Black boxes: how do they work

by Lisa Cardy

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THIS was�a recent question at The AnswerBank and it got some very thorough replies, click here to see them. Here is some more detail for you:

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An Australian scientist, Dr David Warren, invented the so-called black-box flight recorder in 1953. Dr Warren was part of a team trying to establish the cause of a series of British jet crashes when he came up with the idea of the flight recorder.

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As one of the replies pointed out, the box isn't black, it's actually bright orange. It's thought that the term was first coined in the 1960s, when 'black box' was synonymous with a box of gadgets.

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Also it's not really one box, but two. The Cockpit Voice Recorder (CVR) records 30-minute loops of sounds from the cockpit, i.e. it only retains the last 30 minutes. As well as recording crew conversations, the total audio environment (control movements, switch activations, engine noise, airflow noise and passenger announcements) is picked up by a microphone mounted on the cockpit ceiling. The Flight Data Recorder (FDR) records various technical data to help investigators reconstruct what was happening when a plane crashed.

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Flight recorders are situated in crash-protected enclosures, which are designed to survive high impact crashes, intense fires and the high water pressure of deep oceans. The boxes are placed in the most crash survivable part of the aircraft, usually the tail section. The crash boxes are sometimes destroyed in an accident, but the data inside usually survives.

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Each recorder has an Underwater Locator Beacon (ULB)�that is activated when the recorder makes contact with water. It transmits a signal, even from depths of 14,000 feet,�which can be detected with a special receiver.

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Older analogue units use magnetic tape as a storage medium and the newer ones use digital technology and memory chips. The latest designs of recorders use large capacity computer memory chips and are termed 'Solid-State' because they have no moving parts. Because of this, they are far more reliable, require minimal maintenance and can survive harsher fire and impact conditions.