The US Federal Aviation Administration allows airlines to make minor modifications in previously certified aircraft to meet their operational needs, a rule Boeing followed when building the 737 MAX for foreign carriers, aviation analysts said.

Some airlines chose to purchase the "angle of attack" system display for readings of two sensors and a "disagree light" to be activated if the sensors produce conflicting readings while others did not.

Ethiopian Airlines and Lion Air in Indonesia — involved in fatal crashes five months apart — reportedly declined to buy the two add-on safety devices that might have helped the pilots keep the doomed MAX jets in the sky. A total of 346 people died in the two planes crashes; there were no survivors.

However, Ethiopian Airlines CEO Tewolde Gebremariam said it was important not to confuse safety-critical equipment with optional items, Reuters reported on Sunday.

"A Toyota is imported with all the necessary equipment to drive, like the engine and the wheels, but with air conditioning and the radio optional," Tewolde said.

"When Boeing supplies aircraft, there are items which are mandatory for safety, and then there are optional items," he added, noting the angle of attack indicator was optional.

Another Ethiopian Airlines executive, however, criticized the software on the MAX.

"After the crash, it came to our attention that the system is aggressive," said Yohannes Hailemariam, vice-president for flight operations, Reuters reported on Sunday.

"It gives a message of stalling, and it takes immediate action, which is faster than the action which pilots were briefed to take by Boeing," said Yohannes, himself a pilot with more than 30 years' experience, including flying the Boeing 777 and 787.

Boeing also is faced with the first publicly announced cancellation of an order for the MAX 8 aircraft. Garuda Indonesia, the national airline of Indonesia, said on Friday that it told Boeing in a March 14 letter that it wants to cancel a $6 billion order for 49 of the 737 MAX 8 jets, saying passengers have lost confidence in the aircraft following the two deadly crashes. Boeing declined to comment.

The New York Times on Saturday reported that Boeing produced the MAX planes in response to competition in 2011, when longtime customer American Airlines considered placing order with European rival Airbus.

"The competitive pressure to build the jet — which permeated the entire design and development — now threatens the reputation and profits of Boeing, after two deadly crashes of the 737 MAX in less than five months. Prosecutors and regulators are investigating whether the effort to design, produce and certify the MAX was rushed, leading Boeing to miss crucial safety risks and to underplay the need for pilot training," the Times story said.

As to the planes' safety features, "these are buyer-specific options", Robert Mann, president of R.W Mann and Co, an aviation consultant in Port Washington, New York, told China Daily.

"Each carrier has to tell the FAA how it plans to configure and operate each type of aircraft in its fleet. The FAA is comfortable with low-spec and high-spec airplanes — that's evident. There are lots of issues here, but I don't believe any of them are nefarious," he said.

Scott Hamilton, managing director of Leeham Co, an aviation consultancy in Seattle, told China Daily that the FAA should have mandated both safety features.

"I think it was a poor decision not to include both as standard equipment," he said. "There are other safety lights in the cockpit, so why not here? This could overwhelm the crew with too much information, but a properly trained crew should be able to handle multiple warning lights and emergencies. There have been instances where that's been the case."

Hamilton said he did not know the cost of the safety features, but noted it would be an insignificant percentage of a $120 million plane.