US aviation regulators on Monday spelled out new requirements for the Boeing 737 MAX before the grounded jet is permitted to fly again.

The Federal Aviation Administration will require a number of changes to "address the unsafe condition" of the aircraft, the agency said in a notice set for a 45-day public comment.

The jet has been grounded since March 2019 following two deadly crashes.

Some of the changes have been publicly discussed by Boeing, such as upgrades to an anti-stall system that has been seen as a factor in both crashes, and modification to a sensor system that had played a role in the accidents.

A June 2020 test flight of the Boeing 737 MAX, which has been grounded since March 2019
A June 2020 test flight of the Boeing 737 MAX, which has been grounded since March 2019 AFP / Jason Redmond

Other requirements include installing new software and revising the flight manual to mandate new crew procedures.

The requirements do not include new pilot training protocols. Those will be announced later and put out for public comment at a later date, an FAA spokesman said.

The MAX has been grounded worldwide since March 13, 2019, following an Ethiopian Airlines crash that killed 157 people. That catastrophe came just a few months after a Lion Air MAX crash that killed 189 people.

The FAA has said it will only permit the jet to fly again when it is satisfied the model has met all safety concerns. The agency completed test flights on the plane on July 1.