Originally published June 1998
This article is repeating the very obvious and is sophomoric for experienced persons. But, I have been amazed how some persons have been convinced that there is no corrosion, say, on/in a wing.
I have heard of a "salesman" opening an inspection plate and showing that the skin inside was nice and shiny. The skin that you can see is not what corrodes. It is the skin that you cannot see that corrodes. It is where the ribs, stringers, or spars are covering the skin (the faying surfaces).
Have the "salesman" take out some rivets at the trailing edge of the wing near the wing root so that the faying metals can be separated enough to look at the faying surfaces. That is where the corrosion will be.
The situation in the previous paragraph is why I am so adamant about trying to control corrosion on any airplane my grandchildren (and their mother) will fly. (I don't have any control over what their father flys.) (All right, already! I've got the alodine barrels, I'll get the stuff when we get back in July!!)
AC 43-13 (para 252 b. (1)) also warns of corrosion in the tail of a steel tube fuselage. I have heard "war" stories of persons tapping the tubes near the tail skid (wheel to you modern folk) with a plastic hammer (so as not to hurt the pretty fabric) and to have the tubes collapse in a pile of rust.
For a tricycle gear the dangerous corrosion on the steel tube framework is at the low spot when parked. This could be near or at the main gear for a configuration such as the old Piper Tri-Pacer.
On a sheet metal fuselage such as a Cessna 172 look for corrosion in the middle of the fuselage at the low point when it is parked. The most obvious (but hidden) place is the faying surface between the bulkhead (or intercostal) and the belly skin.
Look for drain holes that might be plugged with grass or dirt. Look around the drain holes on the inside for signs of water puddling. Look for corrosion behind various drains, especially relief tubes (assuming the tube hasn't pulled out--Norm!).
Remove wing attach bolts (or any other bolts) and check for rust. Check around the holes they came out of for rust or powdery residue. Check the bushings for corrosion and cracks.
It goes without saying (but AC 43-13 says it anyway) "In general, landplanes do not receive corrosion proofing to the same extent as do seaplanes manufactured as such." Be cautious of a landplane converted to a seaplane.
If there any welds visible, check to see if the paint is coming loose. Sometimes welds are not cleaned thoroughly and the paint loses adhesion. This is a sign of corrosion and can lead to metal fatigue.
Those of you that are wood enthusiasts don't get smug. The locations of corrosion on a metal aircraft were first found as rot (corrosion) on wood aircraft. Another problem with wood structures is "dry rot" underneath the shiny varnished surfaces. A light tapping on the shiny surface could indicate rotten core.
I have a "love/hate" relationship with box spars. For efficient structure, they are great. For corrosion/rot, they are horrible to inspect. It was a rotted wood spar that killed Knute Rockne.
Ask any "old" fireman who were on Hook and Ladder fire trucks in the days of wood ladders about "dry rot." (That could be another lengthy story.) (Hey, Jack Roth, do you know anything about this?)
(Don't you composite guys get smug either. See the next article with Brian Martinez's response about how composites can deteriorate.)
Don't let "Salesmen" use the shiny metal to blind your brain.
1. "Acceptable Methods, Techniques and Practices," AC 43.13-1A; "Aircraft Alterations," AC 43.13-2A
2. Aircraft Corrosion Control," originally published by IAP, Inc. According to "Craig" at 1 (800) 443-9250 the book order number is now JS312630 from Jeppesen Sanderson, 55 Inverness Drive East, Inglewood, Colorado, 80112. It is also available from EAA (1-800-843-3612, stock number 21-37597, $10.95)
Contents of The Leading Edge and these web pages are the viewpoints of the authors. No claim is made and no liability is assumed, expressed or implied as to the technical accuracy or safety of the material presented. The viewpoints expressed are not necessarily those of Chapter 1000 or the Experimental Aircraft Association.
Revised -- 14 March 1999