When the single alternator in a single-engine airplane decides to take the day off in the middle of a flight, the resulting unpleasantness spans the spectrum from inconvenience to terror depending on the weather and the endurance of the aircraft battery—and individual batteries for instruments and portable devices that can help the pilot keep the airplane right side up and heading toward a suitable airport.
While alternators are more reliable than vacuum pumps and some can be expected to perform we’ll beyond engine TBO, they do fail. We’ve had two give out at night in IFR conditions and a brand-new one expire within a month of installation (in VFR).