The 737 MAX-8 presented pilots with a challenge that the company thought could be solved with a system that airline pilots need not know about.

When Boeing pilots were flight testing the new MAX-8 version of the venerable 737 jet they discovered a problem that made the airplane difficult to handle when its speed dropped to a point where it was in danger of triggering an aerodynamic stall, and a loss of control that could lead to a crash.

This is revealed in new reporting by Aviation Week. The report suggests that in order to mitigate the problem Boeing introduced a new system to the flight controls – a system called Maneuvering Characteristics Augmentation System, MCAS, that is at the center of the investigation of the crash of Lion Air Flight 610that plunged into the Java Sea, killing all 189 people on board.

Pilots flying the more than 200 MAX-8s now in service with airlines across the world have said that they were unaware that the MCAS had been installed and were never instructed in how to use it. That would have included the Lion Air pilots. They were also, therefore, unaware of the reasons why Boeing decided to add the MCAS system.

The problems that were revealed in the test flights arose from the adoption of new engines for the MAX series of the 737. They are larger, heavier and more powerful than on earlier models of the jet.

Fixing these engines to the 737’s wings put Boeing’s engineers up against some unique and challenging problems caused by the age of the jet’s basic design, originating in the mid-1960s.

The 737 sits lower to the ground than other Boeing jets. This is because its designers wanted baggage and cargo to be hand-loaded from the tarmac without mechanical assistance, since the airplane was intended to bring jet service for the first time to many small airports not then equipped for that purpose.

This innovation swiftly became pointless as airports became better equipped and, more vitally, the 737 became the best-selling single-aisle jet in history and Boeing’s most enduring cash cow.

However, the 737’s shorter ground clearance, just 17 inches, became problematic as jet engines grew larger. This could have been countered by a new fuselage and normal length landing gear. But although Boeing introduced new wings, tail surfaces and many other upgrades the fuselage and landing gear remained fundamentally unchanged over decades.

A final crunch moment came with the MAX series. The performance of the 737 was greatly enhanced by the new engines, jointly made by General Electric and the French company Safran, providing a new sweet spot for airlines who wanted the improved economies of a small jet that could fly longer routes, often over oceans.

But those virtues were possible only with an increase in the size of the engines. The size of the MAX engines, specifically the diameter of the huge fan blades at the front, is nearly 70 inches, compared to 61 inches on the older engines, and they weigh 849 pounds more.

In order to attach the new engines and still get a safe distance between them and the ground Boeing lengthened the nose wheel by 9.5 inches and, crucially, had to move the engines, inside their bulging nacelles, further forward from the wing.

It now appears that the changes in the 737’s low-speed handling characteristics resulted from this shift in the weight of the engines, as well as the effects of their increased power.

(In response to questions from The Daily Beast, Boeing declined to confirm the details of the Aviation Week report.)

Normally the onset of an aerodynamic stall is indicated by “stick shake” – the joystick, more accurately the yoke, begins to shake and pilots are trained to instinctively increase speed and push the nose down to recover stability.

As a result of the test flights Boeing seems to have decided that the airplane itself should be able to sense this problem and cure it through its automated flight management system, using MCAS to move the horizontal stabilizer to push down the nose. What they apparently did not anticipate was the possibility that an erroneous message from another system, an angle of attack (the pitch of the wings) sensor, could initiate action by the MCAS, unknown to pilots.

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