Indonesian airline Garuda plans to cancel a $6 billion order for Boeing 737 MAX jets because some passengers say they would be frightened to board the plane after two fatal crashes, although industry analysts said the deal was already in doubt.
In the United States, American Airlines pilots prepared to test Boeing Co's planned software upgrade for an anti-stall system on MAX simulators this weekend, saying they want their own safety guarantees on the fix.
Ethiopia and French investigators have pointed to "clear similarities" between the two crashes, which killed 346 people, putting pressure on Boeing and U.S. regulators to come up with an adequate fix.
However, questions have emerged about how much is known about the cause of the Ethiopian crash, 11 days after black box data and voice recorders were found.
None of the parties agreed to comment.
Ethiopia said on Thursday it had started reviewing data with U.S. and French safety investigation authorities.
However, the airline had been reconsidering its order for 49 of the narrowbody jets before the Ethiopian crash, including potentially swapping some for widebody Boeing models.
Indonesia's Lion Air has also said it might cancel 737 MAX aircraft, though industry sources say it is also struggling to absorb the number of planes on order.
No direct link has been proven between the crashes, which killed a total of 346 people, but attention has focussed on whether pilots had the correct information about the "angle of attack" at which the wing slices through the air.
Boeing now plans to make compulsory a light to alert pilots when sensor readings of the angle of attack do not match - meaning at least one must be wrong -, according to two officials briefed on the matter.
Investigators suspect a faulty angle-of-attack reading led the doomed Lion Air jet's computer to believe it had stalled, prompting its anti-stall system, called MCAS, repeatedly to push the plane's nose down.
Norwegian Air played down the significance of the compulsory light, saying that, according to Boeing, it would not have been able to prevent erroneous signals that Lion Air pilots received before their new 737 MAX plane crashed in October.
Boeing must be cautious with how it characterizes the safety alert, risking legal claims by saying it could have made a difference in the crash while not wanting to suggest that the retrofit is meaningless, legal experts said.
The Lion Air plane did not have the warning light installed, and Ethiopian Airlines did not immediately comment on whether its crashed plane had the alert.
But the Ethiopian carrier, whose reputation along with Boeing's is at stake, issued a statement on Friday emphasising the modernity of its safety and training systems, with more than $500 million invested in infrastructure in the past five years.
The Ethiopian crash has set off one of the widest inquiries in aviation history and cast a shadow over the Boeing 737 MAX model intended to be a standard for decades.
Boeing did not comment on the plan to make the safety feature standard, but separately said it was moving quickly to make software changes and expected the upgrade to be approved by the U.S. Federal Aviation Administration (FAA) in coming weeks.
Chicago-based Boeing will also retrofit older planes with the cockpit warning light, the officials told Reuters.
Since the Ethiopian crash, Boeing shares have fallen 14 percent.
Pressure has mounted on the company from U.S. legislators, who are also expected to question the FAA. The company faces a criminal investigation by the U.S. Justice Department as well.
Several lawsuits have already been filed on behalf of victims of the Lion Air crash referring to the Ethiopian accident. Boeing declined to comment on the lawsuits.