Paris - After finding possible survivors and victim remains, the first priority of air crash investigators is to locate the black boxes, a duo of data recorders that can hold vital clues on why an aircraft went down.
Despite their name, the boxes that hold a plane's flight data and cockpit voice recordings are actually bright orange with white reflective strips to make them easier to spot among debris.
Made of hard-to-destroy materials, all commercial planes are obliged to have them on board.
Authorities announced on Tuesday that one of the black boxes of a Syria-bound Russian military plane that crashed into the Black Sea with 92 people on board on Christmas Day, has been found at a depth of 17m, about 1.6km from the shore.
According to experts, black boxes explain the causes of nearly 90% of plane crashes.
The gadget was developed in the 1950s by an Australian scientist, David Ronald de Mey Warren, who helped probe a crash of a De Havilland Comet - the first commercial jetliner.
The first box he designed, inspired by a miniature voice recorder he saw at a trade fair, worked with magnetic recording tape -nowadays they are digitised.
Each black box comprises two units: a digital flight data recorder (FDR) to record the plane's speed, altitude and direction, and the cockpit voice recorder (CVR) to note conversations of the pilots and ground or cabin crew as well as any alarms, engine noise, explosions, pops or thuds.
The FDR can hold about 25 hours of information, the CVR about two hours.
The FDR and CVR can be housed in separate boxes - usually kept at the back of the aircraft where they are more likely to survive a crash - or combined into one.
Mandatory since the 1960s on commercial flights, black boxes are encased in hardened steel or titanium boxes designed to survive high-speed impact and pressure, intense post-crash heat, or lengthy underwater submersion.
Each device is made to resist heat up to 1 100°C for an hour and 10 hours smouldering at a lower 260°C, or to survive underwater as deep as 6 100m.
But they are not indestructible and are often badly damaged.
Analysing black box data can take days or even weeks, depending on their state.
Each box is fitted with an acoustic location beacon or "pinger", which can emit a signal for up to a month.
One device weighs about 5- to 10kg and is the size of a shoe box.