The US Federal Aviation Administration (FAA) has said “additional work” is needed for Boeing’s software update for the 737 Max as a result of the ongoing review of the aircraft’s flight control system.

Boeing had said it was hoping to submit the software update to the FAA for approval by the end of March, but the regulator said it now expects to receive the enhancement in the coming weeks. It added that more work was needed to “ensure that Boeing has identified and appropriately addressed all pertinent issues”.

The software update to the 737 Max’s Manoeuvring Characteristics Augmentation System (MCAS) has being developed after the same aircraft model was involved in two fatal accidents – a Lion Air crash that killed all 189 people on board and an Ethiopian Airlines incident in March in which all 157 passengers and crew died.

Initial investigations have found similarities between the two crashes, with evidence from the Lion Air accident pointing to a possible malfunction of the aircraft’s MCAS. Data retrieved from the aeroplane’s flight recorder revealed its nose had been forced down more than 20 times before it went down.

A statement on the FAA’s website says the regulator “will subject Boeing’s completed submission to a rigorous safety review” and that it “will not approve the software for installation until the agency is satisfied with the submission”.

The news means there is still no timeline for getting the world’s 737 Max fleet – which has been grounded since a few days after the Ethiopian Airlines crash – back in the air.

While the aircraft have been grounded, several airlines have had to make alternative arrangements, with Norwegian saying it would pass the cost of the grounding on to Boeing. American Airlines said last week it would cancel 90 flights a day until 24 April, while Southwest Airlines in the US said the grounding is causing around 130 daily cancellations.

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