Boeing's 737 MAX is not expected to return to service until January 2020 as regulators expand safety checks after the jets were grounded in March following two deadly crashes.

Aviation experts and analysts say they expect the lengthy delay as the Federal Aviation Administration pledges to resolve all safety issues before allowing the planes back in the air, The Wall Street Journal reports.

No official timeline has been confirmed by the company but the checks on software has repeatedly delayed their progress with a growing list of issues for them to look over.

Boeing is now said to be looking at other potential problems, including emergency recovery procedures to electronic components. Some checks are said to be looking at earlier 737 models too.

Boeing's 737 MAX is not expected to return to service until January 2020. The grounded aircraft is seen parked in an aerial photo at Boeing Field in Seattle, Washington, in July

The scene of the Ethiopian Airlines crash near Addis Ababa on March 10. The crash set off one of the widest inquiries in aviation history and cast a shadow over the Boeing 737 MAX model intended to be a standard for decades

The specific software fix to automated system MCAS is said to have been completed and waiting for approval.

But additional concerns have now been raised from studies and tests on the jet in the interim.

And even once the fixes are completed Boeing will need both the FAA to approve them and airlines to carry out their own maintenance procedures, according to experts.

PROBLEMS ON BOARD THE BOEING 737 MAX The 737 MAX was grounded worldwide in March after an Ethiopian Airlines plane plunged to the ground soon after take-off, five months after a similar Lion Air fatal crash off the coast of Indonesia. Concerns were initially raised after a system called MCAS overpowered pilot commands, pushing down the noses of both jets that crashed. Experts then raised concern over pilots being able to manually move a flight-control wheel in extreme circumstances amid more testing. Then in in June further a new flaw was uncovered that is estimated it will take until at least September to fix. Advertisement

American Airlines Group Inc said on Sunday it is extending for a fourth time cancellations of about 115 daily flights into early November due to the ongoing grounding of the jets.

The airline's decision was expected after the FAA, which must reapprove the jets for flight following two fatal crashes, last month uncovered a new flaw that Boeing had estimated will take until at least September to fix.

Boeing has said it will 'provide the FAA and the global regulators whatever information they need,' and will not offer the 737 MAX 'for certification by the FAA until we have satisfied all requirements'.

The 737 MAX, which had been Boeing's fastest-selling aircraft thanks to its fuel-efficient engines and longer ranger, was grounded worldwide in March after an Ethiopian Airlines plane plunged to the ground soon after take-off, five months after a similar Lion Air fatal crash off the coast of Indonesia.

Boeing hopes a software upgrade and new pilot training will add layers of protection to prevent erroneous data from triggering a system called MCAS, which was activated in both the planes before they crashed.

American, the world's largest airline and the second largest MAX operator in the United States, most recently had planned to keep the MAX, which it used on most flights between New York's LaGuardia airport and Miami, off its schedule through September 3.

Boeing is now looking at other potential problems including emergency recovery procedures to electronic components, with some checks looking at cover earlier 737 models

American, with 24 737 MAX aircraft and dozens more on order, is scheduling without the jets through Nov. 2.

'American Airlines remains confident that impending software updates to the Boeing 737 MAX, along with the new training elements Boeing is developing in coordination with our union partners, will lead to recertification of the aircraft this year,' the airline said in a statement on Sunday.

It has been substituting other aircraft for its busiest flights while canceling others and temporarily suspending direct flights between Oakland, California, and Dallas-Fort Worth.

Among other U.S. MAX carriers, Southwest Airlines Co has removed the aircraft from its scheduling through Oct. 1, and United Airlines Holdings until Nov. 3. Southwest is the world's largest MAX operator.