To investigators struggling to reconstruct what happened to TWA Flight 800, nothing is more critical than retrieving the two shoe box-size containers that cradle the flight data and cockpit voice recorders. In airline disasters, the "black boxes" are often the only expert witnesses that survive.

"They tell us a great deal about what the crew was up against," says Bill Hardman, marketing manager for Lockheed Martin Advanced Recorders in Sarasota, Fla., one of the leading manufacturers of "black boxes."

In the case of an explosion, the recorders can show the suddenness of the catastrophe and thus rule out a mechanical failure.

Their name doesn't tell you what they look like: The boxes are bright orange or yellow -- not black -- to make them easier to spot, whether in the muck of the Everglades, ocean waters or on a snowy mountainside. A radio transmitter that broadcasts a signal is part of each box.

Each weighs about 20 pounds and measures 4 by 6 by 8 inches -- about the size and weight of a cinder block. Both of the boxes are fitted as far to the rear of the plane as possible -- because most planes crash nose first and leave their tails relatively undamaged.

The flight data recorder tracks the most recent 25 hours of a plane's performance, providing measurements about speed, altitude, acceleration, thrust from each engine and direction of the flight in its final moments.

The cockpit voice recorder relies on microphones mounted on the flight crew's headsets and on the cockpit ceiling to pick up the sounds inside the cockpit -- including announcements to passengers. The "box" records continuously but saves only the most recent 30 minutes.

Because pilots often die in crashes, the last moments of conversation with each other and air traffic controllers are usually the most important.

What a cockpit crew says and does -- or doesn't say -- can help pinpoint the cause of a crash. After the crash in August 1987 of a twin-engine jet in Detroit, investigators discovered a fatal crew error. Perhaps distracted by bad weather, the pilot forgot to complete a pre-takeoff checklist and thus failed to set the wing panels that provide lift critical for getting the plane off the ground.

But the "boxes" do not guarantee an understanding of events. While the cockpit recorder provided gut-wrenching remarks by the crew of USAir Flight 427 in Pittsburgh two years ago, it only helped rule out some of the possible reasons for the subsequent crash. But the real cause of the disaster -- a crash that killed 132 people -- remains unknown.

The "black box" has changed since Charles Lindbergh's Spirit of St. Louis carried a primitive version on its trans-Atlantic crossing. For years afterward, data recorders -- sometimes tucked inside the airplane's wheel well -- were strictly optional.

In 1947, the Civil Aeronautics Board made them mandatory, but the early devices were so unreliable that a year later the agency dropped the requirement. As technology improved, the board required flight data recorders on large carriers and commercial planes beginning in 1957, cockpit recorders beginning in 1967.

The black boxes are built to survive the most violent impact. Protected by a stainless steel or titanium alloy casing and cushioned by interior padding, all voice and data recorders models are subjected to a series of survival tests.

They are shot out of a 10-inch cannon into a wall, roasted at up to 2012 degrees Fahrenheit for 30 minutes, subjected repeatedly by 5,000 pounds of pressure and immersed in vats of deicing fluid, jet fuel, hydraulic oil and seawater.

Still, flight boxes are damaged -- though rarely so badly that vital information can't be retrieved. Lockheed Martin Advanced Recorders, formerly Loral Data Systems, has designed and built more than 50,000 voice and data recorders, with 600 involved in accidents, Hardman said. Only five of those boxes were destroyed.

In smaller aircraft, the cockpit and flight data recorders are sometimes combined into one "black box," but on typical passenger planes, both boxes are required. In the event one is damaged, investigators can still rely on the other.

Almost all recorders that can't be retrieved or used are damaged not by impact, but by fire. While the recorder's insulation can withstand fires from jet fuel igniting, it cannot tolerate longer-burning blazes, such as a forest fire.

Between 1992 and 1996, 90 flight and voice data recorders sustained fire damage in crashes, according to a National Transportation Safety Board report. In almost half those, the recorder's magnetic tape was either damaged or destroyed.

The latest solid-state recorders, being installed on new aircraft like the Boeing 777, are likely to increase the survival rate of flight recorders to more than 90 percent, according to the NTSB.

Over the years, technology has provided considerable improvements in recorders. When the Boeing 747 was introduced in 1969, the data recorder monitored only five flight parameters. Today, boxes on most planes check at least 17 measurements. The FAA recently proposed rule changes that would require more.

As older jets are phased out, a larger share of the nation's commercial fleet will be equipped with advanced recorders. But the NTSB is pushing to have older jets -- including thousands of 727s, 737s, DC-9s and DC-10s -- equipped with the newer devices.

Pub Date: 7/20/96