Originally Posted by
WLIU
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