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By Odakyu-sen
#404116
Let's share some stories about metal fatigue and the importance of routine maintenance checks.
It's mid-winter down here in Auckland, New Zealand, and there are squalls everyday it seems. I decided to fire up my Mosquito NRG in the garage, just to flush some fuel through the carburetor and keep her turning over.

I have had troubles with the exhaust system and metal fatigue in the past. The new exhaust system is a lot better than the old one. I now safety-wire the two cap screws that hold on the exhaust manifold. I used a 1.5 mm cobalt drill to deal to the high-tensile steel of the bolts. Imagine my surprise when I discovered that one of the bolts had broken in half. The bottom half of the bolt and the offset hex nut were long gone. Only the locking wire was holding the head of the bolt in place.

I decided to replace the other bolt. I found a crack in the offset hex nut. Luckily I had spares from Sweden.

After replacing the manifold blots, I checked the rest of the exhaust and found that the exhaust manifold supporting strap had fractured where it was welded to the exhaust pipe.
Manifold strap break.jpg
Manifold strap break.jpg (1.85 MiB) Viewed 1210 times
It was a lousy weld to start with (only one side of the strap was welded to the exhaust pipe).

The gunk all over the exhaust springs is silicone sealer. It's job is to dampen any spring harmonic vibration and so prolong the life of the springs (which are also safety-wired).

Today I picked up my re-welded exhaust. I'll re-assemble everything tomorrow and fire up the engine. Maybe I have a vibration problem? I haven't dropped, knocked or otherwise abused my harness, and I take great care not to strap it down on my car roof-rack anywhere near the engine or exhaust.

The take-home message is to regularly check all parts of your power harness for cracked welds and lose bolts. Safety wire is your friend.

I invite you to share any of your stories about broken things you discovered in your checks of your equipment. I hope we can all fly safer.
Attachments
Manifold strap close_up.jpg
Manifold strap close_up.jpg (1.8 MiB) Viewed 1209 times
User avatar
By Tormod
#404155
Aside from that very obvious lack of inspection routine you must learn to lockwire bolts. The picture reveals the manifold bolts as unlocked, there might be more. (the heat destroys loctite if that's what you used)
User avatar
By Odakyu-sen
#404156
Thanks for pointing that out. If you read my post carefully, you'll see that I wrote: "Only the locking wire was holding the head of the bolt in place."

I was replacing the safety wire for the cap screws (you pointed out the drill hole in one of them). Originally, there were no holes in the bolt heads. I had to drill them so that they could be safety wired. At the time the photo was taken, I had removed the safety wire (to inspect the bolt).

Do you have a checking routine that you could share with us (and therefore help us to fly more safely)?
User avatar
By Tormod
#404157
I'm a aircraft mechanic so my mantra is: Check everything! But when doing a preflight it's helpful to have a routine. I always go counterclockwise, starting at the same point every time. When coming to the motor you should have a checklist but if you don't at least be sure to check oil level and that everything is secure, Do that by touching, your eyes will follow the fingers and make it easier to spot fails. Physically check cylinder(s), manifold, etc for looseness by grabbing them and try to move them. Propeller must be checked for damage and security. Lockwire must be tight because if a bolt comes loose it's important that the play is minimum. Follow the manufacturers recommendations, don't skip a point because it's "always OK".
User avatar
By Tormod
#404158
And for sharing stories, this is not mine but anyways. A guy in Bodø was doing a pre-flight on a Twin Otter in winter time, turning the props as a part of the check when all of a sudden his glove was pulled of his hand. It turned out that it was stuck to the propeller blade because the blade had a 1 inch crack in its trailing edge and the glove was wedged into that crack. Needless to say, the flight was delayed until the propeller was replaced.
User avatar
By mgforbes
#404215
I was doing a routine check on my trike some years ago, and one of the standard things I do is to grab the prop near the hub and push/pull hard on it. That particular time, I felt a slight "click". Turns out it was a fracture of one of the two saddle mount brackets that hold the engine to the vibration mounts. Failure of both brackets (the other one had a partial crack) would have resulted the in the engine departing the aircraft, probably during takeoff. That could have been....embarrassing....or worse.

I fabricated a new, extra-heavy-duty bracket that's easier to inspect, and I still give everything a good shake, yank and wiggle as part of the standard preflight.

MGF
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