The US Federal Aviation Authority (FAA) said on Monday (April 1) it was expecting to receive the final version of Boeing’s software update “over the coming weeks” for approval, but added time would be needed for “additional work” resulting from an ongoing review of the aircraft’s flight control system. Boeing had previously touted the update for late March.

Last October, Lion Air flight 610 came down in the Java Sea shortly after taking off from Jakarta, killing all 189 people on board. Then in March, Ethiopian Airlines flight 302 crashed en route to Nairobi from Addis Ababa. All 157 people on board were killed.

The 737 MAX was grounded last month following the Ethiopian crash. Preliminary investigations into the two crashes suggest that the same control system – the aircraft’s manoeuvering characteristics augmentation system (MCAS) – is likely to have been active when the aircraft came down.

Crash investigators have been focusing their attentions on the MCAS, which is designed to automatically correct the angle at which the aircraft is flying if it becomes too steep. A preliminary report on the Lion Air crash found an “erroneous” sensor input feeding the MCAS caused the aircraft to nose-dive.

Analysis of the cockpit voice recorder from the Lion Air crash has revealed the pilots searched through the aircraft’s manual to find a solution when they were unable to override the system before the aircraft pitched into the sea. A preliminary report into the Ethiopian crash is due in the coming days.

The update from the FAA comes after Boeing last week unveiled some of the core features of the update, which it said would provide “additional layers of protection if AOA [angle of attack] sensors provide erroneous data”.

All brand-new 737 MAXs will now come with a previously optional MCAS warning system as standard. Boeing has also updated its pilot training to cover the software update, while pilots will also be required to review a flight crew operations manual bulletin, an updated trim fail non-normal checklist, and revised quick reference handbook.

The FAA added: “Upon receipt [of the update], the FAA will subject Boeing’s completed submission to a rigorous safety review. The FAA will now approve the software for installation until the agency is satisfied with the submission.”

A Boeing spokesperson said: "We are working to demonstrate we have identified and appropriately addressed all certification requirements and will be submitting for FAA review once completed in the coming weeks. Safety is our first priority and we will take a thorough and methodical approach to the development and testing of the update to ensure we take the time to get it right."

Tui, which has 15 737 MAX aircraft, issued a profit warning last week, stressing that should the aircraft remain grounded until the autumn, it could cost the airline in the region of €300 million.

On Monday, the European Aviation Safety Agency (Easa) told the Financial Times it was in “permanent contact” with the FAA and Boeing regarding the software update.

A spokesperson said Easa was “working towards having the aircraft return to service as soon as possible” but only “once there is complete reassurance it is safe”.