A cockpit voice recorder is a flight recorder used to record the audio environment in the flight deck of an aircraft as a way of investigating a accident or incidents. This is typically achieved by recording the signals of the microphones and earphones of the pilots' headsets and of an area microphone in the roof of the cockpit. A standard CVR is capable of recording 4 channels of audio data for a period of 2 hours. The original requirement was for a CVR to record for 30 minutes, but this has been found to be insufficient in many cases, significant parts of the audio data needed for a subsequent investigation having occurred more than 30 minutes before the end of the recording. The earliest CVRs used analog wire recording, later replaced by analog magnetic tape. Like the FDR, the CVR is typically mounted in the rear of the airplane fuselage to maximize the likelihood of its survival in a crash.

The AV-557C is a coaxial reel to reel, bi-directional recorder utilizing polyamide base tape as the recording medium. The AV-557C provides continuous four channel recording with a sufficient length of tape to obtain the 30 minute record. In operation a total of eight channels are recorded: four in one direction of tape travel and four in the opposite direction.

The AV-557C automatically changes the direction of tape travel at each end of the tape. The AV-557C automatically erases as it records so that only the last half hour of recording is retained. The AV-557C is designed to record and preserve the last 30 minutes of flight crew communications and conversation, during a flight, to aid in an aircraft incident investigation. The recorded conversations and communications can be completely erased as soon as the aircraft has docked, thus eliminating the possibility of unauthorized access to the recording. Playback is not possible unless the recorder is removed from the aircraft.

The AV-557C is designed to operate with a microphone monitor. The microphone monitor is a remote mounted self-contained unit which is electrically connected to the AV-557C through aircraft wiring. The microphone picks up conversation in the aircraft cockpit. A self-test push button and meter or LED allow activation and monitoring of the CVR self-test circuitry. The AV-557C unit is housed in a standard ½ ATR short case and consists of the following assemblies: tape deck assembly, armored enclosure, dust cover assembly, shock mount assembly, chassis assembly and electronics.

The AV-557C contains a handle for ease of carrying and an electrical connector located at the rear of the AV-557C for electrical connection to the aircraft. The audio inputs, furnished by the aircraft audio distribution/interphone system, are obtained from the pilot, co-pilot, and extra crew member. A fourth audio input is provided by the remote mounted microphone monitor. The differences between the models are the type of dust cover used and the availability of the Underwater Locating Device (ULD) .