The Federal Aviation Administration (FAA) and Boeing on 1 July completed the required re-certification flight tests on the 737 MAX, taking the plane a step closer to FAA approval to return to service.

Dominic Gates of the Seattle Times reports that an extensive to-do list must be accomplished before the plane can receive clearance to fly passengers again, a milestone now expected no sooner than mid-September.

During three days of testing at end June, FAA pilots and engineers evaluated Boeing’s proposed changes to the automated flight control system on the aircraft that was implicated in two fatal crash flights.

The software called the Manoeuvring Characteristics Augmentation System (MCAS) has been upgraded to ensure it isn’t triggered by a single sensor and cannot be activated repeatedly.

Boeing has revamped the entire architecture of the flight control software so that the jet’s systems now use both flight control computers on each flight instead of only one, thus taking input from dual sensors on each side of the plane.

The plane flew tests for two hours on Monday, four hours on Tuesday and finally an hour and 37 minutes on Wednesday.

While completion of the FAA flights is an important step, key tasks remain. The FAA will spend weeks evaluating the data gathered during these flights to assess whether the jet’s systems perform as expected and comply with all safety regulations.

The FAA also will review Boeing’s final design documentation, as will a Technical Advisory Board consisting of experts from a dozen international air safety regulators and NASA.

In parallel, the FAA along with a panel of regulators from Canada, Europe, and Brazil will evaluate minimum pilot training requirements and flight manual instructions, issue a draft report open for public comment, and then produce a final report on the required minimum training standard — which will include time in a full-flight simulator, something Boeing had long resisted for the MAX.

When this is accomplished, the FAA will issue an Airworthiness Directive advising airlines what upgrades they must install on the aircraft, which at that point will have been parked for about 19 months. The agency then will lift the MAX grounding order, conditioned on completion of that work.

Even then, it will probably be another couple of months before U.S. airlines can put the MAX into service. The FAA will have to approve the pilot training programs at each airline, after which the carriers will run thousands of pilots through simulator training.

The FAA must also perform in-person, individual reviews of each of the several hundred MAXs that Boeing built since the grounding but hasn’t delivered.

Even without the complicating factor of the lack of demand for airplanes due to the pandemic, it was expected to take a year or more to get all those aircraft flying again.