Black boxes: crucial to air crash probes

Commercial airliners must carry two black boxes. The Digital Flight Data Recorder contains information 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 kilograms (15-22 lb).

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 exposure to a very high temperature -- one hour at 1,100 degrees Centigrade. They are fitted with a tracking beacon which can emit a signal for about one month.

The signal can sometimes continue for several days beyond that duration, possibly an extra eight to 10 days, according to Angus Houston, the head of the Australian search mission.

A frequency of 37.5 kHz is the most common for aviation flight data recorders, according to experts. It was the level at which a signal was detected by the Chinese ship Haixun 01 on April 5 as it searched the southern Indian Ocean for missing Malaysia Airlines Flight MH370.

Black box beacons have an average range of detection of two kilometres (1.2 miles).

The waters in which the Chinese vessel was searching are 4.5 kilometres (nearly three miles) deep.

So even if the signal were found to come from the plane any recovery operation would be extremely challenging, according to Houston.

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.

In May 2011, after 23 months submerged at a depth of 3,900 metres in the Atlantic Ocean, the black boxes of Air France flight AF447 between Rio de Janeiro and Paris were retrieved with the data intact.