Air France 447: How scientists found a needle in a haystack

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The cockpit voice recorder from Air France 447, as it was found at the bottom of the Atlantic Ocean.

Last weekend, investigators announced that they had recovered the flight data recorder from the wreckage of Air France 447—a jetliner that crashed in the deep Atlantic two years ago. But, while the discovery of the data recorder is recent, the story of how Flight 447 was found goes back a month.

This year's search was the fourth attempt to find the wreckage of Flight 447, and it probably would have been the last, even if the plane hadn't been found. Previous searches had been done by boat, mini-sub, and—back when there was still a chance of catching the audio signal from the plane's black boxes—underwater acoustic sensors. In 2010, scientists from the Woods Hole Oceanographic Institute were brought in to search for the crash site using autonomous robot subs. Still nothing had been found.

On March 22, 2011, the Woods Hole team set out from Brazil to try again. They'd barely been at the search location for a week when they found what they were looking for. On April 3, researchers spotted the plane's debris field, 13,000 feet down, smack in the middle of a massive underwater mountain range.

The success was astounding, but I wanted to know ... what made this search different from the others? What could the team from Woods Hole do that other groups could not, and how did their system work? To find out, I spoke with Mike Purcell, senior engineer with Woods Hole, and the chief of sea search operations for the mission.

Maggie Koerth-Baker: Your team found Flight 447 with the help of an autonomous submarine called the Remus 6000. Can you tell me a little about the history of that sub? What could the Remus 6000 do that previous systems couldn't?

Mike Purcell: The first one was developed in 2001. Really, they have a greater depth limit. There are no other deep water subs that can go to 6000 meters. That's one way it's unique. Also, between the six Remus 6000's that exist out in the world right now, there's probably been more missions done with a Remus 6000 than any other deep water AUV.

To do a search, the Remus 6000 gets a mission program, a track line to swim. It goes into the water and uses various navigation techniques to swim the track line. There isn't anybody actively controlling it. But it's also not as smart as you might think. It's not making decisions based on terrain, other than staying some fixed altitude off the bottom. It can't go around things or avoid stuff that might be in front of it. It does go up over mountain ridges, but the Remus 6000s do sometimes run into things, too. They don't have the full sensor capacity and independent thinking to make decisions that some totally autonomous robot might. One reason that's the case—it's just harder to do that in the water than in the air. We're really limited to one kind of sensor, acoustic sensors, underwater.

MKB: What kind of research do Remus 6000 subs normally work on? Was this search different in any way, from a technological or logistics perspective?

MP: Our lab ... we've been involved in the development of AUVs. We've been making the newer and better ones over the last 15 years. It was only in about 2008 that we started getting involved in operations. We purchased a couple Remus 6000s and we're the operators. They were involved in search for Amelia Earhart's airplane. We did some localization of deep corals in Gulf Stream off of Florida. We mapped the Titanic site with AUV's last year. And then we've now worked on the Air France survey twice, once in 2010 and once in 2011.

Even when we've done these searches for the airplanes there's been a tremendous amount of data collected, and that's been made available, or will be made available in the future, to the science community. What kinds of things can people do with seafloor data? I'm not a geologist, so I'm not totally sure what they might do. But a lot of the seafloor is totally unexplored. We've got about 1500 square miles mapped. And I think there's a lot of interesting geography there in the Mid-Atlantic Ridge where we did this search.

MKB: How many people involved in running one of these searches, and what do they do?

MP: We had three vehicles out there. When we're running three vehicles we have 12 people, working in two 12-hour shifts. There's six people on each shift. And they're doing things like getting the vehicles in and out of the water. Reprogramming the vehicles. Tracking the vehicles. There's usually two AUVs in the water at all times. And there's a guy who is dedicated to processing the data.

MKB: The Remus 6000s had previously been involved in the search for Air France 447, but hadn't found it before. Was there a major location change, or some other shift in how the search was done this time around? Were you involved in deciding where the search would happen?

MP: We were out there first in 2010, and there'd been a pretty big modeling study that guided the search then. Of the entire area, which is 17,000 square kilometers, 7,000 had been what we were search going into this year. The plan was to search it all. There was one difference, we just decided to start close to the last known position of the plane, instead of further away from it. The BEA [Ed: Bureau d'Enquêtes et d'Analyses, the French air safety investigators] identified three search zones, big areas that they wanted us to do in order, and then, from there, we sort of had the freedom to decide where we start in those areas. So we started out based on where we left off last year.

MKB: The mid-ocean ridge, where the search was conducted, has been described as something like an underwater Himalayan mountain range. A lot of reports I've read on it made it sound very foreboding and not like a place where it would be possible to find anything. But WHOI has been doing research on the mid-ocean ridge for decades. Is the scary reputation deserved? What challenges do you face doing research in that location, as an organization that has experience with it?

MP: So, I think this mission was different for us in that we were trying to search such a huge area. We needed our vehicles to swim up and down those mountains. The water out there was 4000 meters meters deep at the deepest spot and very close to that was where we found the wreckage. But just a few miles away it was only 2000 meters meters deep. There are some very steep mountains.

We had a pretty good contour map of the seafloor, but you always learn some new things when you put the vehicle down there. Occasionally it would run into a mountain or cliff. Sometimes, when that happens, it stops the mission. We have a drop weight on the vehicle and it will float back to the surface. Other times a crash leads to problems a couple of missions later&mash;the AUV will suddenly develop an electrical problem. Sometimes, the cliffs fall away so fast that we can't swim down the slope as fast as its falling off, so we don't get good data and have to go back and do it again swimming up the slope. Compared to past experience here, the fact that we were trying to cover so much area was the new challenge.

We can do 100 square kilometers in a day with two vehicles ... but that really depends on geography. If the bottom is flat we can do more. We've done 180 square kilometers in a day with really flat terrain.

MKB: How does a search with the Remus 6000 work, before the point where you know you've found what you're looking for? Are you back on the boat watching live video from the sub, or is it a bit more "blind" than that, so to speak?

MP: Fairly more blind. There's no data transmission back to the ship while the vehicles are in the water. We get status messages—acoustic messages that come in periodically and tell us how deep it is, latitude and longitude, just a status check to tell us whether there's problems. When the AUV gets back, it takes 45 minutes to download the data and then another half hour to process and get a good look at it. During that time the other team is switching out the battery and getting the vehicle ready to go back in. The ideal is that a vehicle is only out of the water for three hours, while somebody is looking at the data to decide where we go from here and are there things we want to look at again. When we're running three vehicles we get a data dump three times a day.

What we get is images of the acoustic data. The vehicle sends out an acoustic ping every second, it goes out 700 meters on every side of the vehicle. That gets logged and turned into an image. It'll do that for about 20 hours at a stretch.

MKB: When your crew first saw what turned out to be the wreckage, what would they have been looking at? Does the image look anything like a wrecked plane, or is this a much more abstract sort of thing?

MP: It's a lot more abstract. It just looks like a bunch of bright returns on an area of no return. We're looking at a 24 inch monitor and we've 1400 meters of space that is being imaged on that monitor. Basically we were looking for a lot of small returns over a big area.

I think it takes a considerable amount of time to get to be good at identifying the data. Lots of people can look at a flat seafloor and see what sticks out. But once you have to start dealing with terrain changes, being able to pick wreckage out of that takes a lot more experience. It's good to have an experienced person looking at this stuff. Our guys probably had 15 years.

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Bright spots on a dark field. How the wreckage looked to Woods Hole data processors.

MKB: How do you tell the difference between stuff that's supposed to be on the sea floor and what you're looking for?

MP: Sometimes it's the strength of the return. Sometimes ... you might have a ridge line that's mostly sediment but some exposed rocks. So you see there's six bright spots coming out of the ridge, but you can also tell there's a ridge there. So you figure it's probably natural. You get a feel for what's natural and what's not natural. Metal tends to have sharp or square edges, That sends back a stronger signal than rounded rocks that have been down there for a million years or more. In this case, the airplane was out in the middle of a very flat area with no natural camouflage that would make it hard to find.

MKB: Did the capabilities of your machinery make the most difference, or did it have more to do with the experience that your operators have conducting this kind of seafloor search?

MP: A lot of people would have found the wreckage where it was, with other vehicles or even towed systems. But when you look at the big picture, the whole area that had to be searched, the Remus vehicles were the best tool able to work in that terrain. And the fact that we had three of them working around the clock. That just increases our search rate. That factor there is both a vehicle thing and it's also the people out there doing the operating.

MKB: During the search you sent progress reports back to victim's families. How do you write a progress report like that? How much different is it from the way you frame findings, and wording you use if you're writing a progress report for something less emotionally charged?

MP: I sort of lucked out. I never had to send a progress report. It was supposed to go out every Tuesday. They gave us a waiver on the first Tuesday because we'd just gotten to the site, and then we found the wreckage on Sunday. We had a BEA representative on board and I talked with him every day. A lot of people had a lot at stake in that search. They've been working at it for a long time. That was more of a factor than the fact that it was a crashed airplane. That sort of changes the attitude. People were serious about the job. They took it seriously and wanted to complete it.

Images: BEA, via Reuters