The Right and The Wrong

Here is a snapshot of some things that Mark is learning and implementing here at MMS Aviation.

To safely secure bolts or screws in place, so they will not come loose due to vibration, they are secured with safety wire. (The Federal Aviation Authority (FAA) document, AC43.13-1B - Acceptable Methods, Techniques, and Practices - Aircraft Inspection and Repair, Chapter 7 Section 7 shows the acceptable method of safetying bolts and washers with safety wire as shown below.)

Whilst working on the Cessna 402B Wing Spar modification (will be blogging this later on in the year once we've moved on a little bit more), it came to our attention the way some of the engine mounts had been safety wired. On the left engine, only one set of bolts that had been safety wired was any good, the other 3 needed attention.

The first had been wired completely wrong. The safety wire should look like a backwards 'S' as shown above in the diagram, so that nether bolt can undo itself. If it tries, to undo, it will tighten the other bolt. The bolts safety wired below would be able to vibrate loose.

This second photo shows the correct 'S' shape, but the twists between the two bolts are far too tight. By twisting the wire too much it makes it susceptible to snapping due to vibration. (The AC43.13-1B document states that there should be between 6-8 twists per inch. This is more like 12-14 twists.)

The third one is okay, it would stop the bolts from undoing but is not ideal as it has wire coming straight out of the bolt on the right. It would have been best if the bolts had been swapped over so that the holes in them worked better for wire locking.

This is the fourth bolt set, and is the only one that was safely safety wired together.

The photo below is after the engine mount had been removed, cleaned up, etched, dipped in alodine and sprayed with primer. The bolts have been torqued and the safety wire has been installed correctly. See how much better it looks now.

This is just a little look at some of the things that we have to keep an eye out for whilst working on the aircraft that come in.