Black boxes: crucial to air crash probes

Black boxes, which record all in-flight data in aircraft, are essential tools in air accident investigations, thanks to which nine out of ten of them can be explained.
Commercial airliners are obliged to carry two black boxes -- the Digital Flight Data Recorder which contains data about the speed, altitude and direction, while the Cockpit Voice Recorder keeps track of cockpit conversations and other sounds and announcements in the pilots' cabin.
Actually orange in colour, with reflective white stripes to make them easy to see, each black box weighs between seven and ten kilogrammes.
Introduced into aviation in the 1960s, the flight recorders are held inside especially solid metal boxes built to survive extremely violent shocks, from intense fires to lengthy immersion in deep water.

They can survive as deep as 6,000 metres (almost 20,000 feet) underwater or when exposed to a very high temperature -- one hour at 1,100 degrees centigrade. They are fitted with a beacon which can emit a signal for one month.
In January 2004, the black boxes of an Egyptian charter flight that crashed off the coast of Sharm el-Sheikh were found after a two-week search, 1,022 metres below water.
Then, after 23 months submerged at a depth of 3,900 metres in the Atlantic Ocean, the black boxes of doomed Air France flight AF447 travelling between Rio and Paris were retrieved, with the data intact, allowing investigators to determine the causes of the June 1, 2009 crash.