A Peek Inside the Black Box—Which Has Probably Saved Your Life
Matthew Greenwood posted on August 19, 2019 |

The black box, or flight data recorder (FDR), is actually bright orange—to make it easier to locate at the site of a plane crash. And most FDRs are not one but two boxes.

The device was invented by Australian rocket scientist David Warren, based on a miniature microphone marketed to businesspeople in the 1950s. The first prototypes were produced in 1957, and in 1960 Australia became the first country to make FDRs mandatory. Today, every single commercial aircraft in operation comes equipped with an FDR.

How a modern flight data recorder works.

Early models recorded only the voices of the pilots and basic flight conditions, such as temperature, heading, altitude, airspeed, vertical acceleration and flight time. Today, FDRs are far more sophisticated, keeping track of many more parameters, including throttle and flight control positions.

The devices also take full advantage of digitization through the use of solid-state memory drives that can be downloaded quickly—making them not only useful after a crash but also helpful for regular maintenance and performance monitoring.

In fact, current FDRs are made up of two boxes: one for recording voice communication and the other for data.

The cockpit voice recorder records sounds from crew microphones, earphones and an area recorder on the cockpit ceiling—as well as any exchanges with ground control or other aircraft. Two hours of audio are recorded, and the device constantly re-records over itself. The recordings are sealed in a moisture- and heat-resistant digital recorder that runs on batteries in case the plane’s electrical systems fail. While it may record the boring banter of a plane’s crew, it can also yield important clues to trained investigators, such as engine noise, alarms and stall warnings.

The flight data recorder keeps track of various measures of a plane’s performance. Its data acquisition unit collects readings on 88 discrete performance indicators—including not only airspeed and direction, but also the movement of individual wing flaps, auto pilot function and fuel levels. It sends snapshots of that data to the FDR for storage. The data is collected every few seconds, though that rate accelerates when a plane is in danger. This data is used by investigators to create a computer model of the flight so they can get a more accurate reading on what happened to the plane before it crashed.

Overall, the FDR has to carry a minimum of 25 hours of data. Not only is this data invaluable to air crash investigators—it’s also used by plane manufacturers and regulators to improve performance and safety. And the information is not encrypted: anyone who finds the device should be able to access the data.

The voice recorder is usually installed in the plane’s tail.
The voice recorder is usually installed in the plane’s tail.

FDRs are built tough, too—after all, their whole purpose is to survive a crash in order to tell investigators what went wrong. They have to go through gruelling tests, including piercing, salt water exposure, intense heat as well as prolonged heat, and extreme pressure—the equivalent of 20,000 feet below the ocean surface. And they can survive a 270-knot crash impact test. The devices can operate between -55°C and 70°C.

It wouldn’t matter if the FDR survives a crash but can’t be found. That’s why its homing beacon is designed to emit its signal for up to 30 days. For maritime accidents, each recorder is equipped with an underwater locator beacon that turns on as soon as the FDR hits the water and can transmit from a depth as much as 14,000 feet.

The black box of United Flight 93, which was hijacked on September 11, 2001. The 9/11 Commission used it to learn that the hijackers crashed the plane to keep the crew and passengers from taking control.

The Future of the FDR

Today’s FDR doesn’t differ that much from the original black box: it’s a recorder in a damage-resistant box. And there are more effective ways of recording and storing data—but upgrading the device is not that simple.

It took investigators two years to find the FDR after an Air France crash in 2009. That prompted the Federal Aviation Administration (FAA) to look at ways to improve the FDR. Recommendations included a mechanism to jettison the device into the air when the plane hits the water (already in use by the U.S, Navy and endorsed by the Flight Safety Foundation) and vastly improving the underwater beacon’s life from 30 days to 90. But while the technology to make such improvements is available, it would be quite expensive to implement them—causing plane manufacturers and regulators to shy away from the option.

Another alternative—in the age of easily available data—is to get rid of the onboard black box entirely, replacing it with an always-on recorder that streams the information via satellite to a secure storage device. Not only would the data never be lost after a crash, but it could also prevent crashes from happening in the first place by anticipating and identifying problems in real time.

But privacy concerns make that approach problematic: it could mean that employees would basically be wiretapped at the office. And while planes automatically send some data, it seems unfeasible for all the planes in the sky to be transmitting massive amounts of information at the same time. Plus, the cost is again prohibitive—airlines would have to retrofit their entire fleets, reserve satellite time, and pay for data storage facilities.

The black box has come a long way from the rudimentary tape recorder of the 1950s prototypes—and they will likely continue to evolve, if slowly. One thing is certain: the data salvaged from past tragedies have led to significant improvements in airplane safety—and most of us have benefited from them without ever realizing it.

Read more about how investigators use FDR data at Update 737 MAX Crash: How Culpable is Boeing?


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