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NTSB black boxes FDR and CVR on table

What is a black box and how does it work?

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Just about everyone has heard of the “black box” on an airplane. The term tends to have strong associations because most of the time when we hear about the black box it’s as a result of an air crash. Here’s a look at how they work.

“Black box” is a common term in popular use but within the industry it is generally referred to as an electronic flight data recorder. That can describe either the CVR (Cockpit Voice Recorder) or the FDR (Flight Data Recorder), or a combination of both. A number of modern black boxes house everything within one unit. Either way, for redundancy’s sake, every aircraft has to have at least two onboard. And they do exactly what they say on the tin: these boxes are essentially heavily fortified hard drives that record everything about a flight on an ongoing basis.

FDR data from Lion Air 610 crash
A readout from the Flight Data Recorder (FDR) on Lion Air flight 610, a 737Max which crashed shortly after departure from Jakarta.

The FDR continuously records a wide array of data (around 700 different parameters) about all aspects of an aircraft as it flies from place to place. The CVR records the conversations on the flight deck and other sounds like radio transmissions and automated alarms, though it deletes all audio older than the most recent two hours of flight. The basic idea is that should any problems arise with the plane – especially if there’s been a serious accident and it’s not possible to speak to the pilots about what happened – the data from the black box can help reconstruct exactly what happened.

In any exploration of this topic, one of the most popular things to point out about the black box is that almost none are actually black. You want to be able to actually find them after an accident, so they’re painted in a bright color: international orange. And yet the name black box persists – maybe because “orange box” doesn’t have quite the same ring.

Easy retrieval is a priority of course, and it may not always be clear where an aircraft has gone down, especially if it happens over remote stretches of ocean. For that reason all black boxes have underwater locator beacons which transmit a signal upon contact with water, at least until the battery runs out – usually after around a month. One problem though is that the radius for the signal is not very wide, and there have been calls for installing stronger beacons that would make it easier to find a remote crash.

Black box FDR from Atlas Air Flight 3591
The recovered flight data recorder of Atlas Air Flight 3591, a Boeing 767-300 cargo jet, that crashed in the muddy marshland of Trinity Bay Feb. 23, 2019. (NTSB photo)

Although the hope is always that the data any given black box contains will never need to be retrieved and analyzed, every recorder needs to be able to withstand the worst case scenario: a catastrophic accident. That means they need to be certified as more or less indestructible, at least up to some very high thresholds. They’re tested by being launched at a concrete wall at a speed of 750 kilometers per hour, and they have to withstand loads of 2.25 tons for at least five minutes, temperatures of 1,100 degrees Celsius for an hour and not only be waterproof but withstand the heavy pressure found at depths of thousands of meters underwater.

BEA investigators analyze a flight data recorder
BEA investigators analyze a flight data recorder.

Consider the fact that after the crash of Air France flight 447 in the Atlantic Ocean in 2009, the black boxes weren’t found until nearly two years later. The wreckage that contained the boxes was submerged at a depth of almost 4,000 meters. And yet, the data and recordings were successfully recovered and proved invaluable for helping investigators to understand exactly what went wrong. These things are clearly built to last.

To accomplish this level of robustness, the solid state memory boards that store the data are housed in several layers of material – often including a steel or titanium outer shell, a thermal block and special insulation. Most of the time they’re kept in the tail of the aircraft where it’s assumed they’ll have a better chance of survival.

The role of these devices in getting to the bottom of major accidents is most well-known but they can be useful for troubleshooting or understanding less severe issues too. The rich set of data points can help to shine a light on what may have gone wrong with a particular system and help engineers isolate and fix that issue, or it can clarify what series of miscommunications or missteps led to an unsafe situation. In either case, specially trained analysts pull the data and analyze it for any and all clues. There are just a handful of agencies in the world with the resources and experience to analyze black box data in complex crashes. One high profile agency is the BEA of France, which was tasked with analyzing the boxes from Ethiopian Airlines flight ET302, the high-profile 737Max crash that led to that aircraft’s ongoing worldwide grounding. Skilled investigators can almost always use what they find in order to paint a very complete picture of what happened, and use that to help make future flights safer.

Kulula 737 with black box markings
A Kulula.com 737-800 seen taking off from Cape Town (CPT) is a rare example of a plane that points out location of its black box in the paint scheme.

Ever since a handful of high profile crashes like the AF447 and MH370 (the Malaysia Airlines 777 that disappeared without a trace) some have called for a move toward broadcasting a livestream of telemetry data from planes instead of storing them on boxes that have to be retrieved. That would give investigators immediate access to the relevant data and bring much-needed answers to the world much faster.

But so far, though there are some satellite-based solutions for live-streaming flight data available, broadcasting the full array of flight data in real time is mostly not seen as a priority. Why? Because it would be an expensive fix for an almost nonexistent problem. Even as thousands of black boxes store millions of data points day in and day out on flights around the world, most of that is never needed because major accidents are so incredibly rare. And accidents where the data is difficult to retrieve or the plane can’t be found are rarer still. A black box is like insurance – it’s great to have, and even better if you never have to use it.

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