I had recently earned my Certified Flight Instructor Instrument rating (CFII). Rob was my student for his long instrument cross-country. Our second of three legs was from Louisville, Kentucky to Indianapolis, Indiana. It was night, with a 2,000-foot ceiling. We were in the clouds at 4,000 feet when Rob noticed that the alternator idiot light was on.
He cycled the alternator switch a couple of times, but the light remained on. We were in the clouds at night and in 20 or 30 minutes we were going to lose all our navigation and communication radios.
I asked Standiford Approach for a direct vector back to Louisville, and the lowest possible altitude to get below the clouds. I shut down all radios except one communication radio, and began a quick descent. We went through the clouds and broke out about 2,000 feet above the ground.
About ten miles away from Standiford Field, we flew right over a small lighted airport. Rob anxiously said, "Aren't you going to land at that airport?"
I said, "No, we'll make it to Standiford okay."
Rob did not say much after that. Standiford Approach was kind enough to turn the lights on high intensity so we could find the airport and the proper runway. All communication was lost on final approach.
After we landed, Rob and I discussed our emergency. He could not believe that I would fly right over an airport when the engine was about to fail. Then I realized what Rob was thinking. If a car's alternator fails, it will power the spark plugs off the battery for 20 to 30 minutes. When the battery is drained, the engine quits. However, on an airplane that is not the case. An airplane has magnetos so the engine drives its own spark. I knew the engine was not going to fail unless something else happened, but Rob thought engine failure was imminent. That is a lesson on magnetos I do not think Rob ever forgot.
Since the alternator repair would have to wait until morning, we spent the night in Louisville. We had only $7 between us. Fortunately I had a MasterCard!