Investigators sifting through the wreckage of the China Eastern Airlines Corp. jetliner that slammed into a hillside two days ago will glean the most conclusive evidence on what caused the crash from the two flight recorders, one of which has already been retrieved from the mangled parts.

The two separate devices, typically referred to as black boxes even though they’re painted distinctive orange to make them easier to see, can store audio recordings from the cockpit and data on hundreds of flight parameters—from speed and altitude to flap positioning and heading—that help investigators recreate the final fateful moments before a crash. 

While the aircraft in question experienced a particularly violent descent that resulted in total destruction of the jetliner and the presumed death of all 132 people on board, the devices—about the size of a large shoe box—are designed to withstand even the worst impacts. They can also function while submerged, emitting an ultrasonic pinging signal. 

The cockpit voice recorder, or CVR, stores conversations between the pilots, their communications with ground-control personnel, and ambient sounds such as engine noise or cockpit sounds. The device typically stores only the last two hours of conversation, providing a narrower data set than the second box: the flight-data recorder, or FDR. This contains no sound recordings but stores the main parameters from at least 25 hours of flight time, allowing for a more technical readout of the aircraft’s performance.

Fire Resistance

Analyzing the boxes is key to discovering why the Boeing Co. 737-800NG jet plunged out of the sky at close to the speed of sound before slamming into a hillside. The pilots didn’t respond to calls from air-traffic controllers after the plane tipped into its nosedive, authorities said. 

Some aircraft data is transmitted via satellite, so even without the recorders investigators know that Flight 5735, from Kunming to Guangzhou, was traveling at an altitude of about 29,000 feet (8,839.2 meters) when it began a sudden descent. The jetliner was cruising at about 595 miles (957.56 kilometers) per hour before the dive, according to data transmitted by the plane and captured by Flightradar24. 

But the recorders will have infinitely more information to help investigators understand what caused the crash. Some aviation experts have said the devices might be badly damaged given the high-energy impact. The retrieved unit has in fact been badly crushed, Xinhua News Agency reported, citing officials at a briefing. It didn’t say which of the canisters was found.

The two devices, built into fortified canisters that can withstand intense heat for at least 30 minutes, start recording when pilots turn on the first engine and remain active until after the plane has come to a complete standstill. 

They are typically housed near the aircraft tail because that part of the structure is considered the strongest section of a jet. While the recorders emit an underwater signal that travels 3 kilometers, the batteries last only a month, giving salvage crews a limited window of opportunity to find them when they’re submerged. 

Long Searches

Black boxes were introduced to commercial aircraft in the 1950s to help investigate crashes. Early devices yielded only limited information and were far less shock-resistant than modern versions, which use solid-state memory devices to record the data rather than the magnetic tape spools used on old versions.

Black boxes have sometimes been recovered long after an accident. One device, from a South African Airways Boeing 747 that crashed in the Indian Ocean in 1987, was found a year later by a miniature remote-controlled submarine at a depth of 13,200 feet, the deepest such salvage operation conducted until that point.

In 1991, a mini-submarine recovered a recorder of a jet flying from mainland Italy to Sicily that crashed into the Mediterranean Sea in 1980, killing all 81 people. The same sub was used in the 2009 Air France 447 crash investigation, a mission that lasted several years before the wreckage and the boxes were finally discovered at the bottom of the Atlantic. 

In that instance, investigators were able to conclude that faulty speed readings had confused the cockpit crew, who mistakenly put the Airbus A330 into a stall from which they were unable to recover.

When Germanwings Flight 9525 went down in southern France in 2015, the voice recordings helped determine that the co-pilot had deliberately flown the Airbus A320 into a mountainside after locking the captain out of the cockpit. 

Conversely, failure to find the boxes can dramatically complicate investigators’ work. In the case of Malaysia Airlines Flight 370 in 2014, what caused the disappearance and crash of them Boeing 777-200 carrying 239 people remains the biggest mystery in aviation history. The plane disappeared en route from Kuala Lumpur to Beijing, about 40 minutes into the flight. After three years, a search for the aircraft was abandoned. 

Some pieces of wreckage eventually washed up on shores thousands of miles away, though the flight recorders remained elusive.