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From the editor: MAX and MCASFrom the editor: MAX and MCAS

Seeking a fix to the fix

No doubt about it, the crashes of two Boeing 737 MAX 8 airliners will continue to dominate aviation news in the coming months. Software and other procedural changes to the MAX’s controversial Maneuvering Characteristics Augmentation System (MCAS) will be made and hopefully no more such tragic accidents will occur.

But the damage is done—to Boeing’s reputation, and to the public’s perception of how the company has faced this crisis. It turns out that the entire host of MCAS-related problems had its origin in the ongoing, epic competition between Boeing and Airbus. In 2010, Airbus came out with a new version of its best-selling A320, the A320neo. The “neo” stands for “new engine option,” signifying that the new engines would be 15 percent more fuel-efficient than those previously used. Airbus began taking orders, and Boeing became concerned about losing market share. It debated whether to design a new airplane to answer to the A320neo, or to modify their existing best-seller, the Boeing 737, with equally fuel-efficient CFM International LEAP-1B engines.

Like most other modern large jet engines, a good bit of that fuel efficiency comes from the LEAP’s high bypass ratio. High-bypass-ratio engines send their thrust not just through the core of the engine, but around the core as well. This gives them larger-diameter nacelles. This larger size presented a problem: The engines wouldn’t fit under the wings. The 737 was introduced in 1967, and was designed with wing-to-ground clearances to accommodate the smaller engines of the day. For the LEAP engines to fit, they had to be mounted farther forward on the wings, and raised.

Boeing was pressed for time and needed to garner some orders for this new airplane before its traditional customer base defected to Airbus. So instead of taking the time to design a new airplane it went with the engine fix for the 737.

But the fix had a problem, and it too needed a fix. At high angles of attack the large engine nacelles acted like lifting bodies, and caused the nose to rise. Boeing’s answer was the MCAS, a system that used input from a single angle of attack sensor (the ship has two) to trigger automatic nose-down pitch forces. Pilots could overcome any MCAS runaways by disconnecting the electric pitch trim and using the manual pitch trim wheels on the center pedestal.

Long after Boeing comes up with hardware and software changes, warning systems, and education programs that will correct the MCAS debacle, pilots and passengers may continue to view the MAX 8 with suspicion.The airplane should have used a subsystem that relied on both angle of attack sensors, comparing their inputs, and annunciating disagreements. A system to do just that was offered by Boeing, but for an extra charge. More problems surfaced once the MAX 8 entered into service, as suggested by preliminary accident analyses of the Lion Air and Ethiopian Airlines crashes. Both of the accident airplanes experienced repeated nose-down MCAS events. The manual pitch trim wheels may not have had enough power to overcome a runaway MAX stabilator. This may have been because of the airplane’s high airspeed. Or was it that the MCAS makes large pitch trim changes quickly? Did Boeing know all this from pre-certification flight testing? In any event, the company certified the airplane under long-standing FAA practices that delegate much certification authority to manufacturers.

Boeing said that the MAX 8 was designed to fly like previous 737 models, so that pilots training to fly the newer airplane wouldn’t need to know about the MCAS. The system wasn’t even mentioned in the MAX’s flight manual. Some airlines did, however, include MCAS training anyway.

Long after Boeing comes up with hardware and software changes, warning systems, and education programs that will correct the MCAS debacle, pilots and passengers may continue to view the MAX 8 with suspicion. There are already those who look askance at increasingly automated systems and digital flight controls, and consider them too complex, quirky, and unreliable. Cobbled-up systems such as the MCAS only reinforce those prejudices.

Concerns over issues such as MCAS aren’t mere intellectual exercises for Turbine Pilot. Two of our contributors—Peter A. Bedell and Chip Wright—fly the 737 MAX 8. A detailed discussion of the system isn’t possible in this space, but as the MCAS investigations continue, look for these writers to air their thoughts on the matter.AOPA

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Thomas A. Horne

Thomas A. Horne

AOPA Pilot Editor at Large
AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.

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