Ryanair Boeing 737 EI-DYH departing London Luton Airport

Inset: A view of a Boeing manufacturing facility Photos: IC

Boeing Co's best-selling jet, the 737 MAX, was grounded globally in March, days after the crash of an Ethiopian Airlines flight that followed a similar Lion Air disaster in Indonesia in last October. A total of 346 people died in the two crashes.

Boeing has spent months updating critical flight control software at the center of both crashes, with the hope of winning Federal Aviation Administration approval for the planes to fly again in the US between October and December.

That timeframe may differ in other countries.

Following are some of the key steps that need to be taken before the 737 MAX returns to the skies.

Software updates and flight tests

Boeing is still finalizing updates to flight control software, which will be followed by a certification test flight. CEO Dennis Muilenburg has said he was targeting the September timeframe for the flight, but on Wednesday declined to give a specific date. Federal officials say it may not occur until October, and the FAA may not give the green light to resume flights until November.

Boeing and FAA pilots have been testing the updated software for months and the FAA is also inviting regular 737 MAX line pilots to run tests.

These tests must be completed before the aircraft is approved for a return to service, the FAA has said.

Pilot training



The FAA's Flight Standardization Board, which determines US pilot-training requirements, must issue new recommendations for what kind of training is needed before US airlines can fly passengers on the 737 MAX again.

The final report is expected in September and will undergo a 30-day period of comments from airlines and pilots.

A draft report in April recommended short, computer-based training and classroom instruction regarding the new software. Simulator training, which some overseas regulators are weighing, would take longer and is more costly.

Certification



Once the software updates, test flights and pilot training recommendations are finalized, the FAA must approve the jets for flight. It has said it will need about 30 days from the time the certification flight is completed before it decides whether to allow flights to resume. The agency has repeatedly said it will not certify the plane until it is safe to do so.

The European Union Aviation Safety Agency said on Tuesday that it will conduct its own test flights separate from, but in full coordination with, the FAA. The test flights are not yet scheduled and depend on Boeing's progress.

Muilenburg has said it is possible that not all regulators around the world will concurrently approve the MAX for flight.

Some officials do not expect the 737 MAX to actually resume flights until early 2020.

Commercial flights



Without the 737 MAX, all airlines that were flying the planes before its grounding have had to cancel some flights, cut routes and use aging jetliners. 

Once the MAX is approved to fly again, airlines will have to install the new software, run a series of maintenance checks on the idled jets and implement the recommended pilot training.

US airlines are hoping to fly the aircraft again later this year or early next year, pending regulatory approval.

Other airlines may have to await approval from their own regulators, some of whom have announced additional checks. 

China, a key importer that was the first country to ground the plane, has not said what - if any - additional checks it will carry out.

Probes

Independently from the 737 MAX regulatory process, Boeing is facing numerous probes by regulators, US lawmakers and the US Department of Justice. It also faces more than 100 lawsuits from victims' families alleging it had designed a flawed airplane.