HELENA, Mont.—Toting a 30-pound rucksack, Mike Rogan squeezes his 6-foot-5-inch frame inside the safety bars encasing a steel ladder and climbs to the top of a 91-foot tower on the side of a Montana mountain just west of here. His mission: to change a light bulb.

In an era of advanced radar and satellite-based navigation, Mr. Rogan has a job that has changed little since the dawn of aviation. He maintains giant lamps used to guide night pilots through the lonely peaks of western Montana. Once part of a national network, the...