I believe most of the aluminum skin is 2024T on most airframes. 

Where is the crack on the lower cowling? 

What about a patch instead of welding? 

Regards,

Airtech99

@airtech99  The area is up next to the firewall and not able to patch unless it would be a scab patch on the outside. Is there a source for your information? 2024t or 2024t3? Not sure what the difference would be if any.

@15018 

• 2024T3 is heat treated and is commonly used on aircraft skin and it also gets used around the engine areas due to the high temp from the engine.
  AC43.13-1B will cover the use of a patch and the rivets to use. 
• Any pictures of the cracked area?

@airtech99 Crack is about 1.25 inch long emanating from the louvre just below the turbocharger extending back toward the firewall. It is a great candidate for welding but not patching as this skin is what mates up to the extension of the firewall. A doubler or patch on the underside would bulge out when cowling is installed.

It’s in the service manual under Figure 4-9 which starts on page 1F7 and ends on 1F8 which is what I’ve attached to this post. It is showing the lower cowling as “5”. On page 1F7 it has the index which states the metal and thickness which is 2024-T3 and is .025” thick. I hope this information helps. 

Here is the index on page 1F7. 

From Pat Barry

Agreed  the typical response is to stop drill the crack. AC43 has a table that states how long a crack  can be before it can be repaired by other means.

I would explore a doubler on the inside of the cowling with flush rivets on the skin.

Aluminum welding takes skill — if your welder is confident then try it but if he fails you will be looking at a reskin which is challenging since the skin is a pressed part with a compound curve. It is possible to make a compound curve with an English Wheel (I have one) but it’s very difficult.

As Airtech 99 asked, how long is the crack and where is it on the skin?

Then I would remove some rivets and slip a small section of aluminum between the skin and the former and try to hold it that way. Drill holes, blind rivets, that will push the skin out a little.

But what you are asking for in your question is what kind of aluminum you need to reskin the lower cowl, and that is an almost impossible task. When Piper made the skin portions they used a press and pressed the part using a compound curve. If you were able to make the skin part you then derivet the old skin and drill the new part and rivet it in place. This is a really big and difficult task and will be very expensive. You will find lower cowls in parts yards for less money than you can make one for.

Alternatively, you can have a scab patch made on the outside since it is practical, inexpensive, and quick.

In the meantime, let me suggest that you call Webco Aircraft and ask what they might suggest. They sell skin material as well.

Pat Barry 

I did 3 scab patches on a the lower cowling of the PA39 that I meant to post before I painted them. I will post all 3 but will have to post the other 2 in 2 other posts. 

Here is another repair. 

And the last one.