Malfunction Junction - Dedicated to my many mistakes
The first in my career:
In the early summer of 1965 I went to work in the family airline business as a radio technician. I already knew a lot about electronics, or so I thought. I had been working on radios since I was eleven and was headed to the university as a freshman in the electrical engineering program that fall.
As a new hire I was given jobs in line with my experience and jobs nobody else liked to do, such as sweeping the floors and cleaning up the shop. One interesting assignment was to repair and overhaul the large backlog of model MG-149G and MG-149H rotary inverters that had been piling up. Inverters were a “time item” that had to be removed from the airplane after so many hours of operation. We had inverters on airplanes coming up for overhaul. Something had to be done because we were out of spares. I ended up doing it.
The inverters were used in our DC-3 and C-46 airplanes. They were a dc motor/ac generator that ran off the airplane’s 28 vdc main bus and supplied 115 vac, 400 Hz for certain radio navigation equipment and instruments.
The overhaul involved completely disassembling the unit and cleaning it up. Since they used carbon brushes, there was plenty of carbon dust to clean out. Once cleaned up, the components were thoroughly examined for anything out of the ordinary. Since I had no idea what was ordinary, the foreman instructed me on how to clean and what to look for. The chemicals we used for cleaning probably are illegal today. The armature required special attention and in some required “turning” on a lathe to provide a smooth, even surface for the brushes. I was shown how to do this and also shown how to use a piece of hacksaw blade, exactly the right thickness, to clean out the grooves in the armature commutator.
The inverter rotor had a bearing in each of the end-bells. Some bearings were sealed and some were open. Open bearings had to be thoroughly cleaned and regreased and not with just any grease. All bearings had to be operated by hand to make sure they were smooth. Upon reassembly, the inverter end-bells, bearings and rotor had to be perfectly aligned. This was done by starting up the inverter on the bench and listening. I was taught what to listen for and how to lightly tap the end-bells with a non-metallic hammer to align the bearings and obtain the required sound (a “smooth whine without the scraping noise” of an out-of-alignment bearing).
As a young engineer-in-training, I soaked up all this knowledge with little problem. It took me awhile to work through the stack of inverters but I got them finished. Our spares rack was now full of shiny, clean MG-149 inverters. Life for me was good, and I occasionally was given other challenging assignments such as troubleshooting and aligning automatic direction finding (ADF) receivers.
But by now I was considered the inverter expert. I was told to remove the No. 1 inverter from the DC-6 over in the hangar and take it to a local shop for overhaul. The DC-6 used a much larger inverter than the MG-149 due to the larger ac load. Our test bench was not setup for the larger inverters. Clearly, this was within my experience and capability. Off I went but not before asking where the inverters in the DC-6 were located. They were located directly behind the radio rack in a closed compartment, No. 1 at the top and No. 2 at the bottom.
When I got to the hangar and climbed into the DC-6, I was all alone in the airplane. I popped the Dzus fasteners that held the compartment covers and exposed the two inverters. So far, so good. I scoped out how they were mounted and the electrical connections. There were two large wires (about 4/0 AWG) for the 28 vdc input and several smaller ones for the 3-phase ac output (the ac system in the DC-6 used a delta configuration with one corner grounded) and on/off control.
I decided to disconnect the large dc wires first and grabbed a combination wrench from my toolbox (adjustable wrenches were absolutely forbidden, and I avoid using them to this day). I reached in and started to unscrew the nut on the positive dc input stud. My wrench hit the enclosure structure and all hell broke loose.
There was a big flash and the wrench instantly turned red. I had shorted the live dc bus to ground. Nobody told me to have one of the airframe mechanics disconnect the batteries in the airplane prior to working on the inverters because, well, everyone knew that with batteries in the airplane, the dc bus was hot all the time. The DC-6 had big batteries with lots of short-circuit current capability (they had to start the huge Pratt & Whitney R2800 engines)
I flinched and quickly grabbed the wrench, but it had welded itself to the aluminum structure. Worse, red-hot molten aluminum and steel had splattered onto the carbon dust that coated everything in the inverter compartment and set it on fire. I was finally able to pull the wrench away. I don’t know how I did that without burning myself. Miraculously, the carbon fires fizzled out and no significant damage was done to anything except my new wrench (picture attached).
I climbed out of the airplane and, still shaking, told the mechanic foreman what happened. He had a pretty good laugh and said he’d have the battery disconnected. After that was done, I removed the inverter without further trouble and took it to the overhaul shop. When I got back to the radio shop and told the foreman there, he also had a good laugh but then said “I should’ve told you.” I realized how close I had come to burning up a perfectly good DC-6 and probably the hangar with it. I went on to college that fall and paid for it with the money I earned in the radio shop. In fact, that’s how I financed my engineering degree, by working there each summer for four years.
Theory is when one knows everything but nothing works. Practice is when everything works but nobody knows why. In our lab, we use both theory and practice: nothing works and nobody knows why.