BOEING has announced changes to the control systems linked to two 737 Max plane crashes that killed a total of 346 people.

As part of the upgrade, Boeing will install as standard a visual warning system, which was previously an optional safety feature.

The 737 MAX planes were grounded worldwide following an Ethiopian Airlines disaster on March 10 that killed 157 people, five months after a Lion Air crash in Indonesia claimed 189 lives.

It is not yet clear when the planes will be allowed to fly.

A Boeing official in Seattle said the timing of the software upgrade was "100 per cent independent of the timing of the Ethiopian accident".

The company says it is taking steps to make the anti-stall system "more robust."

Boeing is under pressure from crash victims' families, airlines, lawmakers and regulators to prove the automated flight control systems of its 737 MAX aircraft are safe.

It also needs to be proved that pilots have the training required to override the system in an emergency.

STANDARD WARNING SYSTEM

The changes also would make standard visual warnings to the pilots if the system had stopped working.

Previously, those warning messages and displays had been optional.

Boeing said it would also change the design of the system so that it no longer relied on a single sensor.

"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.

The Federal Aviation Administration (FAA) said it had not reviewed or certified the software upgrade yet.

The FAA has agreed to significantly improve its oversight of organisations performing certifications on its behalf by July.

US Transportation Secretary Elaine Chao and some lawmakers on Wednesday questioned why Boeing did not require safety features on its top-selling plane that might have prevented the crashes.

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

SAFETY UPGRADE

Southwest Airlines supported Boeing's decision to upgrade safety features.

"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.

Members of the Allied Pilots Association, which represents pilots of American Airlines Group Inc, the largest US carrier, were among the 200 airline customers and others who spent the day at Boeing getting details.

"With the software enhancements, we now have several layers of protection," American Airlines Captain Roddy Guthrie told reporters.

He said it could take two weeks after new training protocols are approved to train all American pilots.

The certification process should not be rushed, the association said in a statement earlier on Wednesday.

COCKPIT SOFTWARE FIX

The fix should be fully vetted and take into account any further information from an investigation into the Ethiopian Airlines crash, the association said.

United Airlines vice president Michael Quiello said the airline was optimistic about the software update, but was counting on the FAA to certify the change.

Airline stocks turned positive after Boeing unveiled the software fix.

Boeing, which is believed to have repeatedly forced the nose lower in the accident in Indonesia last October, would only do so one time after sensing a problem, giving pilots more control.

The system 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 Maneuvering Characteristics Augmentation System – has been pinpointed by investigators as a possible cause in the fatal crashes in Indonesia and Ethiopia.

Current 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.

Ethiopian officials and some analysts have said the Ethiopian Airlines jet behaved in a similar pattern before crashing shortly after take-off from Addis Ababa, but that investigation is still at an early stage.

Boeing's Sinnett said the software had been through extensive testing, including flights with the FAA.

However, he said he could provide no timeframe for when the 737 MAX jets would return to service.

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