There are a handful of cases in which black boxes have not been recovered, and a couple of cases in which the flight data recorder was found but not the cockpit voice recorder, or vice versa.
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“Black box” recording and storage equipment are compulsory on all commercial and corporate flights.
GPS and other tracking devices make it much easier to know where a plane is if it crashes over land. For that reason, the black box does not emit a signal and relies on it being discovered by investigators.
Flight recorders are normally located near the aircraft's tail, as experience has shown that this area generally suffers the least damage during an accident. Flight recorders are designed to survive both high-speed impact and post-impact fire. They are, however, not invulnerable and are sometimes destroyed.