Even after the finding of the two “black boxes," it may be a long time before investigators figure out what caused a China Eastern Airlines Boeing 737-800 to nosedive into the ground last week, killing all 132 people aboard.
Searchers found the first black box two days after the March 21 crash and the second one on Sunday, six days later. Experts say it could take weeks or months to analyze their contents and start to find answers.
WHAT IS A BLACK BOX?
Most commercial jetliners carry two recorders to help investigators try to determine the cause of accidents. Despite their name, they are usually orange to make them easier to find.
The cockpit voice recorder captures the voices of pilots and others in the cockpit, audio alerts and the sound of engines or switches being moved. It stores at least two hours of sound before starting to record over itself.
Flight data recorders constantly capture at least 88 pieces of information, including airspeed, altitude, direction and whether the nose of the plane is pointing up, down or level. They can also record the position of wing flaps and whether the plane is flying by autopilot. They usually record 25 hours of data.
Both stop recording when power is lost. They are designed to withstand fire and powerful impacts and usually are installed in the back of the cargo compartment. They are equipped with radio beacons that activate under water but not on land.
Flight recorders used to use magnetic tape but now are digital, which allows them to hold more information.
WHAT HAVE INVESTIGATORS FOUND?
Searchers found the cockpit voice recorder first and then the flight data recorder. The two orange cylinders, the size of a large, elongated cannister, were dirt-covered. The second one was found under 1.5 meters (5 feet) of soil.
Officials said the exterior of the cockpit voice recorder was damaged but have given no indication whether the recording was intact. Both will be examined and analyzed in Beijing.
WHAT CAN THE RECORDERS TELL CRASH INVESTIGATORS?
The recorders can tell investigators what pilots were doing before a crash, whether they were aware of problems and how engines and other equipment were functioning. They are especially important following disasters such as the China Eastern crash in which no survivors are found and air traffic controllers heard nothing from the pilots.
In the 2018 crash of a Lion Air Boeing 737 Max in Indonesia, the flight recorder showed pilots struggled to keep its nose up, fighting an automated system that pushed it down. An Ethiopian Airlines 737 Max crashed in a similar manner four months later. The crashes led to the grounding of the plane, which is a different model than the 737-800.
WHAT WILL THE INVESTIGATION LOOK LIKE?
China will lead the investigation. It will include a representative of the U.S. National Transportation Safety Board and representatives from Boeing and CFM, the General Electric Co.-Safran joint venture that made the engines. The NTSB has said that talks were ongoing with China to address COVID-19 quarantine requirements.
Investigators will look at the plane's maintenance history, the training and record of the pilots and weather data. They will examine pieces of the wreckage for clues. Even the size of the debris field is important. When wreckage is spread over a very large area, it could indicate the plane was breaking up before hitting the ground.
