The search is ongoing for answers in the crash of an Ethiopian Airlines plane shortly after takeoff for Nairobi on Sunday, killing all 157 people on board.
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Ethiopian authorities said Wednesday that they will send the flight recorders recovered from the plane to an as-yet-unspecified European country for analysis.
The U.S. Federal Aviation Administration said "new information" indicated some similarities with a Lion Air crash in the Java Sea that killed 189 people in October. The agency grounded the jets while investigators determine whether there was a shared cause of the two crashes. The FAA had been criticized for allowing the planes to fly while dozens of countries suspended their use.
U.S.-based Boeing said it has "full confidence" in the 737 Max but supports the decision to temporarily ground all 371 of the planes.
Here are some questions and answers about the crash, the plane and the investigation:
WHAT PROMPTED THE FAA TO ACT?
The Federal Aviation Administration initially declined to ground the 737 Max, cautioning against comparing the Ethiopian Airlines crash with the October crash of a Lion Air 737 Max off of Indonesia or assuming that they are related.
Pressure on the FAA grew as more than 40 countries including the entire European Union and Canada suspended flights by the plane or barred it from their airspace.
On Wednesday, the agency ordered the grounding, saying that new information from the wreckage in Ethiopia, along with satellite-based tracking of the flight path, "indicates some similarities" between the Ethiopia and Indonesia crashes.
The agency said it was ordering the jets' grounding while investigators determine whether there was a shared cause of the two crashes.
Indonesian investigators have not stated a cause for the Lion Air crash, but are examining whether faulty readings from a sensor might have triggered an automatic nose-down command to the plane, which the Lion Air pilots fought unsuccessfully to overcome. The automated system kicks in if sensors indicate that a plane is about to lose lift, or go into an aerodynamic stall. Gaining speed by diving can prevent a stall.
The Lion Air plane's flight data recorder showed problems with an airspeed indicator on four flights, although the airline initially said the problem was fixed.
WHAT ARE FLIGHT RECORDERS AND WHY ARE THEY SO IMPORTANT?
The FAA requires every large commercial aircraft to have a cockpit voice recorder and a flight data recorder, known as "black boxes." The recorders, which can withstand temperatures of 1,100 degrees Celsius and water depths of 20,000 feet, collect information about a flight.
Voice recordings pick up the flight crew's voices, as well as other sounds inside the cockpit. Information from the data recorder can generate a computer animated video reconstruction of the flight. Investigators can then visualize the airplane's altitude, instrument readings, power settings and other details of the flight to help with the investigation, according to the National Transportation Safety Board. Some experts have said the search for answers about what caused the crash could take months.
WHAT IS BOEING DOING IN RESPONSE TO THE CRASHES?
Boeing says it supports the temporary grounding of the entire fleet of Max planes, while reiterating it believes the planes are safe.
Prior to the grounding, Boeing had promised to upgrade some flight-control software "in the coming weeks."
Boeing began working on the changes shortly after the Lion Air crash. It is tweaking the system designed to prevent an aerodynamic stall if sensors detect that the plane's nose is pointed too high and its speed is too slow.
A Boeing spokesman said once updated software is installed, the system will rely on data from more than one sensor to trigger a nose-down command. Also, the system won't repeatedly push the nose down, and it will reduce the magnitude of the change, he said. There will also be more training for pilots.
HAVE THERE BEEN COMPLAINTS ABOUT THE PLANES?
Airline pilots on at least two U.S. flights reported that an automated system seemed to cause their Boeing 737 Max planes to tilt down suddenly. The pilots said that soon after engaging the autopilot on Boeing 737 Max 8 planes, the nose tilted down sharply. In both cases, they recovered quickly after disconnecting the autopilot.
As described by the pilots, the problem did not appear related to a new automated anti-stall system that is suspected of contributing to the Lion Air crash in Indonesia.
The pilot reports were filed last year in a data base compiled by NASA. They are voluntary safety reports and do not publicly reveal the names of pilots, the airlines or the location of the incidents. It was unclear whether the accounts led to any actions by the FAA or the pilots' airlines.
WERE AIRLINE EMPLOYEES AND PASSENGERS WORRIED?
Patrick Smith, a Boeing 767 pilot who writes a column called "Ask the Pilot," says he's been telling passengers who ask that the 737 Max is safe. He also says he hasn't heard of any pilots who worry about flying the plane.
Others didn't want to take any chances. The Association of Professional Flight Attendants, which represents more than 26,000 flight attendants at American Airlines, called on CEO Doug Parker to "strongly consider grounding these planes until an investigation can be performed."
This story has been updated to correct the death toll in the Lion Air crash. It was 189, not 187.