Until recently, I was under the impression that CVRs will start recording as soon as the engines are started and turn off when the engines are stopped.
However, this is not the case given there are only 2 hours of recording time.
In one airplane disaster documentary, the CVR started recording at the gate and when the plane was getting de-iced.
So when does a CVR start recording, is this something that is started manually?
The CVR (and FDR) are continuously recording while the aircraft is in operation. In the days of magnetic tape, this was done with a continuous loop. In digital systems, the same is emulated by automatically returning to the start of the storage area whenever the end is reached. In both cases, the new data always overwrites the oldest data, so the last N hours is always available. When there is a crash, investigators locate where along the loop the accident occurred, and then read all the way around the loop (physical or virtual) until they reach the accident again.
CVRs are normally active as soon as one of the basic electrical buses, like the battery bus, or emergency bus, is energized. So if the airplane has a BATTERY MASTER switch or similar, it is recording audio as you as you switch it on, so pretty much as soon as you climb in and start getting things ready. CVRs will include a G switch to stop recording in a crash since they record on a continuous loop (most newer ones are 2 hours; older ones 30 minutes) so it can't overwrite the 2 hours prior to the crash if it survived and kept running. They also normally have an ERASE button that the pilot can theoretically push, but he/she would be in a heap of trouble if they pushed it without some really compelling reason if an incident occurred, and some airlines may have specific policies on when they can be erased.
Flight Data Recorders, on the other hand, are not started up until some action prior to engine start is initiated, typically when the red Rotating Beacon is activated, which is a standard sign to folks on the ground that you are getting ready to start. On the CRJs, the FDR starts with Beacon, Strobes, or when Weight off Wheels in case you forgot both of the other two somehow, and I'm pretty sure that is typical.
By legal requirement, the CVR must be operated from the start of the first checklist (typically Cockpit Preparation or similar checklist), until after the reading of the final checklist. The CVR can only be erased when the parking brake is set.
If an incident occurs, some aircraft have a marker button on the instrument panel to mark the event, and the CVR circuit breaker will be pulled to preserve the recording, under certain circumstances.
Length of CVR recording may be as little as 2 hours, but most are typically 6+ hours and digital, now. They include area microphones to pick up cockpit sounds such as switches, alarms, etc, as well as microphone channels for the captain and first officer.
Regarding a mishap on a 12 hour flight: if the mishap is severe enough to warrant CVR review, the flight isn't likely to continue for another 6-10 hours; it's probably going to be an air turnback to the point of departure, or a diversion to a different airport.
A mishap in which the CVR will become significant will occuring up until the CVR can no longe record (a crash, for example); the CVR isn't being over-written, because it's no longer recording. The CVR records up to the final moments.
