When we think of aging airplane issues, it’s obvious we’re talking about older airplanes. I tend to think of Beech aircraft built before 1974 as older, since those machines were not zinc chromated when they were built. Here at Air Mod, cleaning and chromating the cabin of an older airplane is typically a monumental task. Fortunately, we also renovate a significant number of newer aircraft that generally don’t require as much clean up and component repair prior to the re-upholstery process.
Newer, however, doesn’t always mean better. Over time, we’ve identified some frequently seen problems in newer airplanes that need to be addressed. Most of these are the result of the factory’s effort to save weight and reduce manufacturing costs. Fortunately, most of the issues that we see in our corner of the Beech world don’t compromise safety. That said, there is one safety-related issue we often find in newer 36, 55, & 58 models, and that is arcing between the feed side of the circuit breakers and the airframe in the upper area of the side-mounted circuit breaker panel. This problem is of particular concern due to the fact that the feed side of the breakers is not protected by any safety device. All the power coming to the main electrical buss is running directly to the place where the arcing is occurring. This configuration unfortunately sets the stage for potential electrical failure or an in-flight electrical fire.
These airplanes are new enough that the cause of this situation is just beginning to surface. So let’s all take a look at the condition of these circuit breaker panels and, as the Lone Ranger used to say, “head ‘em off at the pass”.
Here is what is at the root of the problem. The frame of the circuit breaker panel is vacuum-formed from the same plastic as is used in the window frames. This material is greatly affected by heat, pressure and time. We all know that window frames tend to crack and become deformed at the mounting holes. Well, so do the upper and lower mounting flanges of the plastic circuit breaker panels, which are not reinforced. With no metal reinforcement in these critical areas, the panels are warping and the mounting holes begin to break to the point that the panels are moving around. This allows the breaker feed buss strips to contact the circuit breaker mounting bracket at the top of the panel.
The fix is to first use a heat gun to soften the plastic circuit breaker panel and re-form the stressed areas back to their designed shape. We then flush rivet a piece of formed .032” 2024T3 aluminum in places where the mounting screws are located.
Step two is to fabricate two spacers to keep the upper mounting screws from drawing the now-reinforced circuit breaker panel too tightly against the metal upper mounting flange. Being a found object engineer, I use a piece of -4 aluminum aircraft tubing cut to length. We then flare both ends of the tube to establish a good seating surface, and sand about .020” of material off the two sides to reduce the width of the spacer so it will fit between the tightly located circuit breakers. Finally, we ream the inside diameter of the field-made spacer with a #12 drill bit to accommodate a long #10-32 mounting screw. The use of this spacer system guarantees proper clearance between the breakers and the airframe.
One last extra-insurance step is to rivet a strip of non-combustible insulation material to the face of the upper mounting bracket. Be sure that the original insulating material is still bonded to the cabin skin. Once painted and installed, the circuit breaker panel will look like new and will withstand the ravages of time.
So the next time you’re out at the airport, check to see if there is any movement in your circuit breaker panel, and look at the mounting holes – any deformation or cracks can mean trouble. The licensed mechanics who work for us find these things as we tear down even relatively new airplanes. It pays to look closely.
While we’re on this subject, next time I plan to give away all my tricks for cleaning and detailing circuit breaker panels, sub-panels, switches and dingy knobs. These kinds of improvements make for great weekend projects, and really perk up the looks of an old instrument panel. ‘Til then, fly safe!