Every once in a while, we do things that we regret. When one of those things turns out to be owner maintenance, the damage is often more than a shot to the ego. The following story is an excellent example…
IT WAS JUST A LITTLE OIL LEAK. The tiniest of leaks had developed from the side of the oil sump cover on a Cessna 170. The owner, who is a good pilot and keeps his 170 meticulously maintained, noticed the leak, and wiped it up. Unfortunately, once an oil leak starts, it will seldom stop. This is because the gasket has been breached, and once that happens, well, lets just say that the dam is broken and just doesn’t get better.
OUR OWNER DECIDED TO DO SOME “MAINTENANCE” on his plane. He had all the right tools, and removed the cowl to access the oil pan. He gently snugged up the cover bolts for the oil pan, hoping to stop the leak. He then reinstalled the cowl, and went for a ride.
On his next trip home, he again spotted an oil leak. Once again, he removed the cowling from the plane, and again carefully snugged up the cover bolts on the oil pan, hoping to stop the leak. The bolts were getting a bit stiffer this time, but he was still able to get a flat or two on each one. Finally, he reinstalled the cowling, and a day or three later, went for another ride.
When he reached home, he saw another oil leak. So (you can probably guess this one) he did it all again! This time, he noticed that the sump cover looked a little deformed, because the bolts were taking it up so much that the pan was starting to pucker a little between the bolt holes. He also noticed that turning the bolts was really hard, and correctly realized he was turning a steel bolt into an aluminum casting, so he stopped.
OUR PILOT THEN WENT OVER TO THE FBO, and confessed his actions. The FBO took a look at the plane, and winced at what he saw. The pan gasket was over-crushed, and would not seal. It would have to be replaced, and that meant that the engine would have to be lifted out of the airplane in order to access the gasket.
OUCH! The wallet of our pilot was now smarting quite a bit. The FBO disconnected the motor mounts, drained the oil and pulled the lower cover. They inspected the sealing surfaces, replaced the gasket with a new one, and got the plane back into the air with a minimum cost of dollars and time.
WHAT WENT WRONG?
Per FAR Part 43, owners are allowed to do limited maintenance under the supervision of a qualified mechanic. The qualified mechanic was nowhere to be found while these bolts were being tightened.
Our pilot was torquing the bolts with an open-end wrench and a ratchet wrench, not a torque wrench. The problem here is simple: fasteners — and what they are tapped into — have a specific torque limit. If you take them beyond that limit, either the fastener will “neck” (get thinner at a point) and “yield” (break), or the threads will strip out of the casting (more likely) and require an expensive repair to the case to restore them!
IN THIS CASE, OUR PILOT GOT LUCKY, AND A LITTLE SMART. He stopped when that little voice in his head said ‘Too Much!‘ and got professional help to solve his problem.
Had he gone a turn or two more on the flats of the bolt, he might have stripped out the case, or cracked the oil pan — but he didn’t.
LEARN FROM THIS MISTAKE!
- Want to fiddle with your plane? Either get your A&P Certificate, or simply work under the guidance provided in FAR Part 43.
- Gaskets and aluminum cases won’t take an infinite amount of torque, and how that torque is applied is important. If you don’t do the job right, it may make matters worse, not better!
- There are some things that FAR Part 43 allows the pilot to do. Know your limits and stay within them.
BOTTOM LINE: Our pilot / owner got off lucky this time, and learned a valuable lesson. You can learn this lesson as well, and avoid the same expensive mistake. This should hit home doubly hard for those of us who are former backyard mechanics. By knowing and following the rules, you can avoid causing your own Self-Inflicted Maintenance!
