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When planes crash at sea

[PARIS] Debris and bodies from an AirAsia Airbus jetliner have been located in the Java Sea off Indonesia, but finding and recovering the aircraft could take time and cost millions of dollars.

Here is a recap of similar crashes at sea.

Nine months after it disappeared with 239 people on board over the Indian Ocean on March 8 2014, a Malaysia Airlines Boeing 777 headed to Beijing from Kuala Lumpur, flight MH370, has still not been found. It is not known exactly where the plane might have gone down.

It took 23 months to locate Air France flight AF447, an Airbus A330-230 headed to Paris from Rio de Janeiro with 228 people on board on June 1, 2009. The plane went down in the Atlantic Ocean, but those searching for the plane had a good idea of where to look.

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Recovery operations for the AirAsia flight, which was carrying 162 people, will benefit from the fact that the plane crashed in relatively shallow waters and that debris was found quickly.

When a plane is lost over water, a race against the clock begins to find the "black boxes", which are in fact orange and contain recordings of cockpit conversations and crucial flight data.

Civilian aircraft carry two such devices, the Flight Data Recorder and the Cockpit Voice Recorder, each weighing around seven kilos (15 pounds) and protected by a steel casing that resists water pressure in depths up to 6,000 metres (20,000 feet).

If the recorders are submerged, a high-frequency "ping" is emitted every second for at least 30 days, and it can travel up to two-three kilometres (miles).

There is a wide variety of other devices available to search and rescue teams as well, including satellites, robots and underwater drones equipped with high-definition cameras, sonars, hydrophonic recorders and location buoys.

Statistics have proved useful too. Analysis based on theories developed by Thomas Bayes, an 18th century English scientist, was used by the Woods Hole Group to find the AF447 wreckage.

Once an aircraft is located, salvage ships such as the French "Isle de Sein" send robots down to work at extreme depths.

The Remora robot raised 104 bodies and debris that included the black boxes and engines from the Air France crash site, which lay at 3,900 metres, working from April to June 2011.

Fifty other bodies were recovered shortly after the crash, but 75 remain missing to this day.

The "Ile de Sein" is able to drop cables to a depth of 6,000 metres, and lift up to 10 tonnes at a time.

It has two sister ships, the "Ile de Batz" and the "Ile de Re", all operated by telecommunications group Alcatel Lucent in collaboration with Louis Dreyfus Armateurs.

In January 2004, the "Ile de Batz" recovered the black boxes and debris from a Boeing 737 operated by the Egyptian company Flash Airlines that crashed into the Red Sea after taking off with 148 people from Sharm el-Sheikh.

The "Ile de Re" carried out a similar operation after a regional Air Mooera flight crashed in French Polynesia with 20 people aboard on August 9, 2007.

Salvage operations are expensive of course. The AF447 recovery cost around 35 million euros (US$42 million).

More than US$100 million has already been spent searching for the Malaysia Airlines flight MH370, making it the most expensive on record.