Due to the intense interest in the recorded communications between the pilot of the US Airways aircraft that ditched into the Hudson River on January 15 of this year, and air traffic control, the NTSB has provided a brief explanation of what these communications entail, as well as the difference between the air traffic control (ATC) recording and that of the cockpit voice recorder (CVR).
The ATC recordings capture the radio transmissions between air traffic controllers and pilots on a specific radio frequency reserved for use in aeronautical communications. ATC transmissions are carried over the public airways and can be monitored in real time by anyone with a radio tuned to the appropriate frequency.
The ATC recording primarily captures two types of communications: 1) the radio transmissions between the air traffic controllers and pilots of the numerous aircraft on that frequency, and 2) communications by air traffic controllers made by radio, phone or other direct electronic audio link to other ATC facilities.
In the case of the ATC recordings related to the ditching of US Airways flight 1549, which were released by the Federal Aviation Administration (FAA) this week, the controller at the New York TRACON LaGuardia Departure facility communicated with a total of 14 entities - aircraft and other controllers at ATC facilities - all of which were recorded.
The recording captured by the cockpit voice recorder (CVR) is different from that of the ATC recording. Whereas the bulk of the ATC recording captures the radio transmissions between air traffic controllers and pilots, the CVR, by way of a microphone mounted on the overhead instrument panel and microphones in the headsets worn by the pilots, records all of the sounds inside the cockpit, including the communications with ATC.
In addition to recording all conversations between the pilots themselves and between the pilots and ground and cabin crew, the CVR captures sounds such as engine noise, automated warnings, landing gear extension and retraction, and other sounds that may yield information on the functions being performed inside the cockpit as well as those related to the operation of the aircraft.
A CVR committee, usually consisting of members from the NTSB, FAA, operator of the aircraft, manufacturer of the airplane, manufacturer of the engines, and the pilots union, is formed to listen to the recording. This committee creates a written transcript of the CVR audio to be used during the investigation. This transcript is released by the NTSB either at the time of a public hearing, or if there is no hearing, at the time the accident docket is opened to the public. The timeline for either of these events is usually six months or more into the investigation. In the United States, the actual CVR audio recording is protected by law and is never released to the public.