In the days before digital drives, how did airplane black box recorders work? In this remarkably clear and concise video, "The Engineer Guy," aka Bill Hammack, explains every detail of these lovely old analog devices. It turns out that they worked roughly like seismographs. The altitude- and velocity-measuring instruments carved their data into a special kind of steel allow known as inconel. The 200-foot long steel spool rolled along at six inches an hour, so each box could record 400 hours of data.