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martyb writes:

Initial Findings Put Boeing's Software at Center of Ethiopian 737 Crash:

At a high-level briefing at the Federal Aviation Administration on March 28, officials revealed "black box" data from Ethiopian Airlines Flight 302 indicated that the Boeing 737 MAX's flight software had activated an anti-stall feature that pushed the nose of the plane down just moments after takeoff. The preliminary finding officially links Boeing's Maneuvering Characteristics Augmentation System (MCAS) to a second crash within a five-month period. The finding was based on data provided to FAA officials by Ethiopian investigators.

The MCAS was partly blamed for the crash of a Lion Air 737 MAX off Indonesia last October. The software, intended to adjust the aircraft's handling because of aerodynamic changes caused by the 737 MAX's larger turbofan engines and their proximity to the wing, was designed to take input from one of two angle-of-attack (AOA) sensors on the aircraft's nose to determine if the aircraft was in danger of stalling. Faulty sensor data caused the MCAS systems on both the Lion Air and Ethiopian Airlines flights to react as if the aircraft was entering a stall and to push the nose of the aircraft down to gain airspeed.

On March 27, acting FAA Administrator Daniel Ewell told the Senate Commerce, Science, and Transportation Committee's aviation subcommittee that there had been no flight tests of the 737 MAX prior to its certification to determine how pilots would react in the event of an MCAS malfunction. He said that a panel of pilots had reviewed the software in a simulator and determined no additional training was required for 737-rated pilots to fly the 737 MAX.

What follows is from memory from what I've gleaned from reading several news accounts over the past few weeks. I am not a pilot, so take this with the proverbial $unit of salt.

The design of the MAX version of the Boeing 737 used a larger diameter engine so as to improve fuel economy. Because the original 737 was designed to be low to the ground to facilitate boarding (no jetways back then), it required the engines to be mounted forward and higher than in previous models. This introduced a change in the flight dynamics. Adding throttle in certain conditions would cause the plane to "nose up". Because of the shear size of the engine nacelles, this further increased the lift of the nose (more surface exposed at an angle to the air flow). This would cause further lift and would exacerbate the situation. Boeing wanted pilots to be able to fly the MAX without undergoing expensive retraining. How can they make a different aircraft behave like its predecessor, the 737-NG? The solution Boeing came up with was MCAS which — in certain circumstances — was designed to push the plane's nose back down. So much authority was provided to this adjustment, and its repeated application in some cases, that it could lead to driving the plane downward in spite of the pilot's efforts to maintain level flight. Complicating matters, there was no mention of MCAS in any of Boeing's training materials: pilots were not even aware it was there.

It would be easy to "armchair quarterback" Boeing's decisions. The airline industry was transitioning from its hub-and-spoke system (which favored larger planes) to having a greater number of direct (no layover) flights which favored smaller aircraft. In the meantime Airbus had come out with a new model of more efficient aircraft which fit this flight profile. Boeing could have come up with a clean-slate design for a new aircraft, but that would require several years from design to construction to certification. They elected to modify the 737, instead. As long as it was sufficiently similar (I'm waving my hands around a bit here), it could be sold based on the certification of its earlier models. So, they decided to modify the 737... but not too much so as to avoid the time-consuming recertification process.

I've heard it said, "The longest distance between two points is a shortcut." It is sad that this shortcut appears to have been responsible for two flights crashing shortly after takeoff and killing nearly 350 people.

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