Boeing's chief executive Dennis Muilenburg has joined Boeing test pilots aboard a 737 Max flight for a demonstration of updated anti-stall software.

The planemaker released photos of Muilenburg, also company's chairman and president, getting on board the flight and in the cockpit of a MAX, the aircraft type grounded around the world after two crashes.

It said the flight crew performed different scenarios that exercised various aspects of the software changes to test failure conditions for its Maneuvering Characteristics Augmentation System, or MCAS.

Boeing said the software update worked as designed, and the pilots landed safely at its manufacturing base near Seattle.

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The company said it will conduct additional test and demo flights as ''we continue to work to demonstrate that we have identified and appropriately addressed all certification requirements.''

Boeing's chief executive Dennis Muilenburg took a demonstration flight. Photo / Supplied
Boeing's chief executive Dennis Muilenburg took a demonstration flight. Photo / Supplied

The MCAS system has been linked to crashes of a Lion Air flight and Ethiopian Airlines 737 MAX planes within five months of each other.

Boeing said it would submit its update to the Federal Aviation Administration once the work has been completed in the coming weeks.

''Safety is our first priority, and we will take a thorough and disciplined approach to the development and testing of the update to ensure we take the time to get it right. This demonstration flight is another step in that process.''

Boeing and the FAA are under intense scrutiny for the MCAS and how the aircraft was certified.

At the weekend, pilots from the three US airlines that fly Boeing 737 Max planes tested software changes developed by Boeing.

It was revealed overnight that pilots of an airliner that crashed last month in Ethiopia initially followed Boeing's emergency steps for dealing with a sudden downturn of the nose of their plane but could not regain control, according to a published report.

The Wall Street Journal reported that pilots of the Ethiopian Airlines Boeing 737 Max turned off a flight-control system but still couldn't get the plane to climb.

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They turned the system back on and tried other actions before the plane crashed, the paper said, citing people familiar with preliminary findings of the crash investigation. In a statement, Boeing urged against speculating before the preliminary report and flight data from the plane are released.

The Journal says the pilots' actions are still being evaluated by investigators but could raise questions about assertions made by Boeing and US regulators in the aftermath of another Max crash in October that pilots could regain control simply by following steps to turn off a specific anti-stall system.

Investigators are examining the crashes that killed all 346 aboard the 737 Max 8 jets operated by Indonesia's Lion Air and Ethiopian Airlines, including the role of MCAS, which under some circumstances could automatically turn the plane's nose down to prevent an aerodynamic stall.

The Journal reported that data downloaded from the plane's so-called black boxes indicates that the Ethiopian Airlines pilots followed recommendations to flip two switches that disconnected the system, but the plane kept sinking.

They turned a manual wheel that adjusts the plane's tail, and used electric switches in an effort to climb, but that reactivated MCAS, which continued to push the plane's nose lower.

Ethiopian investigators are expected to release their preliminary findings this week.

Boeing is the focus of investigations by the Justice Department, the Transportation Department's inspector general, and congressional committees. Investigations are also looking at the role of the FAA, which certified the Max in 2017 and declined to ground it after the first deadly crash in October.

Fiji Airways was the only airline with the plane operating in this country and the plane was prevented from flying here by the CAA after regulators in other countries took action.