Features

May 2015 Issue




Of Metal Specifications, Thread Pitch And Torque

A number of years ago, a fellow owner and I both noticed an unusual shimmy in the main gear of our 182 on landing roll out. My first thought was a warped brake rotor, but the vibration didn’t seem to be affected by the application of brakes. Hmm. While it didn’t seem like a big deal, it wasn’t right either, so we had the mechanic take a look.

Since the vibration was in the landing gear, the first place our mechanic checked was the gear saddle bolts, which attach the landing gear to the fuselage. While he was checking the bolts for proper torque, one bolt sheared off. Root cause found.

In the postmortem, we could see that the crack wasn’t new. We saw substantial oxidation through half of the bolt’s diameter, as shown in the top two images at right. At some point in its life, the bolt had developed a crack, which then propagated halfway through the bolt. When we caught it, just half the bolt was carrying the load; it clearly was on its way to failure. The vibrations on landing roll were a signal to us to pay attention to the airframe. Had we ignored the vibrations, the bolt would have eventually failed, putting landing-gear stresses on a single bracket rather than the two the engineers had designed.

That failed landing gear bracket wasn’t my last experience with metal fatigue. Several years later, I decided to change the tailwheel on my Cessna 180, something aircraft owners are allowed to do. I put the parts back together exactly as I found them, not realizing that the original installation was missing required spacers. When I snugged the axle bolt, the lack of spacers resulted in too much stress on the wishbone that holds the tail wheel to the axle. It promptly broke on one of my next landings.

In this case, shown in the bottom image, there was no pre-existing weakness in the metal. I had simply over-tightened a bolt that normally would have been protected by spacers. Old or new, the part was going to suffer a brittle fracture.