How Investigators Pull Data off a Boeing 737’s Black Boxes

In the five days since Ethiopian Airlines Flight 302 crashed a few minutes after taking off from Addis Ababa, killing all 157 people aboard, regulators around the world have grounded the Boeing 737 MAX 8. That’s a reaction to the fact that the circumstances of this disaster match those of Lion Air Flight 610, another 737 MAX 8, which crashed into the Java Sea in October, killing all 189 occupants. But to find out what actually brought down the Ethiopian jet, investigators must get to the vital data stored on the plane’s two black boxes.

Those boxes are the flight data recorder and the cockpit voice recorder. Both use solid state hard drives, with memory cards on a circuit board. The first logs all the details that together provide a picture of everything the plane was doing as much as 25 hours before a crash: the positions of various switches, engine settings, airspeed, altitude, and more than 1,000 other parameters. Examine that data and “you know what happened to the airplane physically,” says aviation consultant Kit Darby.

The cockpit voice recorder is what it sounds like: an audio recording of everything the pilots are saying and hearing. That can often give you the why of a crash, since it provides the pilots’ interpretation of whatever’s going on and how they’re reacting. Paired with the flight data recorder, it can give you the whole picture. “You have to combine the two,” Darby says.

Which is why they’re built to survive practically any type of crash. Honeywell, which manufacturers black boxes, says they can survive an impact of up to 3,400 Gs. (Unless you’re a trained fighter pilot, you can probably handle about 5 Gs before passing out.) They can withstand the pressure exerted by 20,000 feet of water—for two years. They do just fine in temperatures up to 1,100 degrees Fahrenheit as well as ice, so neither a regular nor a wight dragon can destroy them.

For extra security, the boxes ride in the tail of the plane, which is less likely to be damaged in a crash than the rest of the aircraft. They’re equipped with sonar pingers to make them findable if the plane goes down over water and they sink to the ocean floor. (The pingers helped search teams locate the two recorders that sank to the floor of the Java Sea when the Lion Air 737 crashed.) And they’re bright orange, to make them easier to spot.

Ethiopian Flight 302 crashed on relatively flat and open terrain, so investigators found the black boxes within a day. Because special laboratory equipment is required to access their data, the boxes were sent to Paris, where France’s civil aviation investigator, the Bureau d’Enquêtes & d’Analyses, will handle them. They arrived Thursday morning and the technical work of accessing their data began Friday. In a photo released by the French agency, the flight data recorder looks pretty mangled. That’s not surprising for something that’s been in a plane crash—they’re built to survive, not come out looking pretty. And in most cases, the crushed mess not a big problem.

The BEA lab workers start by opening up each box and removing the one-inch layer of thermal protective insulation inside. Once they’ve got the memory board out, they put it under a microscope—some of the components are just millimeters across—looking for damage. Then they put them into an x-ray machine, to inspect what they can’t see with the microscope.

That’s a relatively easy recovery operation, but sometimes black boxes are hard to find and difficult to salvage. Take the case of Air France 447, the Airbus A330 that crashed in the Atlantic Ocean off the coast of Brazil in 2009 with 228 people aboard. It took investigators two years to locate the wrecked plane and its black boxes, which lay under 13,000 feet of water. The fact that an international team spent that long on the hunt tells you just how strongly the sky lords feel about figuring out what causes crashes, and how to prevent it from happening again.

And the fact that the Air France plane’s black boxes made it through their underwater burial at all tells you just how tough-as-mothers they really are. In fact, the flight data recorder came out fine. The cockpit voice recorder was in slightly worse shape. The French team reported a cracked capacitor and resistor, plus damage to two “decoder-type” components. Before doing anything else, the team put the memory cards into an oven to remove any moisture (36 hours for the flight data recorder, 42 for the cockpit voice recorder). Then they removed the damaged bits and soldered in new ones. It’s technical work, but far from impossible.

Other types of damage aren’t so easy to fix, Darby says. “If it’s crushed, you’re not going to be able to read it.” And damage to the memory cards themselves is more worrisome than whatever may happen to the other parts of the circuit board. “That may or may not be recoverable.”

In the case of Air France 447, once the memory boards were fully dried out, the lab workers hooked the cards up to their reader. (They’re one of the few places in the world to have one; the National Transportation Safety Board’s lab in Washington, DC, is another.) They downloaded and synced up the data. Then they got to work figuring out what brought down the jet. A year later, in 2012, they released their final report. The plane stalled at cruising altitude after ice clogged the speed sensors, the autopilot shut off in response, and the pilots mismanaged the situation.

Now that the black box process is starting over in Paris, the results could reveal what brought down Ethiopian Flight 302 and led to the grounding of the world’s Boeing 737 MAX jets. Nothing will happen quickly. Full investigations usually take about 18 months—but preliminary data released before then can start to shed light on what happened.

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