Boeing has taken its most aggressive moves yet to defend its core 737 airliner franchise, developing software fixes to prevent failures of an automated flight control system scrutinised after two deadly crashes in the past five months.

Key points: Boeing says the software update will make the anti-stall system "more robust"

Boeing says the software update will make the anti-stall system "more robust" The company said the upgrade was independent of the Ethiopian Airlines crash

The company said the upgrade was independent of the Ethiopian Airlines crash US airlines want regulators to sign off on the upgrade

Boeing, in the midst of one its worst crises in years, is under pressure from crash victims' families, airlines, US politicians and regulators around the world to prove the automated flight control systems aboard its 737 MAX aircraft are safe, and that pilots have the training required to override the system in an emergency.

A Boeing official in Seattle said on Wednesday the timing of the software upgrade was "100 per cent independent of the timing of the Ethiopian accident", and the company was taking steps to make the anti-stall system "more robust".

Sorry, this video has expired The plane came down near the town of Bishoftu, 50 kilometres south of Addis Ababa

There was no need to overhaul Boeing's regulatory relationship with the US Federal Aviation Administration (FAA) now, the company said.

"We are going to do everything that we can do to ensure that accidents like these never happen again," Mike Sinnett, vice-president for product strategy and future airplane development, told reporters.

A spokesman for the FAA said the agency had not reviewed or certified the software upgrade yet.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 54 seconds 54 s The 737 MAX was one of Boeing's most popular jets.

US transportation secretary Elaine Chao and some politicians questioned why Boeing did not require safety features on its top-selling plane that might have prevented the crashes.

"It is very questionable, if these were safety-oriented additions, why they were not part of the required template of measures that should go into an airplane," she said, adding she was not ready to require that all safety options be retrofitted on existing planes.

Executives with US airlines welcomed Boeing's moves, but want regulators to sign off on the upgrade.

Southwest Airlines, which on Wednesday became the first major airline to formally cut its financial outlook for the year after being forced to pull its MAX fleet of 34 jets out of service, supported Boeing's decision.

"Boeing's software update appears to add yet another layer of safety to the operation of the MAX aircraft," Southwest's certificate chief pilot Bob Waltz said.

The Allied Pilots Association, which represents American Airlines Group Inc pilots, warned the certification process should not be rushed, and the fix should take into account any further information from an investigation into an Ethiopian Airlines crash on March 10.

Airline stocks turned positive after Boeing unveiled the software fix.

CFRA analyst Jim Corridore, who has a "buy" rating on Boeing, said news from the company and the Washington hearing were positive steps toward getting the MAX jets airborne again.

Pilots to get extra training

The world's largest plane maker said the anti-stall system, which is believed to have repeatedly forced the nose lower in at least one of the accidents, in Indonesia last October, would only do so one time after sensing a problem, giving pilots more control.

Sorry, this video has expired Investigators sort through Lion Air debris

It will also be disabled if two airflow sensors that measure the "angle of attack", or angle of the wing to the airflow, a fundamental parameter of flight, offer widely different readings, Boeing said.

The anti-stall system — known as MCAS, or Manoeuvring Characteristics Augmentation System — has been pinpointed by investigators as a possible cause in a fatal Lion Air crash in Indonesia and the one in Ethiopia.

Existing 737 pilots will also have extra computer-based training following criticism that MCAS was not described in the aircraft manual.

Boeing has previously said existing cockpit procedures would cover any example of runaway controls caused by MCAS.

The changes were drawn up in response to the Lion Air crash but are seen as crucial to regaining the trust of pilots, passengers and regulators after the Ethiopia crash prompted a worldwide grounding of Boeing 737 MAX planes.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 40 seconds 40 s US aviation authorities explained the data that prompted the grounding of 737 MAX jets.

Reuters