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

Photo courtesy U.S. Department of Defense

Introduction to How Black Boxes Work

After the crash, they didn't find a single body for five days. Even with military and civilian personnel frantically scouring the seas, it was as if Air France Flight 447 had simply vanished over a remote area of ocean 600 miles from Brazil -- with 228 people onboard. It didn't happen in the early days of the airline industry; it occurred in 2009, on a fancy modern aircraft controlled by a competent company.

Airplane accidents are statistical rarities. But when they happen, they're often fatal, and people want answers as to why their loved ones died.

There are usually 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. Following any airplane accident in the U.S., safety investigators from the National Transportation Safety Board (NTSB) immediately begin searching for the aircraft's black boxes.

It took investigators nearly two years to find the FDR from Flight 447. The box had not only survived impact, but also being submerged under nearly 13,000 feet of salty, corrosive seawater. In the end, the data proved that pilot error had contributed to a stall that eventually caused the crash.

These recording devices, which cost between $10,000 and $15,000 each, reveal details of the events immediately preceding the accident. In this article, we will look at the two types of black boxes, how they survive crashes, and how they are retrieved and analyzed.

An older model flight data recorder from EgyptAir 990, which crashed in 1999.

© Reuters/CORBIS

Black Boxes Begin

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 log much more information about an aircraft's operation.

Older black boxes used magnetic tape, a technology that was first introduced in the 1960s. 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. These days, black boxes use solid-state memory boards, which came along in the 1990s.

Solid-state recorders are considered much more reliable than their magnetic-tape counterparts. 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). 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 such as acceleration, airspeed, altitude, flap settings, outside temperature, engine performance, and cabin temperature and pressure. Magnetic-tape recorders can track about 100 parameters, while solid-state recorders can track a lot more.

For instance, in the Boeing 787, the units can log a whopping 146,000 parameters, resulting in several terabytes of data for every single flight. That incredible load of data is a double-edge sword; it's great for monitoring the aircraft, but it can overwhelm engineers and maintenance personnel. To manage all of that data, they need sophisticated data management software.

Whether the system is an older version or fully modern, 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.

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.

The cockpit voice recorder and the flight data recorder that were retrieved from Comair Flight 5191 in 2006.

© Mark Wilson/Getty Images

Cockpit Voice Recorders

In almost every commercial aircraft, there are several microphones built into the cockpit that listen to flight crew conversation. These microphones also 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).

Microphones send audio to the CVR, which digitizes and stores the signals. In the cockpit, there is also a device called the associated control unit, which provides pre-amplification for audio going to the CVR. The four microphones are place in the pilot's headset, co-pilot's headset, headset of a third crew member (if there is a third crew member) and near the center of the cockpit, to 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 use solid-state storage can record two hours of audio. Similar to the magnetic-tape recorders, solid-state recorders also record over old material.

One of the black boxes recovered from Air France Airbus A330, which crashed into the Atlantic in 2009.

Bureau of Analysis and Research of France (BEA)

Flight Data Recorders

The flight data recorder (FDR) is designed to record the operating data from the plane's systems. There are sensors wired from various areas on the plane to the flight-data acquisition unit, which is wired to the FDR. So whenever the pilot flips a switch or twiddles a knob, the FDR records each action.

In the U.S., 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 hundreds or even thousands more.

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 recorded by the FDR gives investigators one more clue about the cause of an accident.

The flight recorders from Continental Airlines flight 1404, which slid off the runway during takeoff in Denver, Colo. In 2008.

© Mark Wilson/Getty Images

Built to Survive

Airplane crashes are violent affairs. In many such 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, jarring crashes and tons of pressure. In older magnetic-tape recorders, the CSMU is inside a rectangular box.

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 data.

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's a thin layer of aluminum around the stack of memory cards.
  • High-temperature insulation -- This dry-silica material is 1 inch (2.54 centimeters) 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 centimeters) thick. Titanium can be used to create this outer armor as well.

These hardened housings are incredibly important. Without adequate protection, all of the flight data would be destroyed. So to make sure that data stays safe, engineers attack their black boxes with full fury to see if their products can withstand extreme abuse.

Testing a CSMU

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 sample data onto the memory boards inside the CSMU. 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-kilogram) weight with a 0.25-inch (0.64-centimeter) steel pin protruding from the bottom onto the CSMU from a height of 10 feet (3 meters). 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 Celsius) 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.

The Brazilian Navy shows recovered debris of an Air France aircraft lost in mid-flight over the Atlantic Ocean.

Brazilian Navy Handout/Xinhua Press/Corbis

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's because early recorders were painted black, while others think it refers to the charring that occurs in post-accident fires.

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 carrying handle, this cylinder is actually a beacon.

If a plane crashes into the water, the 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, the beacon is activated.

The beacon sends out pulses at 37.5 kilohertz (kHz) and can transmit sound as deep as 14,000 feet (4,267 meters). 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 U.S. when investigators locate a black box, it's 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.

The cockpit voice recorder from Russian Sukhoi Superjet 100, which crashed into the side of a mountain in 2012.

© AFP/AFP/GettyImages

Retrieving 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 National Transportation Safety Board with the readout systems and software needed to do a full analysis of the recorders' stored data.

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 through USB or Ethernet ports. Very often, recorders retrieved from wreckage are dented or burned. In these cases, the memory boards are removed, cleaned up and have a new memory interface cable 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 representatives from the airline and airplane manufacturer, an NTSB transportation-safety specialist and an NTSB air-safety investigator. This group may also include a language specialist from the FBI 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.

Not just for airplanes: The black box pictured here came from a race car that was involved in a crash in Australia in 2006.

© Greg Burke-Pool/Getty Images

The Future of Black Boxes

There are all sorts of potential improvements on the horizon for black box technology. Most obviously, current systems don't record any video of cockpit activity. For years, the National Transportation Safety Board has been trying in vain to implement video capabilities into black box systems, but many pilots steadfastly refuse to allow video, saying such systems violate their privacy and that current data capture is sufficient for accident investigators.

The NTSB continues to insist that there's no such thing as having too much information when investigating plane crashes. At present, video recording is still on hold.

But the technology is more than ready. Airbus, for example, installs a Vision 1000 system in all of its helicopters. The Vision 1000 camera is mounted behind the pilot's head, where it records video of the pilot's actions and the cockpit area, as well as the view beyond the windshield, at four frames per second. It weighs about a half a pound and needs only power and a GPS connection for activation.

Video isn't the only improvement that's found resistance from the status quo. Since 2002, some legislators have pushed for the Save Aviation and Flight Enhancement Act, which would require not one, but two flight recorders, including one that automatically ejects itself from the plane during an incident. Such self-ejecting recorders are easier to locate are less likely to suffer catastrophic damage. So far, though, the law has not passed Congress.

Black boxes aren't just for planes. They're now integrated into many types of vehicles. You may even have one in your car, though you don't know about it. About 90 percent of new cars have event data recorders (EDRs) that track the same kind of data as airplane black boxes. The EDR is ostensibly designed to maintain and monitor the car's safety system, but accident investigators can and do use EDR data to better understand wrecks ... and sometimes, to assign blame after an accident.

When it comes to black boxes mounted to airplanes, it's entirely possible that they'll go by the wayside. Instead of recording to a box, airplanes may soon simply stream all of their essential data directly to a ground-based station. These systems already exist. For example, AeroMechanical Services' FlyhtStream air-to-ground system sends flight data to a home base via satellite.

Such systems eliminate the desperate search for a box that may have been destroyed in a crash, and may be more dependable, too. For the moment, though, black boxes are still very much a necessity each and every day as thousands of planes take to the skies, flying millions of people all over the world.

Lots More Information

Author's Note: How Black Boxes Work

I have a recurring nightmare about zooming through the skies in a doomed jet. Each time, the plane leaves the runway during takeoff and then violently accelerates straight up into the sky. I never get to the end of the dream. Perhaps that's a good thing. Happily, aircraft malfunctions are exceedingly rare – statistically speaking, your car is a whole lot more dangerous. But when planes do fall from the sky, it's a relief to have some idea why...otherwise, engineers and family members would be left agonizing, wondering why innocent people died in such an awful manner. I hope I'm never part of an accident scene where a black box is necessary. Unless, of course, it's just in my dreams. - NC

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