CHICAGO (Reuters) - Boeing Co's BA.N 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 on Oct. 29, 2018. A total of 346 people died in the two crashes.

FILE PHOTO: A worker walks past unpainted Boeing 737 MAX aircraft seen parked in an aerial photo at Renton Municipal Airport near the Boeing Renton facility in Renton, Washington, U.S. July 1, 2019. REUTERS/Lindsey Wasson/File Photo

Boeing has said it is targeting U.S. Federal Aviation Administration approval for the jet’s return to service in the United States by the end of the year, following changes to training and cockpit software at the center of both crashes.

That timeframe may differ in other countries.

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

SOFTWARE UPDATES AND FLIGHT TESTS

Boeing, which has been updating flight-control software, last week gave the FAA a “final software load” and “complete system description” of revisions to the plane that are meant to add layers of protection.

The next step is to complete pilot workload management testing and to have U.S. and international pilots conduct scenarios to determine training requirements before a key certification test flight, FAA Administrator Steve Dickson said.

The FAA 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.

Boeing has run over 800 test and production flights with the new software totaling over 1,500 hours, and earlier this month said it successfully conducted a dry-run of a certification flight test.

PILOT TRAINING

An international Joint Operational Evaluation Board must submit recommendations to the FAA’s Flight Standards Board (FSB), which determines U.S. pilot-training requirements.

Once the new pilot-training recommendations are published, there will be a 15-day public-comment period, during which airlines will be able to propose changes.

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

Boeing has said it is receiving ongoing feedback from pilots around the world, with nearly 90% of MAX operators having participated in simulator sessions with software updates.

CERTIFICATION

Once the test flights and pilot training recommendations are finalized, the FAA must approve the jets for flight. The agency has repeatedly said it will not certify the plane until it is safe to do so.

European regulators expect to clear the 737 MAX in January at the earliest, following flight trials by European test pilots scheduled for mid-December.

Boeing Chief Executive Dennis Muilenburg has said it is possible not all regulators around the world will concurrently approve the MAX to fly again.

COMMERCIAL FLIGHTS

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

Once regulators approve the 737 MAX for flight, airlines must install the new software, run a series of maintenance checks on the idled jets and implement new pilot training, a process that some have said could take one to two months.

U.S. and Canadian airlines have canceled MAX flights into January or February.

PROBES

Independently from the 737 MAX regulatory process, Boeing is facing numerous probes into the development of the aircraft by regulators, U.S. lawmakers and the Department of Justice.

It also faces more than 100 lawsuits by victims’ families, alleging it designed a flawed airplane. Some of them have demanded a full certification review of the 737 MAX.