WASHINGTON • Once again, an almost brand new Boeing 737 Max 8 has crashed not long after take-off, as it flew erratically and pilots asked to return to the airport.

The crash on Sunday morning of a jetliner in Ethiopia bears unmistakable similarities to the Oct 29 tragedy off the coast of Indonesia involving the same plane model, prompting questions about whe-ther a design issue that arose du-ring the earlier accident could be to blame.

The stakes for Boeing and one of its most popular models are enormous. But veteran crash investigators and the airline say there is too little data to draw a direct link between the two at this stage of the investigation.

United States pilot unions, which were critical of Boeing after the Indonesia accident for withholding information on design changes in the B-737 Max 8, were also cautious.

"We would never speculate on the accident or anything at this point because it is just too new," said Captain Jon Weaks, president of the Southwest Airlines Pilots Association.

Ethiopian Airlines Flight ET302 took off from Addis Ababa a little before 8.40am local time and went down about six minutes later, killing all 157 on board, according to a statement from the company.

Its crew had reported an unspecified problem and asked to return to the airport, airline chief executive officer Tewolde GebreMariam said at a media conference.

The plane's initial flight track was very unusual at a time when airliners typically climb steadily to get safely away from terrain and to reach altitudes where engines burn more efficiently.

Instead, the plane twice descended briefly during the first 2½ minutes after take-off, according to tracking data provided by FlightRadar24.com. Its "vertical speed was unstable after take-off", the company said in a tweet.

Mr John Cox, president of Safety Operating Systems and a former airline pilot who has participated in dozens of crash investigations, said: "You want to keep the airplane climbing, even during flap retraction, to get it away from the ground.

"That is what makes the descent so unusual. That is something the investigators would want to look at."

He cautioned against drawing conclusions, however.

"At this point, it is just too early," he said.

As the plane was apparently out of range of FlightRadar24's Addis Ababa ground station, the flight track does not include the last few minutes of the flight, including its final dive.

Photos of the wreckage indicate that it hit the ground at a high speed and broke into small pieces.

Ethiopian Airlines issued a statement cautioning that "further investigation will be carried out to find the cause of the accident".

Last October, Indonesia's Lion Air Flight JT610 dove into the Java Sea about 11 minutes after take-off as pilots struggled to handle a malfunction that prompted an automatic safety device on the plane to repeatedly command a dive.

All 189 people on board died.

While Indonesian investigators have identified multiple failures of the airline's maintenance and raised questions about the pilots' actions, one of the factors under review in the investigation is Boeing's design.

The US Federal Aviation Administration (FAA) is working with the Chicago-based plane-maker on a possible software change to reduce the chances that such a failure could cause an accident in future.

The FAA, which certified the B-737 Max 8 and would most likely take the lead on ordering fixes to the plane if any new flaws are discovered, said it is "closely monitoring developments" in the investigation.

One difference between the two crashes: Indonesian investigators said pilots on prior flights with the doomed jet had reported mechanical problems. The crew of the Ethiopian Airlines B-737 had not reported any mechanical issues on an earlier flight from Johannesburg, the airline said.

The Lion Air plane lost altitude dozens of times before it crashed as the jet's computers, thinking it was in danger of losing control, continually tried to push its nose down.

The pilots countermanded the aircraft's software over and over, pulling it back into climbs, until they failed to do so and it crashed.

The Ethiopian plane's flight track is at least partially similar, said Mr Cox, who flew earlier versions of the B-737 during his former career.

However, for at least the time that it was tracked by Flight-Radar24, it was flying mostly level at an altitude range of 2,350m to 2,620m.

Many other things could have caused the plane to climb and descend, Mr Cox said.

For example, if pilots were handling an unrelated emergency and were planning to return to the airport, they could have deviated from their altitude simply as a result of being distracted by the emergency, he said.

Furthermore, Boeing issued a bulletin after the Indonesian accident alerting pilots that the plane might initiate a dive on its own and reminding them that a pair of switches in the cockpit can disable the motor that pushes the nose down.

"All B-737 pilots have been made aware of the potential and what would cause it," Mr Cox said.

Other carriers say they are monitoring the latest accident.

"We will closely monitor the investigation via Boeing and the National Transportation Safety Board," American Airlines said in a statement. The carrier flies 24 Max 8 aircraft.

The Boeing single-aisle model, revamped with larger engines, is the latest version of a jetliner that formed the backbone of global fleets for five decades. Southwest Airlines is the largest customer, with 31 of the B-737 Max 8 aircraft in its fleet.

"We have been in contact with Boeing and will continue to stay close to the investigation as it progresses," Southwest Airlines said in an e-mail statement.

"We remain confident in the safety and airworthiness of our fleet of more than 750 Boeing aircraft."

BLOOMBERG