Tales from the DAR Side
Attention to detail. That’s what it finally comes down to. All the little things that go into building an aircraft are what take up much of the time and effort. But it’s those little things that can really set the aircraft apart from the rest, so it pays to spend that time and effort as you work on your project.
In my last few columns, I’ve talked a lot about the regulatory requirements – data plates, N numbers, and placards. This month I want to talk a little bit about the nuts and bolts of building an aircraft, as seen through the eyes of the designated airworthiness representative (DAR). The fact is, a lot of the attention to detail I’m referring to is directly related to actual nuts and bolts!
One of the many things a DAR is looking at while performing the airworthiness inspection on a homebuilt is the hardware, things like the proper type of bolt in a particular application, proper application of mechanical safety (cotter pin or safety wire), proper tightening of jam nuts, etc. There are a lot of places on a homebuilt where these items come into play, so there are lots of potential opportunities to miss the details.
In my experience, the most often missed of all these details is the tightening of jam nuts on pushrod ends. I’ve found at least one loose jam nut on almost every aircraft I’ve inspected. In one case, every jam nut on the airplane was loose! Make sure to check all the jam nuts after completing your rod end adjustments! This is a great spot to employ torque seal. This stuff comes in handy for both the builder and the inspector. The builder can use torque seal to mark a connection or adjustment that is complete, and when the inspector sees torque seal on the connection, he or she will know that the builder completed the task. (But the inspector will check to make sure things are tight anyway!) Take time to watch the Hints for Homebuilders clip on torque seal application.
Here’s a good illustration of attention to detail. This builder’s only mistake was getting torque seal on the thread of his rod end on the pushrod that comes into the picture from the lower left. But he did a good job of making sure to tighten jam nuts (and mark with torque seal afterward), and he also remembered to install large-diameter washers on the rod ends to help ensure that they don’t come completely apart should they fail.
Another detail that I often find to be in need of some attention is safety wire installation. I frequently find safety wire that is overtwisted, installed backwards (i.e., not in the tightening direction), or just simply not installed at all. There’s a bit of an art as well as science involved in proper installation of safety wire, so a little practice is a good thing. You’ll probably go through a little bit of safety wire before you get the knack, but it’s worth the time and effort. Check out my article in the April issue of EAA Sport Aviation for more info. You can also find good info on safetying in Chapter 7 of FAA Advisory Circular 43.13-1B, Acceptable Methods, Techniques, and Practices. Here’s a photo of a good-looking safety wire installation on propeller bolts.
Note that in each case, the wire is installed to work against the counter-clockwise rotation of the bolt.
Speaking of the art of safety wire, one place where this is especially true is when safetying turnbuckles. The proper way to do this isn’t readily apparent, and looking at the illustrations in AC 43.13 only goes so far. Take a look at my Hints for Homebuilders video clip on turnbuckle safetying for a good primer on how it’s done. Many builders use the “single wrap” method as shown in the first photo below. I prefer the “double wrap” method simular to what's shown in the second photo below, but either is acceptable.
Another detail where spending a little time with the thinking cap on will pay dividends is routing of lines, wires, and cables. Some builders do a pretty good job of this, but many seem to only think about the single system they’re working on and forget about what’s going to come later. You really want the area under your cowling or behind your instrument panel to look like a well-laid-out road map rather than a bowl of spaghetti. Spend some time thinking about the whole project, including continued airworthiness concerns (in other words, how you’re going to access stuff for maintenance) when routing lines, cables, and wires. A little time spent during the construction process will pay you back handsomely down the road.
Here are a couple pictures showing well-done layouts, one in an engine compartment and the other under an instrument panel.
Remember, building an aircraft isn’t one big project. It’s a series of small projects. Spend time thinking about the little things.