Boeing designed its 737 Max 8 aircraft to rely on a single sensor with a well-known history of failure to measure airflow around the wing, and then failed to include a scenario in its 737 Max flight tests in which that sensor failed, an investigative report by CNN found.

Those angle-of-attack (AOA) sensors are now at the center of investigations into the two Boeing 737 Max 8 plane crashes that collectively killed 346 people in Indonesia and Ethiopia.

The AOA sensors collect data on airlift, speed and flight angle. That information is then transmitted to the airplane’s software system and flight controls. If a defective or damaged sensor sends erroneous data to the plane’s computer system, it could trick the plane into thinking a stall was imminent. The automatic controls would then point the nose of the plane downward to generate more lift under the wings. Faulty AOA sensor readings could repeatedly disrupt the aircraft’s flight with sudden and unexpected maneuvers.

Investigations indicate this is exactly what may have occurred on Lion Air flight 610 and Ethiopian Airlines flight 302, causing the planes to crash. Evidence spotlighting a likely AOA sensor problem in both air disasters has drawn attention to Boeing’s reasoning for designing the 737 Max 8 the way it did, as well as the Federal Aviation Administration’s (FAA) certification of the aircraft.

According to the CNN report, Boeing was aware of the potential for AOA sensors to cause problems in the air. Since 2004, the FAA has received at least 216 reports of AOA sensors failing in flight, requiring repair, replacement or adjustment. One-fifth of those reports involved Boeing aircraft.

Several commercial aircraft in recent years have had to abort takeoffs or perform emergency landings because of faulty AOA sensors. Reports of sensor failures include a spectrum of reasons for the failures, including freezing, improper installation, and damage from lightning or bird strikes.

Despite these vulnerabilities, Boeing designed its 737 Max 8 with an anti-stall system – the Maneuvering Characteristics Augmentation System (MCAS) – which was informed by just one of the airplane’s two AOA sensors.

All Boeing 737 Max 8 planes remain grounded worldwide as Boeing works on a software fix to address the problems that can be caused by erroneous AOA readings. The repair will require both AOA sensors to inform the MCAS, a redundant safeguard that former Boeing engineers and safety experts told CNN should have been part of the airplane’s operating system from the start.

“From the beginning it should have been a fail-safe design, which would have relied on two inputs to make sure that you weren’t sensitive to one failure,” Peter Lemme, a former Boeing flight-controls engineer, told CNN.

Problems caused by faulty AOA sensors could have been avoided had Boeing conducted flight test scenarios for such a malfunction. Pilots flying the new 737 Max 8 planes may not have been sufficiently trained to handle a crisis caused by faulty AOA sensors. While Boeing insists that proper protocols were in place to help pilots overcome repeated nosedives, the manufacturer never disclosed the presence of MCAS in the operating system.

Boeing originally installed MCAS to help its new planes correct a tendency for the planes to lift at too high of an angle in full-thrust situations. That problem stemmed from Boeing enlarging its engines and positioning them higher under the wings. MCAS was designed to automatically correct the plane’s pitch to avoid a stall without pilot input.

Boeing’s design of the 737 Max 8 also raises questions about the FAA’s certification process and how liberally it allows industry to regulate itself. When it was designing the new line of 737s, Boeing was in a rush to compete with European rival Airbus. The multinational European corporation had released its popular A320neo two years before Boeing came out with its new 737s. Both planes are single-aisle models that are widely used on domestic flights.

House Transportation and Infrastructure Committee Chairman Peter DeFazio told CNN his committee is investigating Boeing’s design and the FAA’s certification of the 737 Max and its MCAS system. He said the FAA’s approval of the 737 Max raises concerns as to whether the regulator properly tested and certified other systems on the new model.

The FAA, Department of Justice, and Congress are investigating how Boeing was able to get government clearance for the 737 Max 8 when serious safety flaws were present.

Additional sources:
UTC Aerospace Systems

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