The International Cessna^® 170 Association

My lower cowling, at the bottom, has previously had cracks. Both sides had a doubler added prior to its 2000 paint job (these replaced the two doublers that had been installed at some previous time and both had cracked) This is a 1959 C-172 cowling on a '56 172. The left doubler wasn't made as large as the right side and it has again cracked (did so several yrs ago). I am going to remove the smaller doubler (its on the outside of the cowling as opposed to being on the inside) with one of the same size/design as the one on the right side which has not, to date, cracked again. I would like a suggestion as to the thickness of the aluminum doubler I should use. The ones on either side appear to be .032, but I am not sure. I have some thicker stock--at least 1/2 again as thick (have not measured it yet) and am wondering if that would be better material than the thinner material now on the cracked doubler? It would appear that Cessna needed to have added one more screw attachment on either side to have helped eliminate this condition.

Usually best to make a doubler from same material and thickness, or next up. If you install something to thick, you might just transfer the stresses. You could end up with that heavy doubler lasting forever, but the neighboring material cracking. Also good to fashion the doubler in gusset/triangle shape, rather than rectangle or square for same reason

George included SL 170/172-15 and SK 172-11 to the Maintenance Library. It shows Cessna's original repair for cracks here.

Frank, whatever you use for the doubler, make sure you finish off the edges of the material . Run your finger nail around the perimeter of the material, any rough places need to be smoothed off. Any little nick sets up a stress point for a crack to start. On mine, I also added another nutplate and screw at the bottom.

Ed, I had given some thought to adding another nut plate on both sides of mine as well. Thanks for the tip on making sure the edges don't have any stress points on them. Now I just have to decide which thickness of material to use. Hoping this repair doesn't ever have to be repeated.

Frank

Has your prop been dynamically balanced lately?

NEVER That is on my list of things to do.

That could be the cause of your cracks. If you have it done make sure your prop and spinner are marked so they can be installed in exactly the same position each time they are removed in the future.

Appreciate the input. Our EAA chapter has a prop balancing unit. I just need to get one of the guys to help me check out the prop.

You would probably be shocked at how far out of limits it probably is. By the time most pilots recognize a problem it is usually around 5 times what is considered an acceptable limit. If you can get it done for free, even better.