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Under the deep blue ocean: the search for MH370’s black box

As the effort to find Malaysian Airlines flight MH370 moves inexorably towards the recovery stage, the challenge of finding the plane’s flight recorder (called the “black box” even though it’s actually…

Australian Navy vessel Ocean Shield left Perth yesterday to join the search for missing flight MH370 and its black box flight recorders. AAP/Tim Clarke

As the effort to find Malaysian Airlines flight MH370 moves inexorably towards the recovery stage, the challenge of finding the plane’s flight recorder (called the “black box” even though it’s actually bright orange) on the ocean floor will become a game of blind man’s bluff spread over thousands of kilometres.

The inability to penetrate the darkness of the ocean has long been a major handicap for deep-sea exploration. It is why we know more about the surface of the moon than the bottom of the deep ocean.

In a nutshell, the problem is that seawater is opaque to electromagnetic radiation, such as radio waves, microwaves and X-rays. This means that almost none of our normal techniques to “see” over long distances can be used in the ocean.

As you go deeper into the ocean, it gets darker. People who have dived know that colours are quite different once you get to 20m deep. Red starts to look green and, eventually, everything gets greyer.

Once you get down more than 100m, the ocean is essentially pitch black – leaving almost 4km of darkness in most parts of the ocean – because light is absorbed in seawater.

The bright colour of black boxes doesn’t help when submerged in 4,000m of seawater. AvgeekJoe/Flickr, CC BY-SA

It’s not only visible light that gets lost. All forms of electromagnetic radiation are sooner or later damped out.

Most of our techniques to remotely study objects (the moon, stars, but also clouds in the atmosphere or ships on the surface of the ocean) rely heavily on electromagnetic radiation. This type of radiation forms the basis of radar, GPS and essentially all telecommunication.

But oceanographers who want to study the deep ocean can’t use most common tools, such as satellites or radio waves.

Listen to this

The only signal that can travel through seawater is sound. Sound travels extremely well through seawater; much better than through air. This is why some whales and other marine animals can hear each other over thousands of kilometres.

It’s no surprise, then, that black boxes on aeroplanes have “pingers” that transmit a sound signal. But while sound travels long distances under water, it isn’t a very efficient way of transmitting information: you can’t pack too much information in it and it costs much more energy than electromagnetic radiation.

Another problem is that sound doesn’t travel in a straight line in the ocean and can reflect off sea mountains to create echoes, making it sometimes difficult to locate its origin. The speed and path of sound varies with temperature and salinity and it can bend up or down. In some extreme cases, it can even be reflected in the middle of the ocean.

As sound bounces around under water it can change intensity and create confusing reflections. RA Zingarelli and DB King

In layers where the temperature and salinity change rapidly with depth, these layers act as mirrors and can cause some sounds to be completely lost. Naval submarines are extremely good at hiding themselves underneath these mirrors, making them all but invisible to ships on the surface.

Oceanographers have been struggling with this lack of a good way to transmit signals through the ocean for decades. It means that submarines can’t use GPS to get their position, and that data from subsurface instruments need to be either locally stored on logging devices (which then need to be retrieved) or transmitted via cables to land or ship.

What happens when the pings stop?

If the flight recorders can’t be found using the pinger, it is very likely the ocean floor will need to be scoured using sonar. Again, this is technology based on sound, but is not nearly as accurate as, for instance, radar or photography of the land surface.

A diagram of seafloor sonar ‘pictures’ taken by a ship. NOAA's National Ocean Service/Flickr, CC BY

The real dilemma for the search team is how high above the ocean floor to take the sonar “pictures”:

  • too high up and the plane wreck might be missed because the resolution is too poor
  • too close to the ocean floor and it will take much more time to map the entire area.

In this sense, it doesn’t help that we don’t have any good maps of the ocean floor already. If we would have such maps, we could simply put post-plane crash maps on top of them and see where they differ. These locations could then be scrutinised for possible wreckage, and regions where there are no differences can initially be skipped.

The problems faced by the search time of the MH370 are very similar to those of the Air France AF447 flight that went down in the Atlantic Ocean in 2009. Back then, the search team was not able to locate the black boxes with a pinger signal.

The search took two years, and it was only through months of scouring the ocean floor with submarines and sonar that the black box was finally found.

Let’s hope MH370’s black box can be located sooner and provide some much-needed insight to the flight’s mystery.

Join the conversation

9 Comments sorted by

  1. Zvyozdochka

    logged in via Twitter

    Another problem that I understand is likely, is that these 'black' boxes overwrite or loop only the last 2 hours of data.

    If that's the case, it seems extraordinary in a multi-million dollar aircraft - what cost some extra flash-memory storage?

    1. James O'Neill


      In reply to Zvyozdochka

      There is a systematic campaign in most of the media to spread what at best is misinformation and in most cases disinformation about flight 370. Since 2006 Boeing aircraft have been equipped with an uninterruptible autopilot system that enables any aircraft thus equipped (as was 370) to be remotely controlled. There is another system developed by Raytheon for the FAA called an "Advanced Route Evaluation System" that provides a similar capability. Thirdly, Rolls Royce have the capacity to continually…

      Read more
  2. David Pearn


    Someone needs to invent a 'rattle' driven by currents and not dependent on battery power.
    To commence a search with this particular piece of equipment, without a single confirmed item from the aircraft is simply an exercise in pr unfortunately.

  3. Chris O'Neill

    Retired Way Before 70

    "The problems faced by the search time of the MH370 are very similar to those of the Air France AF447 flight"

    Most likely worse and maybe much worse. AF447 was on course. At best, the course of MH370 is inaccurately known. Taking two years to find it, if it is in the Indian Ocean, might be optimistic.

    1. David Pearn


      In reply to Chris O'Neill

      Exactly, an almost impossible task and the media should start being realistic about a positive search and recovery outcome.
      Searching by side scanning submersibles will, I assume, require a vessel to be on station in the southern ocean for a very long time in heavy seas with no defined track to even start from.
      If the aviation industry finances it, the US navy's submarines might eventually succeed using their detection systems.

  4. Peter Madden


    I feel so sorry for the people directly involved, but like any bad thing, good can be found, and in this case it is the countries that have got together and assist, and I particularly note China and Japan. I think it proves that it you take the politics out of any situation, normal people can work well together.

    1. Raine S Ferdinands

      Education at Education

      In reply to Peter Madden

      Perer, the vast majority of passengers on flight MH370 were Chinese citizens. China responded rather slowly … I would say. Singapore, Vietnam, etc responded immediately (first location proximity). That China was initially cautious about revealing their satalite locations and information pissed me off. If Australians were in large numbers on that flight, I am very certain that Australia would have jumped in immediately and taken a leading role in the search and investigation processes. It took a mini revolt (in Beijing) by the relatives of the lost flight passengers to wake up Chinese officials.
      Agree, however, that "if you take the politics out of any situation, normal people can work well together".
      My heart goes out to all relatives and loved ones of the passengers on board that flight.

  5. Matt Bennett

    logged in via email

    Get REAL! Nothing like AF447' we knew where that baby was.

    The flight recorder is likely to be slightly cactus when it's stumbled upon by an android family out for a Sunday drive on the Indian ocean floor in, say .......2837!

    For God's sake, it's not uncommon for land-based wrecks to go missing for decades!

    Common sense would suggest to me it's likely to be centuries, if not aeons, before any trace is found.

  6. Andrew Beavis

    Mac Guru

    You know, when I worked in marine science 30 years ago I put down a camera fitted with a pinger in the Tasman in about 1km of water and I remember there were many echoes off intervening layers in the sea water caused by temperature and perhaps salinity. What if the pings are causing multiple signals that are carried through a conduit of layers for 100km ? Then as conditions change the ultimate range would alter and the signal would drift over time , depending of the depth of the detector array. The Ocean Shield might not, at the moment be particularly close to the source. If the area can be narrowed down then a towed video can be manually scanned for a debris trail, but at the moment it would be a very remote chance if i am right ..., and I am not sure if a sidescan sonar would work on airplane wreckage or not. My bet would be on visual, but only if the location can be narrowed down.