OK, I've been an airplane owner(and expect to own again), and I've been a 'tinkerer' all my life. I have an EE degree, and mostly familiar with metals, metal working, electrical systems, engines, engine acc, hydro systems, and a lot of other gadgetry we find on vintage planes. I do much of my own work on my planes, and have never caused an accident(only one data point).
Having said that, I have an A&P whom I work with regularly in areas where I am not an expert. Because being an "expert" in terms of aviation is the minimum standard that I want to put my kids in to fly around. Being an expert, and a journeyman together is the best of all worlds and between the two of us, that's what we have.
A few years ago I put my first set of front seat shoulder harnesses in a vintage plane. I'd done very little aviation quality riveting, so I took an online class, and got the A&P, the cleekos, drills, countsink bore, deburr tool, buck bar, and pnuematic hammer, and we went at it. Boy, what an experience for someone who had only done Cherry-max blind riveting before. The more I saw, the more I realized it was an art, and not one that can be just figured out in 20 minutes. Once those rivets were driven, I trusted them with my life(literally) but it's not a job I would have taken on by myself.
Other parts of the plane that I work on regularly are the radios, and interior. I've done a complete interior from scratch on a plane, and yes - I have the burn certs that meet the FAA regs to prove it. Including the thread type used. Even with that, I had the A&P check my work on the frames, and installation. He didn't sign the logbook because he didn't need to for upholstery repair(no one knows how extensive it is until you start taking apart other systems). During this job, I dropped one 1/4" washer into the plane, and just my luck it went into the spar box cavity. The A&P made me fish it out, with a time consuming hunt using a high intensity light, and a pair of tweezers. Lots of busy things going on in the spar box of this plane, and nothing that doesn't belong there gets left behind.
Now, having said the above, let me tell you about a recent experience with an A&P, cause they are not all created equal. On a certain make and model of vintage plane there is a service bulletin about adding a doubler plate to the aileron hinge point where the hinge acts as the bell-crank for actuation. There is supposed to be a doubler on the top and bottom of the inner hinge. So, I'm looking at this plane for sale by an A&P, and I ask him about the doubler plate on the top of the inboard, and also on the top of the outboard hinge, but no doubler plane on the bottom of the inboard hinge where it belongs. He looked at me, and prolly felt like an idiot. He put the doubler plane on the outboard hinge which is free swiveling and forgot to put one on the bottom of the inboard hinge which is actuated. Next, I looked at the flap rigging, cause it's common(but wrong) to 'droop' a flap to correct a skidding plane. This is a poor man's crutch rather than rig it right like the book. Sure enough, the flaps were out of rig, by about 2 deg left to right. Lucky, I had done a rig job, knew what to measure and had the right tools. So, I grab the inboard gear door, and it's loose as a goose on the ground, so it's prolly the same in the air, or even worse. Rather than fix the gear door mechanism and gain back the speed, he just drooped a flap to center the ball, and left the dragging part alone.
Not all A&Ps are created equal. Not all pilots are mechanically inclined. Best of both worlds are a great A&P, and a savvy owner who can take a jug off in a bean field in OK, get it back to the shop and have them put in a new valve and springs, and then give it to the owner who can put the jug back on and fly out of the bean field. Worst case is to have a lackadaisical A&P, and a unskilled mechanical owner(or one who takes on what he doesn't know), and things go bad, and people wind up crashing.
