Roberto Candia/AP
Brazil's Air Force Brig. Gen. Ramon Cardoso talks to the media about search operations for the
missing Air France flight 447 in Recife, Brazil, on Thursday, June 11, 2009.

Deadline in Air France Search Illustrates Importance of Black Boxes

June 23, 2009 06:00 PM
by Liz Colville
As investigators for Air France Flight 447 spend a final week listening for sounds from the plane’s black box, a look at the technology behind the black box and how it’s found.

Black Box Batteries Last One Month

A nuclear attack submarine and two Dutch tug boats carrying listening devices from the U.S. navy have been searching for the black box from Air France Flight 447, Bloomberg reported last week. Four other French vessels have since joined the search, The New York Times reported.

The plane, an Airbus A330, crashed about 600 miles off the coast of Brazil during stormy conditions on June 1, according to The New York Times. All 228 passengers and crew members were killed.

So-called black box recorders have “pingers” that emit signals that can be heard more than a mile away, Bloomberg explained. But the batteries only last one month, so investigators have until June 30 to find their target.
“We have not located the recorders,” Martine Del Bono, a spokeswoman for France’s Office of Investigations and Analyses, told The Times. “They are hearing a lot of sounds. We cannot confirm that it is from the black box.”

Investigators cautioned that the black box could have separated from the “pingers” upon impact with the water.

Background: The technology behind the black box search

“Black box” is actually a misnomer: The box is bright orange and “about the size of a loaf of bread,” The Times explained.

The pingers attached to the black box “have a very specific audio ‘signature,’ emitting an electronic impulse in every direction, once per second,” according to The Times. “At 37.5 kilohertz, the ultrasonic signal is too high-pitched for the human ear to hear, but is distinct from the sounds produced by waves or underwater wildlife.”

Automatic messages transmitted before Flight 447 went down showed that the aircraft’s speed sensors were not working correctly. If found, the black box could tell investigators whether the pilots were prompted to fly at the wrong speed because of this malfunction, Bloomberg explained.

The U.S. Navy has supplied investigators with “towed pinger locators,” which contain a “passive listening device for detecting pingers that automatically transmit an acoustic pulse,” according to a Web page about the devices from the U.S. Navy. A towed pinger locator is specifically designed to find “emergency beacons from downed Navy and commercial aircraft.”

The boats tow the pinger locators at very slow speeds at a maximum depth of 20,000 feet, according to the U.S. Navy. Phoenix, the company that the U.S. Navy usually contracts for deep water searches, has had recent success with the devices, noted Bloomberg.

In March it “recovered parts of a B-52 bomber that went down in 3,500 meters of water off Guam. Last year it found the black box from a Boeing 737-400 Indonesian Adam Air Flight that went down in 1,650 meters of water off Java,” Bloomberg reported.

Reference: How black boxes work

A black box is a loose term that actually describes two devices: the flight data recorder (FDR) and the cockpit voice recorder (CVR), HowStuffWorks explains.

According to the book “Great Mythconceptions” by Karl Kruszelnicki and Adam Yazxhi, available on Google Books, the Wright brothers actually invented a precursor to the black box—a “primitive device to record the revolutions of the propeller” on a plane.

The first black box followed the first commercial jetliner, the Comet, which “began to fall out of the sky—and nobody knew why.” Starting in 1957, it was required on all aircraft weighing more than 20,000 pounds. The earliest devices recorded essential data about the plane using “metal foil and steel wire.” The modern black box “has no moving parts and records directly onto solid state memory,” much like a computer. But older models of black boxes still exist on some aircraft.

The FDR can record up to 24 hours of data about “some 700 different aspects of the plane,” according to “Great Mythconceptions.” The CVR, if using solid-state memory, can contain up to two hours of audio from the cockpit (old data is recorded over in two-hour increments).

Both devices are housed in the tail of the plane, which is usually last to hit the ground or water. They are encased in aluminum, followed by heat-insulating material and stainless steel. They can withstand 2,000 degrees Fahrenheit, 3,400 G of acceleration, salt water and the pressure of the ocean’s depths. “All this high-end engineering means that a Black Box costs about $20,000-30,000,” according to “Great Mythconceptions.”

The black box is run by power generators that derive their energy from a plane’s engines. The FDR gets its data from the front of the airplane, where information from the plane’s many sensors is sent to something called a flight-data acquisition unit (FDAU), HowStuffWorks explains.

The CVR doesn’t just track the voices of the pilots; it can also pick up “any ambient noise in the cockpit, such as switches being thrown or any knocks or thuds,” according to HowStuffWorks. There are usually up to four microphones in the cockpit: on the pilots’ headsets and in the center of the cockpit.

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