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How Black Boxes Work
by Kevin Bonsor

Special thanks to L-3 Communications, Aviation Recorders Division, for its help with this article.
On January 31, 2000, Alaska Airlines Flight 261 departed Puerto Vallarta, Mexico, heading for Seattle, WA, with a short stop scheduled in San Francisco, CA. Approximately one hour and 45 minutes into the flight, a problem was reported with the plane's stabilizer trim. After a 10-minute battle to keep the plane airborne, it plunged into the Pacific Ocean off the coast of California. All 88 people onboard were killed.

Photo courtesy U.S. Department of Defense
The cockpit voice recorder from the downed Alaska Airlines Flight 261, held by the robotic arm of the remotely piloted vehicle that retrieved it

With any airplane crash, there are many unanswered questions as to what brought the plane down. Investigators turn to the airplane's flight data recorder (FDR) and cockpit voice recorder (CVR), also known as "black boxes," for answers. In Flight 261, the FDR contained 48 parameters of flight data, and the CVR recorded a little more than 30 minutes of conversation and other audible cockpit noises.

Following any airplane accident in the United States, safety investigators from the National Transportation Safety Board (NTSB) immediately begin searching for the aircraft's black boxes. These recording devices, which cost between $10,000 an $15,000 each, reveal details of the events immediately preceding the accident. In this edition of HowStuffWorks, we will look at the two types of black boxes, how they survive crashes, and how they are retrieved and analyzed.

Recording and Storage
The Wright Brothers pioneered the use of a device to record propeller rotations, according to documents provided by L-3 Communications. However, the widespread use of aviation recorders didn't begin until the post-World War II era. Since then, the recording medium of black boxes has evolved in order to record much more information about an aircraft's operation.

Most of the black boxes in use today use magnetic tape, which was first introduced in the 1960s, or solid-state memory boards, which came along in the 1990s. Magnetic tape works like any tape recorder. The Mylar tape is pulled across an electromagnetic head, which leaves a bit of data on the tape. Black-box manufacturers are no longer making magnetic tape recorders as airlines begin a full transition to solid-state technology.

Photo courtesy National Transportation Safety Board (NTSB)
The magnetic tape inside the flight data recorder from EgyptAir Flight 990, which crashed on October 31, 1999

Solid-state recorders are considered much more reliable than their magnetic-tape counterparts, according to Ron Crotty, a spokesperson for Honeywell, a black-box manufacturer. Solid state uses stacked arrays of memory chips, so they don't have moving parts. With no moving parts, there are fewer maintenance issues and a decreased chance of something breaking during a crash.

Data from both the CVR and FDR is stored on stacked memory boards inside the crash-survivable memory unit (CSMU). In recorders made by L-3 Communications, the CSMU is a cylindrical compartment on the recorder. The stacked memory boards are about 1.75 inches (4.45 cm) in diameter and 1 inch (2.54 cm) tall.

The memory boards have enough digital storage space to accommodate two hours of audio data for CVRs and 25 hours of flight data for FDRs.

Airplanes are equipped with sensors that gather data. There are sensors that detect acceleration, airspeed, altitude, flap settings, outside temperature, cabin temperature and pressure, engine performance and more. Magnetic-tape recorders can track about 100 parameters, while solid-state recorders can track more than 700 in larger aircraft.

All of the data collected by the airplane's sensors is sent to the flight-data acquisition unit (FDAU) at the front of the aircraft. This device often is found in the electronic equipment bay under the cockpit. The flight-data acquisition unit is the middle manager of the entire data-recording process. It takes the information from the sensors and sends it on to the black boxes.

Source: L-3 Communication Aviation Recorders
Basic components and operation of an aviation recording system

Both black boxes are powered by one of two power generators that draw their power from the plane's engines. One generator is a 28-volt DC power source, and the other is a 115-volt, 400-hertz (Hz) AC power source. These are standard aircraft power supplies, according to Frank Doran, director of engineering for L-3 Communications Aviation Recorders.

Cockpit Voice Recorders
In almost every commercial aircraft, there are several microphones built into the cockpit to track the conversations of the flight crew. These microphones are also designed to track any ambient noise in the cockpit, such as switches being thrown or any knocks or thuds. There may be up to four microphones in the plane's cockpit, each connected to the cockpit voice recorder (CVR).

Photo courtesy L-3 Communication Aviation Recorders
A solid-state recorder

Any sounds in the cockpit are picked up by these microphones and sent to the CVR, where the recordings are digitized and stored. There is also another device in the cockpit, called the associated control unit, that provides pre-amplification for audio going to the CVR. Here are the positions of the four microphones:

  • Pilot's headset
  • Co-pilot's headset
  • Headset of a third crew member (if there is a third crew member)
  • Near the center of the cockpit, where it can pick up audio alerts and other sounds

Most magnetic-tape CVRs store the last 30 minutes of sound. They use a continuous loop of tape that completes a cycle every 30 minutes. As new material is recorded, the oldest material is replaced. CVRs that used solid-state storage can record two hours of audio. Similar to the magnetic-tape recorders, solid-state recorders also record over old material.

Final Words of Flight 261
CVR recordings can hold important clues to the cause of an accident. In the case of Alaska Airlines Flight 261, the conversations between the captain and his first officer pointed NTSB investigators to the plane's stabilizer. This is an excerpt taken from the official NTSB transcript of Flight 261, which crashed on January 31, 2000, off the coast of California. This excerpt contains an exchange between Captain Ted Thompson and First Officer William Tansky and the Los Angeles Route Traffic Control Center (LAX-CTR).

4:09:55 p.m. Thompson: Center, Alaska two-sixty-one. We are, uh, in a dive here, and I've lost control, vertical pitch.
4:10:33 Thompson: Yea, we got it back under control here.
4:11:43 Tansky: Whatever we did is no good. Don't do that again...
4:11:44 Thompson: Yea, no, it went down. It went full nose down.
4:11:48 Tansky: Uh, it's a lot worse than it was?
4:11:50 Thompson: Yea. Yea. We're in much worse shape now.
4:14:12 Public address: Folks, we have had a flight-control problem up front here, we're working on it.
4:15:19 Flight 261 to LAX-CTR: L.A., Alaska two-sixty-one. We're with you, we're at twenty-two-five [22,500 feet]. We have a jammed stabilizer and we're maintaining altitude with difficulty...
4:15:36 LAX-CTR: Alaska two-sixty-one, L.A center. Roger, um, you're cleared to Los Angeles Airport via present position...
4:17:09 Flight attendant: Okay, we had like a big bang back there.
4:17:15 Thompson: I think the [stabilizer] trim is broke.
4:19:36 Extremely loud noise
4:19:43 Tansky: Mayday
4:19:54 Thompson: Okay, we are inverted, and now we gotta get it.
4:20:04 Thompson: Push, push, push...push the blue side up. Push...
4:20:14 Tansky: I'm pushing.
4:20:16 Thompson: Okay, now let's kick rudder. Left rudder, left rudder.
4:20:18 Tansky: I can't reach it.
4:20:20 Thompson: Okay. Right rudder, right rudder.
4:20:25 Thompson: Are we flying? We're flying, we're flying. Tell 'em what we're doing.
4:20:33 Tansky: Oh, yeah. Let me get...
4:20:38 Thompson: Gotta get it over again. At least upside down we're flying.
4:20:54 Thompson: Speedbrakes
4:20:55 Tansky: Got it.
4:20:56 Thompson: Ah, here we go.
4:20:57 End of recording

Click here to read the full transcript (PDF) of Flight 261.

Flight Data Recorders
The flight data recorder (FDR) is designed to record the operating data from the plane's systems. There are sensors that are wired from various areas on the plane to the flight-data acquisition unit, which is wired to the FDR. When a switch is turned on or off, that operation is recorded by the FDR.

Photo courtesy National Transportation Safety Board (NTSB)
The damaged flight data recorder from EgyptAir Flight 990

In the United States, the Federal Aviation Administration (FAA) requires that commercial airlines record a minimum of 11 to 29 parameters, depending on the size of the aircraft. Magnetic-tape recorders have the potential to record up to 100 parameters. Solid-state FDRs can record more than 700 parameters. On July 17, 1997, the FAA issued a Code of Federal Regulations that requires the recording of at least 88 parameters on aircraft manufactured after August 19, 2002.

Here are a few of the parameters recorded by most FDRs:

  • Time
  • Pressure altitude
  • Airspeed
  • Vertical acceleration
  • Magnetic heading
  • Control-column position
  • Rudder-pedal position
  • Control-wheel position
  • Horizontal stabilizer
  • Fuel flow

Solid-state recorders can track more parameters than magnetic tape because they allow for a faster data flow. Solid-state FDRs can store up to 25 hours of flight data. Each additional parameter that is recorded by the FDR gives investigators one more clue about the cause of an accident.

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.

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