WASHINGTON (BLOOMBERG) - US aviation regulators have proposed a long list of fixes to Boeing's grounded 737 Max in one of the most extensive set of requirements the agency has issued following an accident.

The Federal Aviation Administration (FAA) on Monday (Aug 3) asked for public comment on the changes it expects to require to the plane linked to two fatal crashes.

In addition to fixes specific to the system implicated in the accidents, it would mandate broad computer changes to improve reliability, add a warning light that was inoperative in the two crashes and require rerouting electrical wires that do not meet safety rules.

The release of the FAA proposal shows that, after 16 months of the plane's grounding and a series of investigative reports and congressional hearings, aviation regulators are satisfied that the fixes will allow the plane to safely resume service. Flight tests of the redesigned systems by FAA were completed July 2.

The agency "has preliminarily determined that Boeing's proposed changes to the 737 Max design, flight crew procedures and maintenance procedures effectively mitigate the airplane-related safety issues" revealed in the crashes, it said in a summary report it included with the proposal.

The step is a welcome milestone, but needs to be approached with caution because other critical regulators in Europe and Canada have not acted, said Mr Richard Aboulafia, an industry analyst with Teal Group.

"We'll see what the commentary period brings and we'll see, most importantly, what the international partners do," Mr Aboulafia said.

While the European Aviation Safety Agency has not yet been able to conduct its own test flights of the Max, Boeing has demonstrated compliance under the European standards, the FAA said in its summary of the review.

The FAA's proposal for fixes and a preliminary report on its findings from its own internal investigation provide the most detailed accounting to date by the agency on the plane's original shortfalls and what went wrong in the two crashes.

The actions would cost US airlines about US$1 million (S$1.4 million) for the 73 planes registered in the country, the FAA said.

The agency did not estimate how much it would cost to make the required changes on the several hundred jets registered in other countries and did not account for Boeing's costs.

The Chicago-based manufacturer may cover some of the airlines' repair costs under warranty, FAA said.

The FAA also proposed requiring a test of the sensor - known as an angle of attack vane - that failed in the crashes and giving each aircraft a test flight before it is permitted to return to service.

The fixes outlined by FAA apply only to the Max models, but the agency said Boeing is considering some changes to the 737 Next Generation family, which preceded the Max but were not susceptible to the same failure. Boeing has sold more than 7,000 Next Generation planes, meaning costs could be higher if upgrades are eventually required.

"We're continuing to make steady progress towards the safe return to service, working closely with the FAA and other global regulators," Boeing said in an e-mailed statement. "While we still have a lot of work in front of us, this is an important milestone in the certification process."

The public has 45 days to comment on the FAA's plans. That means the plane most likely cannot get the official go-ahead to return until October at the earliest.

With airlines having to retrain pilots and perform maintenance on the grounded fleet, it will take weeks or months longer before the planes begin carrying passengers.

The changes listed by the FAA track with those that have been discussed for months, indicating no new problems were found in the later stages of the agency's review.

A pair of crashes - each tied to the same flawed system that malfunctioned and repeatedly dove the planes - killed 346 people less than five months apart.

The first occurred on Oct 29, 2018, in the Java Sea shortly after takeoff from Jakarta. The second was on March 10, 2019, in Ethiopia. The plane was grounded worldwide three days after the second crash.

The FAA said that a feature, known as Manoeuvring Characteristics Augmentation System (MCAS), had failed to meet safety requirements because it could fail with a single malfunctioning sensor and its repeated attempts to push down a plane's nose could overpower pilots who did not react properly.

MCAS was installed to meet FAA safety requirements, the agency said.

In both crashes, a malfunctioning sensor that erroneously reported the planes were pointed too high began automatically and repeatedly pushing down the noses of the jets.

Boeing's engineers assumed that pilots would know how to disable the motor driving down the nose and the procedure for doing so is taught to all 737 pilots. But both crews apparently became confused during the emergencies and neither was able to do so.

The redesigned system will have multiple protections to prevent such crashes in the future.

Instead of firing repeatedly, it can only activate once in the new design. It checks data from two sensors instead of one. And its power to dive a plane has been reduced.

In addition, there will be training for pilots and documentation of how the system works to help crews prepare for failures in the future. Boeing is also revising eight emergency procedures in its flight manuals for the plane, FAA said.

Originally, pilots were not told about the system. Both the company and the FAA did not realise it could pose a danger.

The accidents also prompted a re-examination of the 737 Max's flight-control computer, which led to a redesign of that system as well in an attempt to make it more resilient against failure.

Finalising new pilot training will be done separately by an FAA panel that will also give the public a chance to comment on the changes.