So What Are Black Boxes?

Within days of the crash of ValuJet Flight 592, federal investigators were able to pinpoint the exact moment when things started to go wrong.

The reason: information gleaned from the plane's flight data recorder.

While they have yet to determine the precise cause of the crash, investigators of the National Transportation Safety Board say this flight data will make up the backbone of their case. For this reason, two flight recorders are required equipment aboard all commercial aircraft. The shoebox-sized recorders - one for the plane's major components and the other for cockpit voices - are built to survive the most violent impacts and are often the sole witnesses in a devastating air accident.

Divers are still searching for the voice recorder of Flight 592 in the muck of the Everglades, but when it is found, every click and pop will be examined. Along with hearing cockpit voices, engineers can listen to background noise and determine engine speed, and expert pilots can distinguish the sounds of different warning signals and the low hum of a landing gear being lowered.

While early "black boxes" were in fact black, today's versions are painted orange so they can be found more easily. Modern data recordings can tell investigators hundreds of details, from flap position and smoke alarms to the airspeed, altitude, and direction of a flight in its final moments. NTSB investigators feed all this data into a computer transcript that includes the precise local time for each event.

All black box designs are put through a series of survival tests. A high-strength steel case and interior cushioning help a recorder pass an impact test, in which the box is shot out of a 10-inch cannon into a wall. One-inch thick insulation is added to allow recorders to remain functional even after being roasted at up to 2012 degrees F. The boxes also are immersed in vats of deicing fluid, hydraulic oil, jet fuel, and seawater to see if they resist corrosion.

Despite all this innovation, many flight boxes are damaged. Older magnetic tape recorders, such as the one aboard Flight 592, have more moving parts that require maintenance, and they can be vulnerable to heat. But while half of the tape-based recorders over the past 30 years have been damaged or destroyed by fire, manufacturers of newer recorders say that more-durable computer memory chips will help raise that survivability level to around 90 percent.