My 310 is annualled every March. This year, the engines had reached 500 hours SMOH, so I decided it was time to pull all four magnetos and perform the 500-hour inspection as called for in the Bendix magneto service manual. The annual also involved overhaul of a sticky wastegate, replacement of a prop control cable, and a bunch of other minor items.
When the annual was done and the logbooks were signed off, I took the airplane up for a local test flight. Everything checked out perfect, except that the right engine fuel flow was a touch low at takeoff power, so I tweaked it up a quarter-turn at the completion of the test flight. During the next two weeks, I made a few short flights in the airplane, and everything (including the fuel flows) seemed flawless.
My first longish X-C after the annual was a 2.5-hour IFR early-morning flight from my home base in central California to visit an associate in Tucson, Arizona. The sky was cloudless, the flight was beautiful and the airplane behaved perfectly. That night, I departed Tucson IFR for the return flight to California. It was a particularly dark night, with no moon, few ground lights, and high cirrus clouds obscuring the stars. The autopilot was on nav-track and alt-hold, there was hardly any ATC chatter, and almost nothing for me to do. I trekked across the vast Arizona desert at 200 knots, my eyes staring into the featureless blackness, my ears listening to the smooth drone of the engines and some static in the radios.
Wait a minute! My radios never had static before! What’s going on here?
I checked each of the radios in turn. The ADF sounded fine, and the LORAN performed flawlessly. But there was definitely noise in both comm radios that wasn’t there before. In fact, the noise was loud enough that I had some trouble understanding the dialogue on Albuquerque Center frequency.
I decided to switch into troubleshooting mode. Varying engine RPM a bit caused the sound of the radio noise to change. It sounded to me as if it might be ignition noise of some kind. I richened the mixtures a bit and then shut off the magneto switches one at a time. When I switched off the right mag on the left engine, the noise in my headset went away completely. None of the other three mag switches had any effect on the noise. I pulled out a Post-It Note and wrote “radio noise: left engine/right mag” on it. Recalling that all four mags had been pulled during the annual, I figured that perhaps the shield wire on that particular P-lead and broken off during the reinstallation, and that neither I nor my IA had noticed it.
I arrived home about midnight, thoroughly exhausted by the long day. I briefly considered pulling the cowling on the left engine and checking the P-lead, but I was much too tired. The next morning, I couldn’t stop thinking about the ignition noise while I was taking my morning shower. I had a very busy workday planned that day, and didn’t really have time to fool with the airplane. But I also had several more flights planned later in the week, and didn’t want to continue to suffer with noisy radios. Should I take the time to drive down to the airport, pull the engine cowling, and check out the P-lead? Or should I get my work done? I tried to work for awhile, but my mind kept drifting. Finally, I decided to bite the bullet, put on my work-on-the-airplane clothes, drive to the airport, and take a look at that dumb P-lead.
45 minutes later, I was in the hangar, the toolbox was in place, and the left engine cowling was off. Using a flashlight and mirror, I took a look at the P-lead on the right mag. The shield looked intact, and its crimp lug properly grounded to the lower-left mounting stud on the magneto distributor cap. Darn, my ungrounded P-lead theory up in smoke!
Then I noticed something: the upper-right mounting stud was protruding bare from the mag distributor cap, with no nut or lockwasher to secure the cap. Gosh, how’d that happen? Could my mechanic possibly have forgotten to install that nut or to tighten it down properly when the mag was re-installed? Not likely…this particular fellow is a real perfectionist, and makes fewer errors than any other A& P I’ve ever worked with.
Nevertheless, the nut was unaccountably missing from the upper-right stud. I found an appropriate nut and lockwasher in the parts room, installed it on the protruding stud, and tightened it down. But I was not a happy camper, because I still hadn’t found anything that accounted for the ignition noise in my radios.
A little voice told me that I should double-check the tightness of the other three studs on the magneto. I put my socket over the upper-left stud, and discovered to my shock that the nut was not even finger-tight! The lower studs were difficult to reach, but I finally found a combination of extension shaft and wobbly-socket that would do the trick. Both lower nuts were loose, too. The nut on the lower-left stud (to which the P-lead shield was grounded) was so loose that I could wobble the P-lead grounding lug. At last, I found the cause of the radio noise! My face broke out into a big grin.
I tightened all four nuts, and then checked the left mag. All four of its nuts were tight. I decided to uncowl the right engine and check its mags. The eight nuts on those two magnetos were also tight.
As I was replacing the cowlings and locking up the toolbox, I got to thinking. Suppose I had decided to do my work and put off checking the radio noise problem for a few more days. Would the other three distributor cap nuts have backed off completely like the first one did? Would the distributor cap or ignition harness have separated from the magneto? Would I have wound up having to replace the whole magneto rather than just tightening a few loose nuts? In a worst-case scenario if this happened on takeoff, would the left engine have gone into detonation and destroyed itself? Would it have taken the airplane (and me) with it?
The lesson here is simple: if you notice a change from the norm, no matter how trivial, act immediately to determine its cause. Don’t put it off. A seemingly inconsequential thing like static in your headset (or a twitch of your oil pressure needle) might be a vital clue to an impending catastrophic failure.
There’s also another lesson: even the best mechanics occasionally screw up (as they will be the first to admit). So be careful out there!