They run automatically from the point at which the aeroplane is capable of moving under its' own power to the moment it cannot.
Not with the Boeings I fly. The CVR now runs anytime there is power on the aircraft and for ten minutes after power is removed.
At least that is what we are now told.
Until quite recently the aircraft manuals had the customary boilerplate language about a 30 minute recording. Now it is at least two hours and there is definitely still a circuit breaker in the cockpit to disable the CVR. In fact, if the crew is involved in a reportable event, we are required to pull the CVR breaker and notify maintenance so that the recording can be harvested.
Hmmm. Wonder if the CVR still runs for ten minutes after you pull the breaker? In a commuter plane overrun into the EVAS they did the shutdown checklist down to the CVR breaker, got interrupted and the cell phone calls to ops and the union were both in the NTSB transcript.
Discussions here and elsewhere leave doubt in my mind whether the legally mandated erase function is any more effective than in the analog CVR days. 'Erased' or overwritten CVR conversations in the past were sometimes 'recovered' using some closely held forensic techniques.
I agree that there might not be much on the MH 370 CVR if it is recovered. However the idea that non-volatile memory chips in cameras, tablets and phones of the pax and crew could yield clues certainly has recent precedents in accident investigations.
See for example:
http://dms.ntsb.gov/public%2F55000-5...7%2F550800.pdf