Sunday's devastating plane crash in Ethiopia could renew safety questions about the newest version of Boeing's popular 737 airliner.
The Boeing 737 Max 8 operated by Ethiopian Airlines crashed shortly after taking off from the capital of Addis Ababa, killing all 157 people on board.
The plane was new. The weather was clear. Yet something was wrong, and the pilots tried to return to the airport. They never made it.
In those circumstances, the accident is eerily similar to an October crash in which a 737 Max 8 flown by Indonesia's Lion Air plunged into the Java Sea minutes after takeoff, killing all 189 people on the plane. But safety experts cautioned against quickly drawing too many parallels between the two crashes.
William Waldock, an aviation-safety professor at Embry-Riddle Aeronautical University, said suspicion will be raised because the same type of plane appeared to crash the same way — a fatal nosedive that left wreckage in tiny pieces.
"Investigators are not big believers in coincidence," he said.
Waldock said Boeing will look more closely at the flight-management system and automation on the Max. But he noted that it is very early, and more will be known after investigators find and analyze the Ethiopian plane's black boxes.
Alan Diehl, a former National Transportation Safety Board investigator, said the similarities included both crews encountering a problem shortly after takeoff, and reports of large variations in vertical speed during ascent, "clearly suggesting a potential controllability problem" with the Ethiopian jetliner.
But there are many possible explanations, Diehl said, including engine problems, pilot error, weight load, sabotage or bird strikes. He said Ethiopian has a good reputation, but investigators will look into the plane's maintenance, especially since that may have been an issue in the Lion Air investigation.
By contrast, the Ethiopian Airlines CEO told reporters that a maintenance check-up did not find any problems with the plane before Sunday's flight, "so it is hard to see any parallels with the Lion Air crash yet," said Harro Ranter, founder of the Aviation Safety Network, which compiles information about accidents worldwide.
"I do hope though that people will wait for the first results of the investigation instead of jumping to conclusions based on the very little facts that we know so far," he said.
Boeing representatives did not immediately respond for comment. The company tweeted that it was "deeply saddened to learn of the passing of the passengers and crew" on the Ethiopian Airlines Max airplane.
The Chicago-based company said it would send a technical team to the crash site to help Ethiopian and U.S. investigators.
A spokesman for the NTSB said the U.S. agency was sending a team of four to assist Ethiopian authorities. Boeing and the U.S. investigative agency are also involved in the Lion Air probe.
Indonesian investigators have not stated a cause for the Lion Air crash, but they 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.
Days after the Oct. 29 accident, Boeing sent a notice to airlines that faulty information from a sensor could cause the plane to automatically point the nose down. The notice reminded pilots of the procedure for handling such a situation, which is to disable the system causing the automatic nose-down movements.
Pilots at some airlines, however, including American and Southwest, protested that they were not fully informed about the new system. Boeing Chairman and CEO Dennis Muilenburg said in December that the Max is a safe plane, and that Boeing did not withhold operating details from airlines and pilots.
Diehl, the former NTSB investigator, said the Ethiopian Airlines pilots should have been aware of that issue from press coverage of the Lion Air crash.
The 737 is the best-selling airliner in history, and the Max is the newest version of it, with more fuel-efficient engines. The Max is a central part of Boeing's strategy to compete with European rival Airbus.
Boeing has delivered about 350 737 Max planes and has orders for more than 5,000. It is already in use by many airlines including American, United and Southwest.
The Lion Air incident does not seem to have harmed Boeing's ability to sell the Max. Boeing's stock fell nearly 7 percent on the day of the Lion Air crash. Since then it has soared 26 percent higher, compared with a 4 percent gain in the Standard & Poor's 500 index.