What is a black box and how do you find one? Searching for the last remains of ...

These devices (known as 'crash-survivable memory units' in the official jargon) could hold vital information, but the batteries powering their locator beacons only lasts for around thirty days.

Once these die the black boxes will lay silent on the bottom of the ocean floor, making their discovery even more unlikely. But what exactly are search teams looking for and just what do they hope to find?

What is a black box?

Black boxes are specialized devices designed to record the final moments of a flight and survive the extreme heat and pressures of a crash. There are usually two black boxes located in the rear of the plane (usually the last point of impact) each recording a streams of information: the first, the Cockpit Voice Recorder (CVR) stores the last two hours of conversation in the cockpit, and the second, the Flight Data Recorder (FDR) keeps a larger set of data outlining the altitude, airspeed and direction of the flight for the last 25 hours. Modern recorders include much more data - about everything from fuel levels to the position of wing flaps.

What are they made out of?

The 'crash survivable memory unit' is actually a steel cylinder containing the memory boards that hold the actual data. This connects to the various audio compressors that feed the information into the heart of the black box. There are various layers of materials protecting these memory boards, with an outer case made out steel or titanium covering layers of dry-silica insulation (to protect against heat) and an inner aluminium housing.

How tough are they?

To test the survivability of black boxes engineers subject them to a series of tests. These include crushing the unit in various places with pressures of up to 5,000 psi (pounds per square inch), firing it down an air cannon to create impact forces of 3,400 Gs (1G is Earth's gravity) and cooking it in a fire for one hour at a temperature of 2,000 degrees Fahrenheit. (1,100 degrees Celsius).

In order to test the units' ability to survive in water they are also immerse in pressurized tanks of salt water for 24 hours and non-pressurized tanks for 30 days.

How do investigators find them?

Black boxes are painted bright orange (the actual shade is 'international orange') to help with visibility and are equipped with water-activated beacons that send out ultrasonic pulses when the boxes hit water. These send out a pulse every second for thirty days after activation, transmitting their signal across ranges of 14,00 feet (4,267m). Two boats are currently searching for this beacon: one, a US submarine, and the second an Australian ship, the latter towing an underwater locator behind it close to the ocean floor.

Will they find MH370's black box?

It's possible but seems increasingly unlikely. Flight MH370 has been compared to Air France flight 447, which went missing over an ocean in 2009 for similarly mysterious reasons. It took two years for investigators to find the flight's black box, and even then the discovery was described as a 'minor miracle', with the device happening to land on a sandy area at the bottom of the ocean floor, rather than among mountains crags and valleys.

Why aren't black boxes better?

Although you can't really beat black boxes in terms of survivability, on the face of it there does seem to be a lot of ways they could be improved. If your smartphone can record hours of video and beam that to your computer from anywhere in the world, why can't black boxes send out more of their information? Ejectable black boxes that float on the ocean's surface are already in use by the US Navy, so why don't commercial flights have those?

Professor Krishna Kavi of the University of Texas suggests that it is ' sheer institutional intertia ' that is holding the industry back, with pilots fearing that advanced black boxes would lead to round-the-clock monitoring of their work. Others suggest that airlines' profits are simply so razor-thin that they don't want to invest in a technology that hardly makes for a comforting sales pitch: 'if you crash, at least they'll know why!' Incidents like the disappearance of flight MH370 might help to push the industry into action, but even then, change is likely to be slow.

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