Air France tragedy: How to hunt for deep-sixed black boxes

They record cockpit communication. They log instrument readings. They chronicle an airliner's behavior, tracking 400 on-board processes at once.  They are the so-called black boxes airliners carry. And they will be prized witnesses in the investigation into Monday's Air France Flight 447 tragedy.

If investigators can find them.

You can read more about the plane's disappearance here, here, and here.

For a sense of what a search might involve, let's check in with Chris German. He's the chief scientist for the deep-submergence group at the Woods Hole (Mass.) Oceanographic Institution. If you need to go deep – either in person or remotely – his group's your team.

In this case, the French are sending their own ship and submersible. Nevertheless ...

Step 1. Figure out where the plane went down.

Colleague Dave Scott points to a retired Air Force meteorologist who has tried to reconstruct Flight 447's path. You can read about that here. Searchers have found debris and identified it as belonging to the airliner. Using satellite data on currents and weather information regarding winds, experts will try to work back to an estimated point of impact. That's where the hunt for the boxes will begin.

Step 2. Listen for the box's telltale ping. Bill Reavis, a spokesman for Honeywell International Inc., which built the black boxes for the airliner, explained in a quick phone chat that these boxes are able to withstand depths greater than 20,000 feet. And for 30 days, devices attached to the boxes will emit pings on an underwater "here I am" frequency. The ping is outside the range of human hearing, but it's easily detected by underwater listening devices – if nothing stands in the ping's path.

What could disrupt it? A boundary formed by differences in water temperature, for instance. Essentially, if the pinger lies below about 1,000 feet, ship-board sonar won't be able to find it, Dr. German explains.

That's where robotic underwater vehicles, either towed or untethered, come in handy. In 2007, for instance, the US Navy research ship Mary Sears found a black box – a cockpit voice recorder – belonging to a downed Philippine airliner. The crash killed all aboard, and the box came to rest in 5,600 feet of water. The Mary Sears found it by towing a device designed to pick up pings.

Step 3. Once you know you're in the neighborhood, pinpoint the box's location. In the case of Flight 447, if Dave Scott's blogger is right, the plane went down in about 10,000 to 13,000 feet of water. That's the good news – a workable depth. Scientists send robotic vehicles down that far all the time, or at least when funding is available.

The challenge: The estimated site is not far from the mid-Atlantic ridge. This is relatively rugged undersea terrain. It's subject to tugging, hauling, squeezing, and fracturing from tectonic forces at work along the ridge, where is where fresh crust wells up to form Earth's solid skin.

Towed or free-traveling vehicles can spot items three or four inches across on the bottom. But the robotic subs German works with can only cover a swath of seafloor perhaps 150 feet on either side of them. One sub could cover perhaps two or three square miles in a day.

This puts a high priority on pinpointing the ping as accurately as possible as soon as possible.

Once the box is located, searchers can use remote vehicles to pick them up and return them to the surface.

"It's not that we don't have the tools to do it," German says. "But it's going to take some pretty smart people to put all the pieces together to get the job done."

And a postscript:

In a chat with Richard Healing, a former member of the US National Transportation Safety Board, he cautions about getting too wedded to any explanation for the crash before those boxes are retrieved and analyzed. Some folks, for instance, have focused on lightning as a possible trigger.

But Mr. Healing explains that airliners are designed to dissipate lightning strikes.  If the plane had been hit by a bolt, the cockpit crew would have radioed in the strike; bolts don't take out a airliner's entire electronics systems.