Because the system’s operation was transparent to pilots and its troubleshooting relied on a common procedure (the 737’s runaway stabilizer checklist debuted in 1967 and had not changed since 2013), Boeing decided not to include a description of the MCAS in flight training or flight crew operations manuals. Boeing did not keep it a secret—the MCAS was included on a list of changes from the 737NG during high-level customer briefings and covered in MAX maintenance documentation. But most airline pilots had no idea it existed.
Why did Boeing develop the MAX instead of a clean-sheet successor to the 737?
3 | Angle of Attack Vanes
The MCAS’s primary data sources are the MAX ’s two AOA sensing vanes, one on either side of the nose. Boeing designed the MCAS to receive input from only one of the sensors during each flight. The left and right sensors alternate between flights, feeding AOA data to the FCC and the MCAS.
This is a condensed version of a 5,300 word in-depth article published in the August 19, 2019 issue of Aviation Week & Space Technology.
Sean Broderick, Senior Air Transport Editor, takes a deep dive into the Boeing 737 MAX crisis and asks: what happened, and what now?
Subscribers can access the full article here.
If investigators have discovered a link between the AOA failures, it has not been made public.
Investigators have not explained what happened to the Lion Air JT610 AOA vane, but the report suggests faulty maintenance played a role. A left-side vane removed from PK-LQP during an overnight stay in Denpasar, Indonesia, following an Oct. 27 flight has been analyzed, but investigators have not released any details. Digital Flight Data Recorder (DFDR) data from JT610 and the previous flight the day before, JT43, showed left-side AOA value disagreements during taxi-out.
But ET302’s AOA values were similar until shortly after takeoff, when the left vane’s value changed suddenly and significantly. One theory is that a bird or some other object struck the vane, but nothing released by investigators besides the DFDR data readout supports this.
A Nov. 10 message from Boeing to MAX operators shed more light on the system described in the earlier bulletin, using the MCAS’ name. Airlines disseminated bulletins to their pilots. For most of them, it was the first time they had heard of the MCAS.
Still, confusion persisted. Pilots at one U.S. major airline were told by their safety committee that the MCAS could be countered by applying opposite control-column input to activate the column cutout switches, which was not true.
This is a summary. Subscribers can access full details of the MCAS design here.
Boeing is targeting “the September time frame” for handing over the long-awaited changes to the FAA. The FAA will be the first regulator to lift its ban, and at least some are expected to follow in short order—likely days.
For carriers with just a few MAXs, the return-to-service timeline could be just a few days. But for larger carriers, it will take a month or more—a function of their ability to take only so many aircraft in short order, Boeing’s available manpower to support the service returns, and—crucially—ensuring pilots have completed the latest training.
What else on the MAX will change?
How are suppliers handling this?
1 | Leap Engines and Pitch-up Moment
The MAX’s larger CFM Leap 1 engines create more lift at high AOA and give the aircraft a greater pitch-up moment than the CFM56-7-equipped NG. The MCAS was added as a certification requirement to minimize the handling difference between the MAX and NG.
Why was FAA the last to ban the MAX?
4 | Stabilizer Deflection
When threshold AOA is reached, the MCAS commands 0.27 deg. of aircraft nose-down stabilizer deflection per second for 9.3 sec.—a total of 2.5 units of trim. When the FCC reads the AOA as back to below threshold, the MCAS is reset, and the aircraft’s trim returns to the pre-MCAS configuration. Inaccurate AOA data will trigger the MCAS every 5 sec. until the data is corrected or the system is disabled.
Where did the MCAS come from?
“It’s a good thing we knew what to expect. Otherwise tunnel vision from the ‘airspeed unreliable’ event could have blinded us to the subsequent MCAS nose-down trim input."
The MAX Saga
One Question At A Time
Pilots Say MAX MCAS Software Updates Prove Effective In Simulator Demo.
2 | MCAS Activation
The system activates when the aircraft approaches threshold AOA, or stickshaker activation, for the aircraft’s configuration and flight profile. The MAX flight-control law changes from speed trim to the MCAS because the MCAS reacts more quickly to AOA changes.
“It’s like a world where there are just two car companies. Boeing will come out of this.” - Bank of America Merrill Lynch aerospace analyst Ron Epstein
The longer airlines are without their MAXs, the higher the the cost to Boeing will be. But long-term damage to Boeing’s finances could be limited by the fact that current MAX customers don’t have a lot of options. They cannot simply switch orders to Airbus because the A320 family is largely sold out through 2022. And China’s new narrowbody—the Comac C919—is unproven and behind schedule in flight-testing.
Will any other manufacturers benefit from Boeing’s problems?
Using modeling and wind-tunnel data, Boeing linked the MCAS activation to two factors: G forces and AOA data. The system’s authority was limited to 0.6 deg. of stabilizer movement per MCAS activation cycle. No limit was put on the number of times the MCAS could activate; it would trigger whenever data fed to it determined that it was needed.
BY SEAN Broderick
Stephen Brashear/Getty Images
How was the MCAS designed?
How will the MCAS change?
More 737 MAX questions answered
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As the 2010s began, Boeing was looking hard at developing an all-new narrowbody to replace the aging 737, which first flew in 1967. The company’s leaders downplayed the belief that a rumored reengining of the A320 by Airbus would force Boeing to follow suit. “Not if we’re convinced a new airplane will be coming at or near the end of the decade,” then-CEO Jim McNerney told Aviation Week in mid-2010. “I think our customers will wait for us.”
While Boeing is making progress on changes needed to convince regulators that the latest iteration of its venerable narrowbody is safe to fly, the timing remains fluid.
The road ahead is littered with unanswered questions following the crashes of Lion Air Flight 610 (JT610) in October 2018 and Ethiopian Airlines Flight 302 (ET302) this past March.
As with every major accident, the lessons learned go far beyond addressing the primary causal factors. With two accidents to learn from, the MCAS may be close to being updated, but the MAX story’s influence on aircraft design, certification, and pilot training is only starting to be felt.
Why didn’t pilots know about the MCAS?
Two fatal MAX accidents in five months was enough for many regulators to take action. FAA Acting Administrator Dan Elwell said that the agency’s review of MAX in-service data, including what little was known about the second accident, “provides no basis to order grounding the aircraft, nor have other civil aviation authorities provided data to us that would warrant action.”
That changed overnight. Working with Transport Canada, the FAA reviewed what it termed “refined” satellite data that helped map Ethiopian Airlines ET302’s flightpath. The data was enough to convince the Canadians to ban the MAX. A few hours later, the FAA became the last regulator to issue a ban.
The groundings—or, more accurately, operations bans that cover a regulator’s airlines and the airspace it governs — remain in place and must be removed one by one, at each regulators’ discretion.
5 | Disabling the System
Pilots can interrupt the MCAS in two ways: via the yoke-mounted electric trim switches, or using the STAB TRIM CUTOUT switches on the center console. The trim switches interrupt the MCAS for 5 sec. and establish a new stabilizer trim reference point. Toggling both cutout switches de-powers the MCAS and the speed-trim system.
Will the MAX situation change aircraft certification and global regulatory cooperation?
What about Boeing’s finances?
Why are the AOA vanes failing?
When will the MAX return to service?
Will MAX pilots be getting special training before they fly again?
Early in the MAX’s development, Boeing discovered that the heavier engines presented a stability and control issue: Their larger nacelles created more lift at high angles of attack (AOA) and high airspeeds. Boeing’s solution: Expand the 737NG’s speed trim system (STS) by adding the MCAS—software code that expands how the horizontal stabilizer is automatically adjusted as the aircraft approached its threshold AOA or stickshaker activation.
When did the world learn about the MCAS?
Mid-August marked five months since the last Boeing 737 MAX fleet’s revenue flight and customer delivery as well as—most significantly—its second fatal accident.
By mid-2011, seven months after the Neo’s launch, Airbus had won more than 1,000 orders and commitments. Boeing’s hand was finally forced by the prospect of losing one of its marquee customers, American Airlines.
“The Neo’s success in the marketplace did point out unmistakably that customers would embrace a lower amount of capability quickly,” McNerney reflected in 2012. “So that was a factor. We added it all up and decided to move with the MAX.”
How MCAS Worked Before The Grounding