How does an airplane black box work?

The flight data recorder logs hundreds or even thousands of measurements, such as altitude, speed, heading and engine settings, many times per second. The cockpit voice recorder captures pilots’ conversations, radio calls and cockpit sounds; new aircraft must now keep at least 25 hours of audio instead of the older 2-hour standard. Both are usually mounted in the tail, which tends to survive crashes best, and their memory is sealed inside armoured casings. They are tested to withstand an impact of 3,400 times the force of gravity and fire of 1,100 degrees Celsius for an hour. Each unit carries an underwater locator beacon that sends out pings for at least 30 days, and newer beacons last 90 days.