing with 4130 steel tubing, but this is a military manual not an FAA manual.

[Steve]

AC43-13A All repairs shall be as strong or stronger than the original. In terms of mat'l. 4130 is a approved and is a direct replacement for 1025. Steve

[Lowell Sterchi]

I looked at my manuals, 7AC & L-16's. Only thing different is the L-16B
has added fittings for the hyd. brakes (7CCM). The L-16 is the 7BCM, without the greenhouse. As memtioned earlyer, 4130 steel tubing is a
replacement for 1000 series tubing. If you check closely at the tubing,
you will see all the high strength areas use 4130 tubing, it will give you
an ideal of the loads in that area.

[s2d]

AC43-13A All repairs shall be as strong or stronger than the original. In terms of mat'l. 4130 is a approved and is a direct replacement for 1025. Steve

AC 43.13-1B Section 4. METAL REPAIR PROCEDURES

4-50 b. - - - - - - The Repair must be equal to the original structure, but not stronger or stiffer, which will cause stress concentrations or alter the resonant frequency of the structure.

I Know 4130 is the standard replacement for 1025, but where specifically is data attesting to such.

When an FAA guy walks up to your project and asks where you got the data to do what you are doing, what can you show him?

[jerrymjr]

Its details like this that have killed the One Time Field Approval because the mod has to go to an Aircraft Certification Office. What it will come down to is, what will the FSDO guy think. Does he feel its fine and lets it go under 43-13B or feels an engineer needs to look at it (or have DER approval). Here, we stick in 4130 on the drawings without hesitation for tube frames. This is different when you have to bend the stuff, hehe.

Really, the 43-13 guideline just wants you to use your head a little. In an integrated all tube frame structure, like 7-450, swapping to 4130 will do very little to affect the resonant frequency in any adverse way. Back then, it was a huge cost advantage to use 1008-1025 carbon steels. Now days, the weak stuff costs just as much.

Basically, the snag of that statement is to prevent some of the overkill repairs from making problems elsewhere. Like riviting on a huge doubler and causing fatigue issues where the doubler stops.

[s2d]

I fully inderstand what you are saying Jerry, however in talking to my current FSDO rep is is pretty obvious and (he even admits) that common sense is slowly going out the window. If it isn't in black and white they won't let you do it. Hopefully it won't get that bad on the tubing issue but I wouldn't bet on it.

[jerrymjr]

Hmm. If thats how he feels, then only a DER or having it passed on to the ACO will get the job done. If you want it done with any speed, obviously the former is better (though not free like the latter). Usually the latter will kick it back to you for substantiation, and you might be able to demonstrate that yourself without the DER. I don't know that this would need a compliance checklist; its kinda simple.

Either way, I would start drawing up what you want to do. For either method of approval, you will need a drawing with a unique number of some sort. They need data to approve as they cant just approve an airplane. For one-time mods, you don't need a high quality print....back of a clean napkin!!

Some of you already know this, of course.