Most people call them "black boxes," but aircraft flight data recorders and cockpit voice recorders should be called magic boxes.
The two boxes from EgyptAir Flight 990, pinpointed Friday on the bottom of the Atlantic by the USS Grapple, may allow investigators from the National Transportation Safety Board to quickly learn the cause of a crash that so far remains a mystery. And the faster they know why 217 people died, the faster they can prevent a similar crash.
If Flight 990's voice and data recorders contain usable information, investigators will know far more than just what the pilots said and did. They will know the pilots' emotional state, whether the plane responded as the pilots intended, and even whether the plane was hit by an explosion and exactly what explosive was involved.
Safety board investigators are often called "tin kickers," and it is true that an experienced investigator can often walk through a field of shredded metal and broken bodies and read it like a book.
But as new-generation airliners begin to dominate aviation, crash investigation has gone digital, creating new opportunities to delve into a plane's secrets. Many computer chips in the cockpit and the engines have "nonvolatile memory" that can be deciphered after a crash.
But the plane's cockpit voice recorder and flight data recorder are the mother lode.
For most of the history of commercial aviation, recorders were relatively unsophisticated. Voice recorders were hardly more than tape recorders, and data recorders ranged from rolls of tinfoil to analog instruments capable of recording only a small number of parameters such as airspeed and altitude.
Following the crash of USAir Flight 427 at Pittsburgh on Sept. 8, 1994, safety board Chairman Jim Hall began a crusade to force airlines--as the Federal Aviation Administration has since ordered--to upgrade flight data recorders because the Boeing 737's recorder had only 11 parameters and was of limited value.
The EgyptAir flight data recorder, by contrast, contains 74 measurements of airplane movements, control surface movements, control positions, altitude, aircraft speed, outside wind speed, engine operation and warning system activation. The cockpit voice recorder contains the last 30 minutes of cockpit sounds.
They are the most protected piece of any jetliner. Modern recorders are no larger than a thick disk that easily fits in the palm of the hand. They are encased in plastic and surrounded by electronic equipment that also serves as a buffer for the internal disk and is packed into an orange-colored container that can withstand crash forces and fires. The orange box "is basically a dust cover," said one investigator.
Crash investigation is replete with examples of crashes solved by flight and voice recorders, sometimes by only one sound, one phrase or one electronic clue.
On May 26, 1991, a Lauda Air Boeing 767 suddenly dived into the jungles of Thailand, crashing with such force that even the well-protected flight data recorder was destroyed. But the cockpit voice recorder survived.
On the voice recorder, just at the end, one pilot of the Austrian airliner said "thrust reverser" in German. That gave the tin kickers the clue about where to concentrate their attention in the shredded wreckage of the first Boeing 767 to crash. The thrust reverser, a form of engine brake, had deployed in flight, setting up air currents that destroyed the lift on one wing and sent the plane flipping into a dive.
On Feb. 6, 1996, a Birgen Air Boeing 757, carrying German tourists, took off from the Dominican Republic. Within minutes, the pilots seemed to lose touch with reality and, in confusion, dived into 7,600 feet of sea water.
Recovery of the plane and bodies was not possible at that depth, but a remotely operated submarine retrieved the recorders. The plane fell for a simple reason: a blockage in the plane's pitot tube, a small probe that helps determine the plane's airspeed.
The plugged pitot tube fed confusing readings to the plane's instruments, leaving the crew disoriented over the dark ocean.
The value of the information from a flight data recorder is obvious. But the value of the cockpit voice recorder goes far beyond a mere recording of pilot voices.
A voice recorder gathers information from four microphones, one next to each pilot, one in the instrument console and an "area mike" that gathers sound overall.
The console mike is sensitive to vibration in the plane's airframe, and since vibration travels through the airframe faster than sound will arrive at the other mikes, it is possible to pinpoint some loud sound--such as an explosion--by measuring the minuscule time difference between sound and vibration.
Explosives leave distinctive sound patterns, allowing the safety board laboratory staff to determine whether dynamite, plastic explosive or something else was used.
The final sound on the cockpit voice recorder of Trans World Airlines Flight 800 in 1996 initially stumped investigators because it fit no known explosive. Further work determined it was more characteristic of the explosion of a fuel-air mixture. The cause of the Boeing 747's breakup was a fuel tank explosion.
Another mystery was the sound of two "thumps" on the voice recorder of USAir Flight 427 just as it rolled and dived into a hillside near Pittsburgh. Investigators knew that the plane ran into turbulent air in the wake of a plane flying four miles ahead, but that didn't necessarily explain two thumps.
But when the safety board's recorder expert, James Cash, rode a test flight to determine the effect of such wakes on a Boeing 737, he heard the distinctive two thumps as the plane hit the wake at a certain angle.
Another field of cockpit voice recorder research is to determine the emotional state of the crew and its performance. Pilots often speak in a deliberate monotone, but they are still subject to stress and fear.
In the USAir 427 crash, investigators used speech characteristics to gauge the crew's stress level, a technique pioneered by Russian aviation authorities, to estimate the force being applied by the crew to controls.
Sometimes voice and data recorders together can solve a mystery.
In the May 1996 Valujet onboard fire and crash into the Everglades, a loud bang from the cargo hold was caught on the voice recorder but was not readily identifiable.
At exactly the same moment on the flight data recorder, the plane appeared to rapidly gain several dozen feet in altitude and just as rapidly descend while sharply speeding up and then sharply slowing down, something physically impossible.
The answer? A tire in the cargo hold had overheated in the fire and exploded. It created a brief pressure surge inside the airplane, affecting the altimeter and airspeed indicators.
Voice and data recorders are only one part of a crash investigation. The safety board's laboratory also performs metallurgical and other tests, and nothing has replaced the tin kicker for interviewing witnesses, collecting samples, studying damage patterns, reviewing maintenance records or making the final judgment of why people died.