EXPLAINER: How 'black boxes' hold vital clues to air disasters

IIndia's Aircraft Accident Investigation Bureau (AAIB) has begun its investigation into the cause of the crash of an Air India plane bound for London with 242 people on board moments after it took off from the western city of Ahmedabad, the country's Civil Aviation Minister said on Thursday (June 12).

Key to the investigation will be the plane's 'Black Box' flight recorders.

The boxes are not actually black but high-visibility orange. Experts disagree how the nickname originated but it has become synonymous with the quest for answers when planes crash.

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There are two recorders: a Cockpit Voice Recorder (CVR) for pilot voices or cockpit sounds, and a separate Flight Data Recorder (FDR). Some devices combine both functions.

They are mandatory on civil flights and the aim is to preserve clues from cockpit sounds and data to help prevent future accidents, but not to determine wrongdoing or liability.

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In broad terms, investigators say the FDR helps them analyse what happened and the CVR can - though not always - start to explain why. But experts caution that no two probes are the same and virtually all accidents involve multiple factors.

(Production: Thomas Holdstock)