Large commercial aircraft and some smaller commercial, corporate, and private aircraft are required by the FAA to be equipped with two "black boxes" that record information about a flight. Both recorders are installed to help reconstruct the events leading to an aircraft accident. One of these, the Cockpit Voice Recorder (CVR), records radio transmissions and sounds in the cockpit, such as the pilot's voices and engine noises. The other, the Flight Data Recorder (FDR), monitors parameters such as altitude, airspeed and heading. The older analog units use one-quarter inch magnetic tape as a storage medium and the newer ones use digital technology and memory chips. Both recorders are installed in the most crash survivable part of the aircraft, usually the tail section.

Each recorder is equipped with an Underwater Locator Beacon (ULB) to assist in locating in the event of an overwater accident. The device called a "pinger", is activated when the recorder is immersed in water. It transmits an acoustical signal on 37.5 KHz that can be detected with a special receiver. The beacon can transmit from depths down to 14,000 feet.

The CVR records the flight crew's voices, as well as other sounds inside the cockpit. The recorder's "cockpit area microphone" is usually located on the overhead instrument panel between the two pilots. Sounds of interest to an investigator could be engine noise, stall warnings, landing gear extension and retraction, and other clicks and pops. From these sounds, parameters such as engine rpm, system failures, speed, and the time at which certain events occur can often be determined. Communications with Air Traffic Control, automated radio weather briefings, and conversation between the pilots and ground or cabin crew are also recorded.

The FDR onboard the aircraft records many different operating conditions of the flight. By regulation, newly manufactured aircraft must monitor at least twenty eight important parameters such as time, altitude, airspeed, heading, and aircraft attitude. In addition, some FDRs can record the status of more than 300 other in-flight characteristics that can aid in the investigation. The items monitored can be anything from flap position to auto-pilot mode or even smoke alarms.

With the data retrieved from the FDR, the Safety Board can generate a computer animated video reconstruction of the flight. The investigator can then visualize the airplane's attitude, instrument readings, power settings and other characteristics of the flight. This animation enables the investigating team to visualize the last moments of the flight before the accident.

Both the Flight Data Recorder and the Cockpit Voice Recorder have proven to be valuable tools in the accident investigation process. They can provide information that may be difficult or impossible to obtain by other means. When used in conjunction with other information gained in the investigation, the recorders are playing an ever increasing role in determining the Probable Cause of an aircraft accident.