The US Federal Aviation Administration (FAA) has proposed four design changes to address the unsafe condition of Boeing 737 Max aircraft.

The agency sent a notice of proposed rulemaking (NPRM) for the grounded aircraft’s airworthiness directive (AD) to the US Office of the Federal Register.

The design changes are updating flight control software, installing updated MDS display processing computer (DPC) software, revising crew operating procedures and changing the routing of horizontal stabiliser trim wires.

Along with these changes, the agency has also proposed that operators perform an angle of attack (AOA) sensor system test and an operational readiness flight before each aircraft is returned to service.

The FAA is also planning to make an operational readiness flight mandatory after the design changes have been implemented.

An operational evaluation will also be conducted to assess the impact of the proposed changes on pilot training. This will be jointly carried out by FAA, Agência Nacional de Aviação Civil (ANAC) Brazil, Transport Canada Civil Aviation (TCCA), and the EASA.

The proposed AD will affect 73 aircraft of US registry and estimates the average total cost per operator to be $7,650 to comply with the AD.

A Boeing spokesman was quoted by BBC as saying: “We’re continuing to make steady progress towards the safe return to service, working closely with the FAA and other global regulators.

“While we still have a lot of work in front of us, this is an important milestone in the certification process.”

In June, FAA head Stephen Dickson confirmed that Boeing 737 MAX aircraft will return only after ‘a comprehensive and rigorous review process’.

Meanwhile, aerostructures designer and manufacturer Spirit AeroSystems decided to place some of its Boeing 737 MAX programme staff on a 21-day unpaid temporary layoff/furlough in June.