What is a black box in an aircraft?

The Cockpit Voice Recorder (CVR) and the Flight Data Recorder (FDR), also known as the "black boxes", record the events and status of a flight in a continuous loop throughout the duration of the flight. The design of the FDR and CVR are such that, once the device becomes full, it begins to overwrite the already recorded information, starting with the oldest information.

The CVR is a recording device with microphones in the cockpit, and has inputs from the radiOS, allowing it to record all spoken Words and noises in the cockpit, as well as all air traffic control (ATC) communication - both transmitted and received. The CVR also records all communications over the aircraft's internal phone and public address system. The cockpit microphones of the CVR also record ambient noises, and this feature has proven useful in some crash investigations.

Most CVR devices record the last 30 minutes of the cockpit noise, as required by the Federal Aviation Administration (FAA). In a recent proposed change to the requirements, the FAA has indicated it may require all aircraft operators to replace their existing CVRs with ones capable of retaining the last 2 hours of the cockpit audio recording.

The FDR records the status of the aircraft's operations and conditions. Modern FDRs are required to record a minimum of 88 parameters, including factors such as altitude, speed, engine thrust, autopilot status, landing gear position, stall warning, windshear, cabin pressure, the amount of force the pilots excerpt on the flight controls, and so on. The FDR must record the last 25 hours of aircraft operations. Many digital FDRs record hundreds of parameters for the last several days.

The black boxes are designed to withstand extreme mechanical shock, heat, and water depth, and thus survive all but the most extremely violent crashes. They have proven so valuable in crash investigations that, after the survivors have been rescued and the remains have been removed, location and recovery of the CVR and FDR are the highest priority tasks on a crash scene.

The FDR is used to analyze the flight characteristics of the aircraft. Modern investigation software creates an animated reconstruction of the aircraft's flight leading up to the moment the recorder stopped, which is usually (but not always) the moment of impact. This time-coded animation is especially useful to understand the actual trajectories, speeds, attitudes, and courses the airplane followed in its final minutes. It is also invaluable in identifying, or ruling out, mechanical failures or system anomalies.

By analyzing both the CVR and FDR together, the investigation team can recreate the airplane's and crew's conditions and activities with a remarkably high degree of accuracy and realism. This recreation in turn unearths important insights into the aircraft's operations, as well as the crew's situational awareness and effectiveness.