Boeing

It blames a "culture of concealment" at Boeing, but says the regulatory system was also "fundamentally flawed".

Boeing said it had "learned many hard lessons" from the accidents.

But families of the victims accused the company and the regulator of continuing to hide information.

The US report is highly critical of both Boeing and the regulator, the Federal Aviation Administration (FAA).

The firm said it had inadvertently made an alarm feature optional instead of standard, but insisted that this did not jeopardise flight safety.

All 737 Max planes were grounded in March after an Ethiopian Airlines flight crashed, killing 157 people.

Five months earlier, 189 people were killed in a Lion Air crash.

The worldwide fleet of 737 Max planes totalled 387 aircraft at the time of the grounding.

But it's still not certain when the planes, that were grounded worldwide this month, will be allowed to fly.

Investigators have not yet determined the cause of the accidents.

As part of the upgrade, Boeing will install as a standard a warning system, which was previously an optional safety feature.

Neither of the planes, operated by Lion Air in Indonesia and Ethiopian Airlines, that were involved in the fatal crashes, carried the alert systems, designed to warn pilots when sensors produce contradictory readings.

Announcing the deal along with Boeing at the final trade day of the Singapore Airshow, Air Niugini chairman Sir Frederick Reiher said the carrier needed the planes "urgently".

The airline currently has a domestic service, as well as flights to Australia, Singapore, Fiji, the Philippines, Solomon Islands, Hong Kong, Vanuatu and Japan. It is planning to fly to China in the near future.

Dinesh Keskar, regional senior vice president for sales at Boeing Commercial Airplanes, said the aircraft had its first test flight in late January and was "performing exceptionally well".