Built to Survive In many airline accidents

How Black Boxes Work

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Built to Survive
In many airline accidents, the only devices that survive are the crash-survivable memory units (CSMUs) of the flight data recorders and cockpit voice recorders. Typically, the rest of the recorders' chassis and inner components are mangled. The CSMU is a large cylinder that bolts onto the flat portion of the recorder. This device is engineered to withstand extreme heat, violent crashes and tons of pressure. In older magnetic-tape recorders, the CSMU is inside a rectangular box.

Source: L-3 Communication Aviation Recorders

Using three layers of materials, the CSMU in a solid-state black box insulates and protects the stack of memory boards that store the digitized information. We will talk more about the memory and electronics in the next section. Here's a closer look at the materials that provide a barrier for the memory boards, starting at the innermost barrier and working our way outward:

Aluminum housing - There is a thin layer of aluminum around the stack of memory cards.
High-temperature insulation - This dry-silica material is 1 inch (2.54 cm) thick and provides high-temperature thermal protection. This is what keeps the memory boards safe during post-accident fires.
Stainless-steel shell- The high-temperature insulation material is contained within a stainless-steel cast shell that is about 0.25 inches (0.64 cm) thick. Titanium can be used to create this outer armor as well.

To ensure the quality and survivability of black boxes, manufacturers thoroughly test the CSMUs. Remember, only the CSMU has to survive a crash -- if accident investigators have that, they can retrieve the information they need. In order to test the unit, engineers load data onto the memory boards inside the CSMU. L-3 Communications uses a random pattern to put data onto every memory board. This pattern is reviewed on readout to determine if any of the data has been damaged by crash impact, fires or pressure.

There are several tests that make up the crash-survival sequence:

Crash impact - Researchers shoot the CSMU down an air cannon to create an impact of 3,400 Gs (1 G is the force of Earth's gravity, which determines how much something weighs). At 3,400 Gs, the CSMU hits an aluminum, honeycomb target at a force equal to 3,400 times its weight. This impact force is equal to or in excess of what a recorder might experience in an actual crash.
Pin drop - To test the unit's penetration resistance, researchers drop a 500-pound (227-kg) weight with a 0.25-inch steel pin protruding from the bottom onto the CSMU from a height of 10 feet (3 m). This pin, with 500-pounds behind it, impacts the CSMU cylinder's most vulnerable axis.
Static crush - For five minutes, researchers apply 5,000 pounds per square-inch (psi) of crush force to each of the unit's six major axis points.
Fire test - Researchers place the unit into a propane-source fireball, cooking it using three burners. The unit sits inside the fire at 2,000 degrees Fahrenheit (1,100 C) for one hour. The FAA requires that all solid-state recorders be able to survive at least one hour at this temperature.
Deep-sea submersion - The CSMU is placed into a pressurized tank of salt water for 24 hours.
Salt-water submersion - The CSMU must survive in a salt water tank for 30 days.
Fluid immersion - Various CSMU components are placed into a variety of aviation fluids, including jet fuel, lubricants and fire-extinguisher chemicals.

During the fire test, the memory interface cable that attaches the memory boards to the circuit board is burned away. After the unit cools down, researchers take it apart and pull the memory module out. They restack the memory boards, install a new memory interface cable and attach the unit to a readout system to verify that all of the preloaded data is accounted for.

Black boxes are usually sold directly to and installed by the airplane manufacturers. Both black boxes are installed in the tail of the plane -- putting them in the back of the aircraft increases their chances of survival. The precise location of the recorders depends on the individual plane. Sometimes they are located in the ceiling of the galley, in the aft cargo hold or in the tail cone that covers the rear of the aircraft.

"Typically, the tail of the aircraft is the last portion of the aircraft to impact," Doran said. "The whole front portion of the airplane provides a crush zone, which assists in the deceleration of tail components, including the recorders, and enhances the likelihood that the crash-protected memory of the recorder will survive."

After a Crash
Although they are called "black boxes," aviation recorders are actually painted bright orange. This distinct color, along with the strips of reflective tape attached to the recorders' exteriors, help investigators locate the black boxes following an accident. These are especially helpful when a plane lands in the water. There are two possible origins of the term "black box": Some believe it is because early recorders were painted black, while others think it refers to the charring that occurs in post-accident fires.

Underwater Locator Beacon
In addition to the paint and reflective tape, black boxes are equipped with an underwater locator beacon (ULB). If you look at the picture of a black box, you will almost always see a small, cylindrical object attached to one end of the device. While it doubles as a handle for carrying the black box, this cylinder is actually a beacon.

Photo courtesy L-3 Communication Aviation Recorders
A close-up of an underwater locator beacon

If a plane crashes into the water, this beacon sends out an ultrasonic pulse that cannot be heard by human ears but is readily detectable by sonar and acoustical locating equipment. There is a submergence sensor on the side of the beacon that looks like a bull's-eye. When water touches this sensor, it activates the beacon.

The beacon sends out pulses at 37.5 kilohertz (kHz) and can transmit sound as deep as 14,000 feet (4,267 m). Once the beacon begins "pinging," it pings once per second for 30 days. This beacon is powered by a battery that has a shelf life of six years. In rare instances, the beacon may get snapped off during a high-impact collision.

In the United States, when investigators locate a black box it is transported to the computer labs at the National Transportation Safety Board (NTSB). Special care is taken in transporting these devices in order to avoid any (further) damage to the recording medium. In cases of water accidents, recorders are placed in a cooler of water to keep them from drying out.

Photo courtesy U.S. Department of Defense
U.S. Navy Lieutenant Junior Grade Jason S. Hall (right) watches as FBI Agent Duback (left) tags the cockpit voice recorder from EgyptAir Flight 990 on November 13, 1999.

"What they are trying to do is preserve the state of the recorder until they have it in a location where it can all be properly handled," Doran said. "By keeping the recorder in a bucket of water, usually it's a cooler, what they are doing is just keeping it in the same environment from which it was retrieved until it gets to a place where it can be adequately disassembled."

Retrieving the Information
After finding the black boxes, investigators take the recorders to a lab where they can download the data from the recorders and attempt to recreate the events of the accident. This process can take weeks or months to complete. In the United States, black-box manufacturers supply the NTSB with the readout systems and software needed to do a full analysis of the recorders' stored data.

Photo courtesy L-3 Communication Aviation Recorders
This portable interface can allow investigators quick access to the data on a black box.

If the FDR is not damaged, investigators can simply play it back on the recorder by connecting it to a readout system. With solid-state recorders, investigators can extract stored data in a matter of minutes. Very often, recorders retrieved from wreckage are dented or burned. In these cases, the memory boards are removed, cleaned up and a new memory interface cable is installed. Then the memory board is connected to a working recorder. This recorder has special software to facilitate the retrieval of data without the possibility of overwriting any of it.

A team of experts is usually brought in to interpret the recordings stored on a CVR. This group typically includes a representative from the airline, a representative from the airplane manufacturer, an NTSB transportation-safety specialist and an NTSB air-safety investigator. This group may also include a language specialist from the Federal Bureau of Investigation and, if needed, an interpreter. This board attempts to interpret 30 minutes of words and sounds recorded by the CVR. This can be a painstaking process and may take weeks to complete.

Both the FDR and CVR are invaluable tools for any aircraft investigation. These are often the lone survivors of airplane accidents, and as such provide important clues to the cause that would be impossible to obtain any other way. As technology evolves, black boxes will continue to play a tremendous role in accident investigations.