The Boeing 737 MAX is the latest variant of the most successful passenger aircraft ever. Many airlines ordered the fuel-efficient jet. Nevertheless, some serious defects have come to light, since the Lion Air accident in Indonesia from October last year. Several aviation authorities in the world have ordered airlines to ground all Boeing 737 MAX aircraft, after the Ethiopian Airlines crash on Sunday.

The 737 MAX is the fastest-selling airplane in Boeing history. Boeing received more than 5,000 orders within 5 years for the Boeing 737 MAX. And the list of airlines that order the type is getting longer and longer.

After American Airlines, Southwest Airlines and United Airlines also Air Canada, Aeromexico, GOL, Norwegian, Ryanair, TUI fly, Jet Airways, Turkish Airlines, Flydubai, Lion Air, Ethiopian Airlines and dozens of others followed.

The 1st aircraft was taken into service in May 2017 by Malindo Air from Malaysia, owned by the Indonesian Lion Air Group. Lion Air itself was involved in a fatal crash of one of its brand new aircraft, a B737 MAX 8. Last Sunday a second B737 MAX 8 – which was in service of Ethiopian Airlines for nearly half a year – crashed, killing 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 the Lion Air crash in October, where a B737 Max 8 plunged into the Java Sea minutes after takeoff, killing all 189 people on the plane.

Safety experts took note of the similarities but cautioned against quickly drawing too many parallels between the two crashes.

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.

“There were no defects prior to the Ethiopian Airlines 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 aircraft manufacturer 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 sent 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 4 to assist Ethiopian authorities. Boeing and the U.S. investigative agency are also involved in the Lion Air investigation.

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 Lion Air 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 a new system that could automatically point the plane’s nose down based on sensor readings. 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.

The American Federal Aviation Administration (FAA) said that the Boeing 737 MAX 8 can continue to fly. The aircraft is “airworthy”, the aviation authority announced on Monday. The FAA is still busy collecting data and is in contact with international aviation authorities.

According to Reuters, Boeing confirmed late on Monday it will deploy a software upgrade to the 737 MAX 8, a few hours after the Federal Aviation Administration said it would mandate “design changes” in the aircraft by April.

Boeing did not reference Sunday’s Ethiopian Airlines crash in connection to the software upgrade. The statement did express the company’s condolences to the relatives of the 157 people who died, however.

The aircraft manufacturer said in the aftermath of the Lion Air Flight crash in October, it has for several months “been developing a flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer.” The software upgrade “will be deployed across the 737 MAX fleet in the coming weeks,” it said.

Singapore and Australia suspended operations of all Boeing 737 MAX aircraft in and out of their airports on Tuesday, while Indonesia and China grounded their fleets of the Boeing 737’s latest model after it suffered a 2nd fatal crash in less than 5 months.

Nearly 40% of the in-service fleet of 371 Boeing 737 MAX jets globally is grounded, according to industry publication Flightglobal, including 97 jets in the biggest market, China.

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(Top photo: pjs2005 [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)])