How To Find a Plane at the Bottom of the Ocean
When You Don't Know Exactly Where to Start

by Jim Dawson and Devin Powell

As search crews found debris fields Tuesday in the area where Air France flight 447 apparently crashed into the Atlantic Ocean, John Perry Fish was waiting for the phone to ring.

Fish's company, American Underwater Search and Survey on Cape Cod, was involved recovering pieces of the 1996 TWA flight 800 crash in the Atlantic and the 2002 crash of China Airlines flight 611 over the Taiwan Strait.

He described the difficulties and procedures needed to find the remains of Air France flight.

"The water is deep in that region," he said, "some 7000 meters deep in the deepest parts, but averaging about 4000 meters. It is near the mid-Atlantic ridge, [an undersea mountain range] which runs from Iceland to the south Atlantic."

FINDING THE SPOT

The first problem in finding the debris and black boxes from the plane, he said, is that the aircraft was not being tracked on radar when it disappeared, "so you don't know exactly where to start. If you have a radar track, you can plot an area of a couple miles out from that point and start searching."

Without the radar, he said, the task is to find the floating debris and do "hindcasting," which traces the path of debris backwards as it floats on the ocean currents.

"They'll have to work with flotsam," he said. "You get the wind and current data and work back. You have to distinguish between the light material that is on the surface and exposed to the wind, and the buoyant material, which is floating, but just under the surface and not exposed to the winds."

Chris German, the chief scientist for the deep submergence group at the Woods Hole Oceanographic Institute on Cape Cod, said that even with two debris fields located miles apart, the backtracking can be done.

"You look at the ocean currents and wind and determine where the debris was 10 hours before, then 10 hours before that. You do that all the way back to when you think the crash occurred." Fish said that the hindcasting could trace out the path up to 30 days back in time.

SEEKING THE DEBRIS

Fish said that once a point of impact has been estimated, the search begins for a debris field on the ocean bottom. That can be done by towing an unmanned sonar-mapping submersible at the end of a miles-long cable or by sending newer, autonomous submersibles that map the ocean bottom on out their own, which come back to the surface with their data.

The unmanned vehicles can only scan a strip about a kilometer wide at a time. And they have to be close to the bottom to see objects smaller than a small car, German said.

"In rougher terrain, you are looking at surveying a couple of square miles per day," he said.

The task is made harder by undersea currents, German said. He once lost a submersible craft off a ship near Bermuda, he said.

"It fell in and went to the bottom and we knew where it went in," he said. "But there are deep-sea currents that affect objects as they sink to the bottom."

He found his lost submersible, he said, but it was at the far edge of the projected search area, pushed there by the deep currents.

Once the debris field is located, a vehicle that can detect the pings of the black box can enter the search, Fish said.

The box, actually orange and about the size of a loaf of bread, is a technology that "hasn't changed much in decades," said Michael Thompson, a technical manager at Honeywell, which supplies black boxes to airlines.

It records the sounds of the pilot's voice and the engine noise, as well as data on the plane's altitude, airspeed, and heading.

"It takes some luck to find the box in the debris," said Thompson.

HEARING SOUNDS FROM THE DEEP

Attached to every black box is an underwater location device, also called an acoustic pinger, that can function up to depths of 20,000 feet (about 3.8 miles), said Thompson.

When a sensor on this device detects water, it activates the pinger, which sends out short pulses of sound in every direction, once every second. These sounds are about as loud as an electric drill would be above water.

"Under normal conditions, they have a range of about a mile," said Tom Greenacre, president of Dukane's Seacom division, a company that manufactures the pingers.

The ultrasonic acoustic pings are too high-pitched for a human ear to hear, at 37.5 kilohertz. This frequency was chosen to stand out against the types of sounds produced by waves, weather, and wildlife, said Thompson.

To detect them, ships and submersibles use underwater microphones called hydrophones.

The environment around the black box can distort the pings. Ambient noises can mask the sound, or wreckage from the crash can absorb the pings if the plane ends up on top of the box.

The top layers of water in the ocean where the temperature changes sharply can bend or refract a pulse of sound. Like a prism bending a ray of light, these "thermoclines" can send a pulse off course and confuse listening devices.

So any device trying to detect the ping has to be lowered below the thermoclines.

"It's highly dependent on the nature of the debris ... there are just so many variables," Greenacre said.

LOCATING THE BOX

The battery of the black box lasts for 30 days. If the search takes more than 30 days and the black box batteries are dead, said Fish, the box can still be found "but you have to sort through the debris."

And that would be a difficult task, said German, who has spent time exploring volcanic thermal vents on the mid-Atlantic ridge.

"It's very dark down there," he said. "Even with lights and optimal equipment, you can only see tens of feet."

June 4, 2009

Copyright © 2009 Inside Science News Service