A Better Black Box

by Mark Alpert, staff writer

In the tragic wake of an airline crash, one of the highest priorities of the accident investigators is to retrieve the aircraft's black boxes. The Federal Aviation Administration requires all large commercial aircraft to be equipped with two such devices: the cockpit voice recorder, which records the flight crew's voices and other sounds in the cockpit, and the flight data recorder, which monitors the plane's altitude, airspeed, heading and other instrument readings. Because this information can be vital to the investigation of an air disaster, the recorders must be designed so that the stored data can survive virtually any crash.

The first flight recorders were introduced in the late 1950s. The devices became known as black boxes, and the name endured even after the government required airlines to paint the boxes bright orange to make them easier to locate after a crash. The newest recorders employ flash-memory chips, which can store data for several years without the need for power. The best solid-state recorders can hold about 80 megabytes—much less than the memory of most personal computers but enough to store two hours of voice recordings from the cockpit or a full day's worth of readings from the plane's instruments. These recorders also contain circuit boards that process and compress the data, but only the memory chips are enclosed in the crash-survivable unit inside the box. This unit is covered with thick steel armor so that it can withstand a crushing impact shock. Beneath the steel is a layer of thermal insulation designed to protect the memory chips from the high-temperature fires that often ignite after a jet accident.

Today's black boxes are far more capable and crashworthy than earlier models, but the FAA is still seeking improvements. It recently expanded the list of instrument readings that must be stored in flight data recorders. The agency has also proposed that each cockpit voice recorder be equipped with a backup power supply so that it can continue recording even if the aircraft's electrical systems fail.

BLACK BOXES are placed in the aircraft's tail because that section usually experiences the least force in a crash. The plane's instruments relay their readings to a flight data acquisition unit, which sends the signals to the data recorder.

 

 

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SOLID-STATE RECORDERS are more rugged than earlier devices because they contain no moving parts. In the model shown here, the flash-memory chips are arrayed in a stack of memory boards at the center of a cylindrical crash-survivable unit. UNDERWATER LOCATOR BEACON, which is bolted to the crash-survivable unit, is activated if the recorder is submerged in water. Also known as a pinger, the device can transmit an acoustic signal from depths as great as 14,000 feet.

 

 

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CERTIFICATION TESTS ensure that flight recorders are crashworthy by simulating the extreme conditions of an aviation catastrophe. In the crash impact test (a), a gas gun shoots the recorder into an aluminum target, producing a maximum force of 3,400 g's. In the penetration resistance test (b), a 500-pound weight with a hardened steel spike is dropped on the recorder from a height of 10 feet. In the static crush test (c), an actuator applies 5,000 pounds of pressure. In the deep-sea submersion test (d), the recorder must survive 24 hours in a chamber filled with pressurized seawater. In the fire protection test (e), it is subjected to flames of 1,100 degrees Celsius.

a) CRASH IMPACT

b) PENETRATION RESISTANCE

c) STATIC CRUSH

d) DEEP-SEA SUBMERSION

e) FIRE PROTECTION

DID YOU KNOW ...

 

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The earliest flight data recorders used steel foil as their storage medium. The crude devices made marks on a moving roll of foil that was housed in a crash-survivable box. Unfortunately, the foil could be embossed only once, which limited the amount of data it could hold. In 1965 airlines were required to install cockpit voice recorders that used magnetic tape for storage, and this medium soon became the predominant one for flight data recorders as well. Solid-state recorders using memory chips became prevalent in the 1990s.

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Nearly 100,000 flight recorders have been installed in commercial aircraft over the past four decades. The prices of the latest models generally range from $10,000 to $20,000. Their survival rate has greatly improved in recent years as the FAA has raised the certification requirements. Although older recorders using magnetic tape were susceptible to fire damage, no solid-state device has been destroyed in an accident to date.