Thanks for the lead-in to the question I’ve always wanted to ask: How do the boxes “know” to stop recording at the time of the crash? I mean, you wouldn’t want to recover the CVR only to hear the sounds of the recovery team opening it up. If they have battery backups, as suggested here, then termination of the plane’s power due to the crash can’t be used. Do they have impact or motion sensors? Or, as one might guess from reading that Scientific American article, is it that only the memory parts are enclosed in the crash-proof boxes, and so the crash rips away all power input, including the batteries? Or something else?
WAG: Possibly they have impact sensors. The Emergency Locator Transmitter on an aircraft can either be turned on manually or it can be triggered automatically by high, sudden g loadings. It would make sense that the recorders could be turned off by the same means.
Any one know the correct answer? 
Just on general principle, I was under the impression that having equipment on a plane that cannot be turned off, under any circumstances, is a Bad Thing.
An electrical fire in a black box would certainly be an ignominious cause of a crash. I realize that they’re probably designed so that it’s extremely unlikely, but no component can be built totally immune to failure.
If it’s really true that the black boxes cannot be shut off in any way, is it because it was judged worth the risk?
I heard that they found the black box in PA. Why do they call it a Black Box when it is actually orange? Were they black until they actually had to find one?
I asked a friend of mine yesterday who is a Dash 8 First Officer, and he said that there is a circuit breaker in the cockpit for the CVR (we didn’t discuss the FDR), and also the CVR can be manually erased from in the cockpit.
Disclaimer: This refers to the Dash 8 only and also the aircraft are operated on surveillance flights without passengers, so commercial jets may be different.
AS far as the “black box” thing goes it would seem that it is a term coined by the media/public and refers more to it’s mysterious inner workings than the colour.
I mentioned this in the other thread I references, JCThunder. The term “black box” is a reference to the function of the device rather than its color. A “black box” is a device which has predictable and reliable external functions but mysterious or unknown internal mechanisms.
How Black Boxes Work
by Kevin Bonsor
Introduction to How Black Boxes Work
Topics covered
Recording and Storage
Cockpit Voice Recorders
Flight Data Recorders
Built to Survive
After a Crash
Links
"Cockpit Voice Recorders
In almost every commercial aircraft, there are several microphones built into the cockpit to track the conversations of the flight crew. These microphones are also designed to track any ambient noise in the cockpit, such as switches being thrown or any knocks or thuds. There may be up to four microphones in the plane’s cockpit, each connected to the cockpit voice recorder (CVR). "
etc etc etc etc
I work as an engineer for a company that builds big ol’ jet airliners. I’m also a private pilot.
Cockpit voice recorders CAN be turned off from the cockpit of most commercial airplanes. It is usually against regulations to do so but is permitted by some airlines (pilot unions agreements and legal hoohah permit this).
There was a very recent crash (I forget the exact airline, will look it up later) where a pilot did exactly that before deliberately suiciding a jetliner (again I forget the exact model, will look it up later) into the ocean.
The flight data recorder on the other hand is independent of cockpit control and cannot be normally turned off, although it is simple to manually disable should one know the correct place in the airplane to go poking around.
The recorders know when to turn themselves off because they are powered by the airplane’s electrical system. Usually a catastrophic crash means that along with the airplane disintegrating and burning, electrical power loss also happens 
There is seldom any worry that one will be running and overwriting the data after a crash. They do have batteries that power their ping beacons to enable search parties to find them however.
Transponders don’t carry voice information. They ping out an altitude and an identifier, that’s it. All aircraft flying in Class A airspace (over 18,000 feet) are required to have a transponder. Below that it depends on specific airspace class. Most of the geography of America does not require a transponder, only in denser areas or near medium-ish to large airports.
GargoyleWB: "There was a very recent crash (I forget the exact airline, will look it up later) where a pilot did exactly that before deliberately suiciding a jetliner (again I forget the exact model, will look it up later) into the ocean. "
Boeing 737, Silk Air (a division of Singapore Airlines), 12/20/97 in Indonesia. The CVR recorded the captain locking the copilot out of the cockpit, then its breaker was pulled (and only that one), and the plane went into a steep dive at max power. The final report has not yet been issued, and may never be, due to Indonesia’s insistence that there had to be a mechanical failure.
The FDR can be shut off by pulling the breaker for the main bus it’s on - the attempts by the Swissair MD-11 pilots near Halifax to fight the fire instead of landing included that step, which has hampered the investigation.
FWIW, the breaker panel is next to the captain’s left knee in most larger aircraft. For the transponder to be shut off that way, presumably one of the hijackers would already have to have been in that seat, and be at least able to recognize the labels.
Not only can the CVR be turned off, but it can also be erased by the pilot. I’m pretty sure that erasing the CVR is part of the shut-down checklist on most jets. I remember an incident years ago where some pilots got an airplane in major trouble through stupidity, and even more trouble when they ‘accidentally’ erased the CVR before the authorities could get to it.
From what I remember of the incident, these clowns had a debate over whether or not deploying a couple of degrees of slat would improve high-altitude cruise speed. So they decided to make every passenger an inadvertant test-pilot and try it out. But to do that, they had to pull a breaker that prevented the slats from retracting. So they did that, and while our intrepid aviators were examining the flight instruments the Flight Engineer returned to the flight deck, noticed the breakers were out, and popped them back in. This caused one slat to retract, and one to stay deployed due to wind forces. The airplane apparently did a complete barrel roll on its way to losing some 20,000 ft of altitude before the pilots managed to regain control. So when they landed, they erased the CVR to cover their tracks.
Anyone else remember this incident? I think I read about it in FLYING about 10 years ago.
Sam Stone - If you could remember some of the details, like when it occured, what type of plane, where the incident may have occured, we could look for it in the NTSB Aviation Database. I would be interested to know if you can conjur up any more details.
On the 727 incident. Those pilots were later exonerated. They found a failure that would allow uncommanded movement of those slats, and implemented a fix. Someone eventually flew some test flights which proved the fuel savings rumor was bogus. Also, it caused a medium buffet that the passengers would have noticed, but which none reported.
They did erase the CVR as part of the normal post-landing checklist. They said they were shaken and did it out of habit. I’m not certain, but I believe the modern electronic CVRs are not erased as part of the normal procedure.