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Thread: Old Sheet Metal Cross Reference

  1. #11

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    Aaron Novak hit the nail on the head. His answer is right on as far as the FAA is concerned. The only thing I would suggest is that AVPacer try to verify that the "3S" sheet is the original Piper-installed material and not a later field-fabricated replacement part. Sometimes the maintenance manual specifies the materials used in various parts of the structure.

  2. #12

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    If you look at AC43.13-2B, Chapter 4, Section 4 Metal Repair Procedures, 4-53 Selection of Aluminum for Replacement Parts, you see that there is a alternative allowed as "If another alloy is being considered, refer to the information on the comparative strength properties of aluminum alloys contained in MIL-HDBK-5.". So MIL-HDBK-5 gives you the properties of modern alloys.

    Then in 4-58 Repair Methods and Precautions for Aluminum Structure states in paragraph b Repairs to Aluminum Allow Members, "Make repairs to aluminum alloy members with the same material or with suitable material of higher strength.".

    So the fine print of this approved data allows for materials substitutions done with appropriate justification. You can not just willy-nilly substitute one type of aluminum for another, BUT you can substitute a higher strength aluminum for a lower strength one. That is the basis for my comment that you can use 2024T3 for most parts.

    You will note that all of the approved repair data in the paragraphs that follow my reference above refer to the use of 2024 sheet. I do not believe that is an accident.

    You are unlikely to get Piper to tell you what material was used for a flap skin. That said, there are folks out there who will sell you FAA-PMA replacement parts and you can do the repair with those parts and not worry about justifying the material.

    Best of luck,

    Wes
    N78PS
    Last edited by WLIU; 12-14-2012 at 07:38 AM.

  3. #13
    Aaron Novak's Avatar
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    Quote Originally Posted by WLIU View Post
    If you look at AC43.13-2B, Chapter 4, Section 4 Metal Repair Procedures, 4-53 Selection of Aluminum for Replacement Parts, you see that there is a alternative allowed as "If another alloy is being considered, refer to the information on the comparative strength properties of aluminum alloys contained in MIL-HDBK-5.". So MIL-HDBK-5 gives you the properties of modern alloys.

    Then in 4-58 Repair Methods and Precautions for Aluminum Structure states in paragraph b Repairs to Aluminum Allow Members, "Make repairs to aluminum alloy members with the same material or with suitable material of higher strength.".

    So the fine print of this approved data allows for materials substitutions done with appropriate justification. You can not just willy-nilly substitute one type of aluminum for another, BUT you can substitute a higher strength aluminum for a lower strength one. That is the basis for my comment that you can use 2024T3 for most parts.

    You will note that all of the approved repair data in the paragraphs that follow my reference above refer to the use of 2024 sheet. I do not believe that is an accident.

    You are unlikely to get Piper to tell you what material was used for a flap skin. That said, there are folks out there who will sell you FAA-PMA replacement parts and you can do the repair with those parts and not worry about justifying the material.

    Best of luck,

    Wes
    N78PS
    Wes,
    I believe what they are stating is basicly common sense. If you have to substitute a material, go up in strength, not down. However this as I read it does not grant the ability to change material without formal approval. The key here is that using 2024 in place of 3003 would be a deviation from the manufacturers approved design. Tensile strength alone is but a small part of the materials properties. Changes in stiffness of a control surface can lead to flutter issues and component life issues just to name a couple. In any case, if one wanted to do a material change, do your homework, get it approved and fly safely.

  4. #14
    Matt Gonitzke's Avatar
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    Quote Originally Posted by Aaron Novak View Post
    Wes,
    I believe what they are stating is basicly common sense. If you have to substitute a material, go up in strength, not down. However this as I read it does not grant the ability to change material without formal approval. The key here is that using 2024 in place of 3003 would be a deviation from the manufacturers approved design. Tensile strength alone is but a small part of the materials properties. Changes in stiffness of a control surface can lead to flutter issues and component life issues just to name a couple. In any case, if one wanted to do a material change, do your homework, get it approved and fly safely.
    Changing a control surface skin from 3003 to 2024 of the same gauge will have no effect on the stiffness of the structure, as all aluminum alloys have essentially the same elastic modulus.

  5. #15

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    Substituting for obsolete material with that of greater strength is a minor alteration.

  6. #16

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    Quote Originally Posted by Matt Gonitzke View Post
    Changing a control surface skin from 3003 to 2024 of the same gauge will have no effect on the stiffness of the structure, as all aluminum alloys have essentially the same elastic modulus.
    Exactly! 3003/5052 alloys are good for welding and compound forming, but as a tradeoff, have mediocre strength.

    2024T3 has excellent fatigue properties but its forming is generally limited to simple bending with adequate bend radiis - unless you go thru a heat treat and aging process (then it is called 2024T42) starting with 2024T0. If you are forced into the agony of this, you probably are making an alteration that must be approved as a major substitution.

    But flat and simple bent parts can easily substitute 2024T3 for 3003 and get the benefit of better fatigue resistance and better machining and drilling. A lot say 2024T3 will crack when 3003 won't, but that flatly isn't true in most cases unless there is a considerable locked in stress or a too-sharp bend in the installed finished part. The fatigue and cracking properties of 3003/5052 are inferior to 2024T3. DAMHIK.

  7. #17
    Aaron Novak's Avatar
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    Quote Originally Posted by Matt Gonitzke View Post
    Changing a control surface skin from 3003 to 2024 of the same gauge will have no effect on the stiffness of the structure, as all aluminum alloys have essentially the same elastic modulus.
    Elastic modulus is one part yes. The point I was trying to make is that from an engineering perspective, 2024 is not a direct substitute for 3003. Im still thinking that this being a flight surface repair, plus a change in material/design that it would fall into the Major Repair reguardless (or alteration depending how the IA wants to do it). This is where the IA and local FSDO come in. Of course if you just were to make a new part to the original specifications, then its just a major repair and the thing can keep flying for another 50 years. Sometimes more harm than good can come from "upgrading" materials.
    Last edited by Aaron Novak; 12-16-2012 at 01:03 PM.

  8. #18

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    Interesting discussion. It touches on some legal and safety aspects of airplane repair. I don't expect it's too helpful for Daryl though. Anyway Daryl, there's one more reference you may want to check. It's from the Code of Federal Regulations, Part 43, Maintenance, Preventive Maintenance, Rebuilding and Alteration. You can access it from the FAA website:

    Appendix A to Part 43—Major Alterations, Major Repairs, and Preventive Maintenance
    (b) Major repairs —(1) Airframe major repairs
    (xxi) Repairs involving the substitution of material

    Regardless of what you read on this forum, it's your IA who will have to approve your work. It may be a good idea to discuss the question with him/her before you get too far into it.

  9. #19

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    Hey, Dude said "repair". Didn't say re-skin or replace, or rebuild. Nobody has asked yet what what type of repair. If he's asking for a material for a simple patch I would think it would be much less important than a repair of a greater magnitude. I wouldn't have any problem with 2024 T-3 for a simple patch or a doubler for a small crack if no forming were required. Let's not make a grandfather clock out of a PA-20.

  10. #20
    Aaron Novak's Avatar
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    True the scope of the conversation did go beyond the OP question. However it must be kept in mind that "information" given in a forum is open to the eyes of all that read it, and so when there is a statement of questionable nature given, it is the obligation of others to counter or correct it. If it were not for this there would be more falsehoods that truths floating around than there already are.

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