The Federal Aviation Administration (FAA) on Monday published its final list of required design changes to the Boeing 737 MAX, as well as changes to operation and maintenance procedures and to proposed pilot training, that must be completed for the jet to return to passenger service.

The design changes include new software to limit the flight-control system that caused two MAX crashes that killed 346 people, a new cockpit alert to tell pilots if a sensor that initiated those crashes is faulty, as well as the rerouting of some wiring on the planes to forestall a potential similar failure being triggered by an electrical short.

The FAA said it completed all approvals associated with the design changes itself, delegating none of the oversight work to Boeing.

“Through a thorough, transparent, and inclusive process, the FAA has preliminarily determined that Boeing’s proposed changes to the 737 MAX design, flight crew procedures and maintenance procedures effectively mitigate the airplane-related safety issues that contributed to the Flight 610 and Flight 302 accidents,” the FAA said.

The publication sets in motion the final steps toward allowing U.S. airlines to fly passengers on the MAX again before the end of this year.

The FAA documents will be published in the Federal Register in the coming days, which will start the clock on a 45-day period for public comment, during which airline operators, industry experts and members of the public can provide feedback on the FAA proposal.

After some weeks, the FAA will respond to the comments and issue its final Airworthiness Directive, approving the MAX to fly again. That clearance is expected in October. After that, U.S. airlines will take a month or two to modify and test their airplanes and train their pilots.

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A separate FAA report that will lay out the minimum training standards for MAX pilots is still pending.

Fixing the flaws

The proposed FAA Airworthiness Directive that would reverse the March 2019 grounding of the Boeing jet will require installing new software to correct the flaws in the flight control system — the Maneuvering Characteristics Augmentation System (MCAS) — that went wrong on the two MAX crash flights in Indonesia and Ethiopia.

The changes mean MCAS will activate based on inputs from two angle of attack sensors rather than the single sensor in the original design, failure of which initiated the catastrophes.

And if MCAS is triggered, it will activate only once, not multiple times as on the two crash flights, and the magnitude of the system’s deflection of the horizontal tail will be limited so that the pilots can counter it if needed by pulling back on the control column.

The FAA said it evaluated Boeing’s preliminary individual safety assessment on the updated flight control systems and that the new designs “meet all reliability and integrity safety requirements.”

Outside experts believe that these fixes, developed by Boeing, should prevent a recurrence of the scenarios on the crash flights.

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The FAA also proposes requiring an additional software update that will provide a new instrument panel alert to the pilots when there is a disagreement between the two Angle of Attack sensors.

In addition to these software and system changes, the wiring that controls the horizontal tail of the airplane — known as the horizontal stabilizer — must be rerouted to separate bundles of wires. This is to avoid an electrical short circuit that might swivel the horizontal tail uncommanded by the pilot, causing a potential outcome similar to an activation of MCAS.

Once each of the MAXs has been modified with the new software, crew alerts and wiring changes, the FAA mandates that the airline must complete a system test on the Angle of Attack sensors. This test is projected to take a week.

Because the angle of attack sensor that triggered the Lion Air crash was a replacement device that had been miscalibrated when installed on the aircraft, the FAA is also mandating a change to MAX maintenance procedures “to include a final independent check to ensure the repair” is properly installed.

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Finally, every airline must also perform an operational readiness flight on each MAX jet before it carries any passengers.

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Changes to pilot emergency checklist

The FAA directive will also require revision of the existing pilot manual for the MAX to incorporate new and revised flightcrew procedures.

One key change is to an emergency checklist that the pilot must follow if the horizontal tail is continuously pushing the jet’s nose down, known as the runaway stabilizer checklist.

A week after the first MAX crash in October 2018, that of Lion Air JT 610 in Indonesia, Boeing issued a bulletin to pilots telling them that what had gone wrong was essentially a runaway stabilizer and pointed to that checklist as the appropriate response.

However, just over four months later, the pilots on Ethiopian Airlines flight ET 302 tried to follow the runaway stabilizer checklist and found it impossible.

As instructed, the Ethiopian pilots hit the cutoff switches that killed power to the horizontal tail, stopping its movement. But then they found they couldn’t physically move the tail to a nose-up position by rotating a manual wheel in the cockpit.

These pilots had allowed the plane to gather too much speed, which increased the forces on the tail and jammed it in position. The pilot couldn’t budge the manual wheel.

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In the FAA’s new instructions, that critical checklist is significantly updated with warnings never given to the Ethiopian pilots.

First, pilots are instructed that before hitting the cutoff switches they need to use the main electric stabilizer thumb switches on the control column to adjust the jet’s pitch and reduce the forces on the tail.

Separately, they are cautioned that in certain conditions, when the horizontal tail has moved to an extreme position, it may take “a two-pilot effort” to physically turn the manual control wheel.

A final note in the revised checklist advises that “reducing airspeed” will lower the forces loaded on the tail, “which can reduce the effort needed to manually trim.”

The new version of the checklist, developed by Boeing, does not attempt to instruct pilots in a old aviator technique for handling a runaway stabilizer that was in the flight manuals decades ago, sometimes referred to as the “rollercoaster” method.

Similar to the way a fly fisherman reels in a fish, this involves a pilot easing hold of the control column, even letting it go, to relieve the forces on the jet’s tail, then cranking the manual wheel, and repeating this many times.

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However, this technique requires a lot of pilot skill and nerve. Using it in a flight simulator that re-created the conditions on the Ethiopian crash flight, a pair of U.S. pilots managed to pull out of the dive, but lost 8,000 feet of altitude before doing so.

Both MAX crashes happened within minutes of takeoff, before the planes had climbed to that altitude.

Instead then, Boeing makes do with the standard runaway stabilizer instructions, now enhanced with warnings about the order of the steps and the pitfalls to avoid.

The FAA offered assurances that it has checked that the manual stabilizer trim wheel will function as required if these instructions are followed, even by a crew of smaller or less physically strong pilots.

“Simulator predictions validated by Boeing flight testing, and overseen by the FAA, were used to calculate the required trim wheel forces,” the FAA said. “A mockup of the stab trim wheels was used to measure the force capability of appropriate pilots.”

Public comment invited

The FAA said its final plan to unground the MAX incorporates the recommendations from several investigations into the MAX crashes and from the independent Technical Advisory Board of experts from around the world and various government agencies set up to review the proposed corrective actions.

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“This thorough review has taken more than 18 months and included the full-time work of more than 40 engineers, inspectors, pilots, and technical support staff,” the FAA said. “The effort represents more than 60,000 FAA hours of review, certification testing, and evaluation of pertinent documents. This has so far included approximately 50 hours of FAA flight or simulator tests and FAA analysis of more than 4,000 hours of company flight and simulator testing.”

Once the FAA document is officially published in the Federal Register, anyone can comment on it, either online at the Federal eRulemaking Portal —regulations.gov — or by mail to the address listed in the document.

The FAA said it is actively supporting concurrent assessments of the MAX by foreign aviation regulators. However, each authority will make its own assessment of the FAA’s findings and its own decision on clearing the MAX to fly again.

For already delivered 737 MAXs, the airline must incorporate all FAA-mandated design changes and maintenance actions before the aircraft can fly.

For any future delivered 737 MAX airplanes, Boeing is required to incorporate the same mandatory actions.