Boeing has said it has developed software fixes on its 737 Max planes to prevent failures of an automated flight control system that is under investigation after two fatal crashes in the past five months.

The US aerospace company, in the midst of one of its worst crises in years, is under pressure from crash victims’ families, airlines, lawmakers in Washington and regulators around the world to prove that the automated flight control systems of its 737 Max aircraft are safe, and that pilots have the training required to override the system in an emergency.

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

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

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

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

The FAA said it had not yet reviewed or certified the upgrade. It has agreed to significantly improve its oversight of organisations certifying on its behalf by July, the US transport department’s inspector general, Calvin Scovel, told a Senate panel on Wednesday.

The US transport secretary, Elaine Chao, and some lawmakers 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.

Southwest Airlines Co, 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, 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.

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,” the American Airlines captain Roddy Guthrie told reporters. He said he was confident flying the 737 Max in its current state.

It could take two weeks after new training protocols are approved to train all American pilots, Guthrie added.

The certification process should not be rushed, the association said in a statement earlier on Wednesday. The fix should be fully vetted and take into account any further information from an investigation into the Ethiopian Airlines crash, it said.

The 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.

Boeing, the world’s largest planemaker, said the anti-stall system, which is believed to have repeatedly forced the nose lower in the crash in Indonesia last October, would now only do so once after sensing a problem, giving pilots more control.

The system will also be disabled if two airflow sensors that measure the angle of the wing to the airflow offer widely different readings, Boeing said.

The anti-stall system – known as MCAS, or Manoeuvring Characteristics Augmentation System – had been pinpointed by investigators as a possible cause in the fatal crashes in both Indonesia and Ethiopia.

Extra computer-based training is being planned for 737 pilots following criticism that the system 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 Ethiopian Airlines crash prompted a worldwide grounding of 737 Max planes.

Ethiopian officials and some analysts have said the Ethiopian Airlines jet behaved in a similar pattern before crashing shortly after takeoff 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 jets would return to service.