If we add a video recorder in the cockpit to complete FDR/CVR, will we add a blackbox?
Before answering, let's look at these terms:

FDR - Flight Data Recorder

FDRs record all flight data, every button pressed, every control input etc. They record throughout the duration of the flight.

CVR - Cockpit Voice Recorder

CVRs record all voice communications through the radios and the chat pilots have between themselves. They record only the last 30-120 minutes.

So what benefit will adding a camera bring?

After an accident (God forbid) the investigators are not interested to see what was the color of pilot's shirt or was he picking his nose before he pressed a button. In addition, you would need several cameras to record every single action of the pilots, and even then it would be hard to figure out as some buttons are placed very close to each other.

FDRs solve this problem as any input from the pilots are recorded. CVRs capture all communications so the motive or reason behind any input from pilots are be found out.



I suggest you to read some NSTB investigation reports (here or here) to get a detailed idea how in-depth knowledge the investigators can get by these two instruments.