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How Black Boxes Help Solve Plane Crash Mysteries

Flight recorders are built to survive and testify. Here’s how investigators use them.

How Black Boxes Help Solve Plane Crash Mysteries
The flight data recorder for China Eastern Flight 5735 immediately after it was located by searchers. Photo courtesy of Chinese News Network/Wikipedia

It might seem obvious that we need flight recorders to find the causes of air disasters, but until recorders were developed and mandated more than 50 years ago, the causes of some crashes remained mysteries even after skilled investigators tried for years to solve them. Such is the case, most famously, for Malaysia Flight 370, which disappeared in March of 2014 under impossibly complex and inexplicable circumstances. The cause of the disappearance and presumed crash of the Boeing 777 wide-body jetliner, which was indeed equipped with both a flight data and cockpit voice recorder, remains a mystery in part because the recorders, which are likely at the bottom of the Indian Ocean, one of the deepest in the world, have never been found.

More recently, the disastrous March 2022 crash of China Eastern Flight 5735 is a case in point for why flight recorders are both necessary and limited in how they can help.

It can be difficult or impossible to pinpoint the cause of crashes without the data that these hardened boxes record and safeguard even after a crash. Since the United States began mandating data recorders in 1967, they have helped solve many dozens of high-profile crashes. In many of those cases, the recorders were central to investigators finding a probable cause for the mishap.  

There are currently two kinds of recorders widely mandated—flight data recorders (FDRs) and cockpit voice recorders (CVRs). The United States chief accident investigating body, the National Transportation Safety Board, has for years been pushing for a third type—the cockpit video recorder.

Physically, recorders are impressive feats of engineering. Housed inside a thick steel body and embedded in a hard, wax-like substance, the solid-state (no moving parts) electronic recorders are designed to withstand massive G forces. In the China Eastern crash, the 737 was descending straight down at speeds fast enough that it began to shed parts. Still, the recorders survived, and technicians were able to extract data from them. While it’s such a big number that’s hard to make sense of, recorders are built to withstand more than 3,000 Gs, or in other words, 3,000 times the force of gravity. This is roughly equivalent to a plane crashing straight down into a solid surface at better than 250 knots. Few crashes, including the China Eastern mishap, meet or exceed these G-force limits.

A flight data and cockpit voice recorder as typically installed in a turbine aircraft.
A flight data and cockpit voice recorder as typically installed in a turbine aircraft. Photo courtesy of YSSYguy/Wikipedia

As time has passed and technology has improved, flight data recorders have become more sophisticated. Modern units can record more than a thousand of what are referred to as “parameters,” all of them related to the plane’s flight status. These include the obvious details: How fast was the plane going? Which controls were the pilots manipulating; how many G forces was the plane subject to; and how were the many, complex systems of the aircraft performing? 

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Voice recorders are, in contrast, dirt simple. Constructed nearly identically to FDRs, CVRs have one job, to record the sounds from the cockpit, chiefly but not solely the voices of the crew members on the flight deck. Investigators have used audio extracted from CVRs to solve mysteries based on the sounds the airplane’s equipment made, such as flaps, landing gear or wind speed, picked up by the cockpit mics during the crash sequence.

Based on early reports, investigators and analysts working on the crash of China Eastern Flight 5735 have succeeded in extracting data from the FDR. That data has shown that one of the pilots manipulated the flight controls—literally by pushing forward (probably as hard as he could) to dive the plane into the ground, thereby killing himself and another 131 innocent passengers and crew members.

One might assume that the voice recorder would have been even more helpful, but this is not always the case. In emergency situations, pilots are often so focused on trying to avert disaster that they say literally nothing until, in many instances, they scream in terror. This is why the NTSB declines to release the actual audio captured by the CVR in crashed airplanes, opting instead to publish transcripts of the recordings.

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While their probes into air crashes are aided greatly by flight recorders, investigators want more. They do indeed want cockpit video recorders, which would do just what it sounds like they would, and they would like units with very long recording lives. If the flight recorders for Malaysia 370 are ever found, it’s unlikely that they would be of much help, as both units were designed to record over themselves in a loop. At a minimum, investigators want recorders that have a longer duration than the longest possible flight in an aircraft.

Based on the case of Malaysia 370 again, investigators also want recorders that won’t sink to the bottom of bodies of water, that is if the crash they are documenting ends in the water. High on the hit list, too, are recorders that can send locating signals more powerfully and for longer than current recorders are capable of doing.

Today, when recorders can be found and their data extracted (which is almost always) they provide investigators with tools their predecessors 50 years ago could only have dreamed of in their quest to explain air disasters, and in the process, propose changes to rules that would prevent similar accidents in the future.

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