As said above it, and the cockpit voice recorder, come on automatically when an engine is started (normally rising oil pressure is the switch) and cannot be turned off by the crew, except by pulling a circuit breaker, until the parking break has been set and the engines shut down. Once this has occurred the cockpit voice recording, which is normally a 1 or 2 hour loop, can be erased by the crew so that you can get rid of the rude comments you just made about the Chief Pilot. The flight data recorder cannot be erased by the crew.