Boeing’s 737 Max plane has been certified safe by the US Federal Aviation Administration (FAA), meaning it could soon return to service.

Two 737 Max crashes – a Lion Air flight in Indonesia in October 2018 and an Ethiopian Airlines flight in March 2019 – killed a total of 346 people. In September a report from the US House of Representatives came to scathing conclusions about the conduct of Boeing and the FAA, the agency regulating the plane.

However, the FAA’s announcement marks the beginning of the end of a crisis for Boeing.

What was the problem with the 737 Max?

Both the Lion Air and Ethiopian Airlines flights crashed shortly after takeoff, and investigators quickly found similarities when they examined the planes’ black box flight data.

The pilots in both crashes struggled with the manoeuvring characteristics augmentation system (MCAS), which had been installed on the 737 Max to prevent the plane from stalling if it climbed too quickly.

The system was designed to push down the nose of the plane when it rose too high. That system depended on a sensor on the outside of the plane that measured the aircraft’s so-called angle of attack. However, a faulty sensor caused the planes to dip their noses repeatedly, eventually forcing the planes into a dive.

How has it been fixed?

Boeing has changed the software running the planes so that they take account of two sensors instead of one, according to the FAA. The anti-stall system will no longer kick in repeatedly, meaning a pilot should be able to take control again.

The manufacturer has also updated its training materials so that pilots are aware of the potential issue, and it will install as standard on all of its models a warning that will go off if a sensor fails.

Is it now safe?

The regulators and Boeing certainly think so. Steve Dickson, the head of the FAA, said he would be “100% comfortable” putting his family on a 737 Max when they re-enter service. Boeing said it would be “one of the safest airplanes ever to fly”.

Given the prominence of the crisis, it is likely to be one of the most heavily scrutinised. The 737 Max has now completed about 1,400 test and check flights and more than 3,000 flight hours.

When will it come back in to service?

It is unclear exactly when the first 737 Max will take off on a commercial flight, but American Airlines has scheduled its first one for 29 December.

In the US, the FAA has mandated various steps that Boeing has to carry out, including training pilots in its software, making the required technical changes and carrying out routine maintenance for planes that have been grounded since last year.

The European Aviation Safety Agency (EASA) has a similar process. It will soon make a formal proposal to unground the 737 Max, followed by a 28-day consultation period. After that the regulator will examine responses, meaning the first flight in Europe is not likely to be until the end of the year or early January.

What about the UK?

There is an added complication for the UK: Brexit. The UK Civil Aviation Authority usually follows the lead of EASA in certifying planes as safe to fly, but whether it will continue to do so depends on trade negotiations between the UK and the EU.

If there is no deal and the plane has not yet been certified by 1 January, the end of the post-Brexit transition period, it could lead to complications in the recertification process.

Will I know in advance if I am flying on a 737 Max?

Many airlines let their customers know what model of aircraft they will be flying when they make a booking, but there is no obligation to do so. Two 737 Max customers in the British Isles are preparing to fly the plane when the ban is lifted: Tui and Ryanair. Tui said it would let customers know beforehand what model is scheduled, and let customers change flights free of charge if they feel uncomfortable.

Ryanair, on the other hand, said it was not practical to inform its passengers of what model they would get, because it allocated its aircraft the day before the flight. Ryanair will not waive flight change fees for customers who do not want to fly on the 737 Max, according to Which?, the consumer group.

Rory Boland, the Which? travel editor, said: “This is the first step to the Boeing 737 Max returning to service worldwide, but many passengers may still be uncomfortable taking flights on these planes given their past record.

“Airlines that plan on flying these aircraft should give passengers with existing bookings the option of transferring to another flight for free, while operators should also make clear which planes will be used for future bookings, so people can make an informed choice before travelling.”

• This article was amended on 19 November 2020 to make clear that the MCAS system was designed to prevent an aerodynamic stall - a loss of lift under the wings – rather than an engine stall as an earlier version stated.