The best clues to what caused the crash of Air France Flight 447 could lie inside two devices, each the size of a couple of shoe boxes, some 3 miles deep in the Atlantic Ocean.
The tough, little "black boxes'' can withstand massive impacts and send a "ping'' noise from depths of 20,000 feet that ships with underwater microphones can use to locate them.
"It's extremely rare for (investigators) not to be able to extract the data, even if they're damaged,'' said George Bibel, a University of North Dakota mechanical engineering professor and author of the book Beyond the Black Box: the Forensics of Airplane Crashes.'
On Tuesday, military pilots found areas of floating debris about 600 miles off Brazil's northeast coast that officials thought might be from the Airbus A330 jet carrying 228 people. France sent a research ship that can deploy unmanned underwater vehicles to the site.
Crew members sent no radio distress calls, leaving investigators precious few leads. That makes the black boxes "the single most important element in solving that accident,'' said John Cox, an airline safety consultant and former pilot who lives in St. Petersburg.
The boxes, not black but painted bright orange for visibility, are the cockpit voice recorder and flight data recorder. Many planes still record with magnetic tape. But the Air France jet had modern versions that store data on memory chips, said Bill Reavis, a spokesman for Honeywell, which made the plane's recorders.
The voice recorder should have stored the last two hours of pilot conversations, audio alarms and other sounds in the cockpit. The data recorder should have kept 400 performance parameters, from the jet's air speed and direction to settings on its flaps and engine temperatures.
Black boxes must pass brutal tests to ensure they survive a crash. Researchers smash them with a 500-pound weight with a quarter-inch steel spike dropped from 10 feet. They cook them in fire at 2,000 degrees Fahrenheit for an hour and plop them in saltwater for 30 days.
Titanium or steel armor surrounds the mechanisms. The boxes are designed so water pressure at the bottom of the sea makes the cover close tighter to protect stored data. Aircraftmakers usually install black boxes in the tail, considered the safest location in a crash.
If a plane crashes into the water, a homing beacon transmits an ultrasonic ping that can be picked up by sonar and underwater hydrophones. The signal carries to the surface from nearly 4 miles underwater for 30 days.
Finding the boxes isn't always easy. Homing beacons have torn off from recorders. Layers of water at different temperatures, called thermoclines, can deflect beacon signals sideways, said Reavis of Honeywell.
Seas near the possible debris site are estimated to be around 15,000 feet deep, or just under 3 miles. That would make recovery of the black boxes among the deepest attempted by salvage crews, said Ted Lopatkiewicz, spokesman for the National Transportation Safety Board.
In 1989, a private contractor's underwater robot went 3 miles under the Indian Ocean to pluck the voice recorder from the wreckage of a South African Airways 747. U.S. Navy salvage crews have recovered black boxes from all recent water crashes investigated by the NTSB, said Lopatkiewicz.
The last black boxes not found were from the two hijacked jets that hit the World Trade Centers on Sept. 11, 2001. Investigators assumed they were incinerated by the intense heat, he said.
Researcher John Martin contributed to this report, which includes information from Times wires. Steve Huettel can be reached at firstname.lastname@example.org or (813) 226-3384.