Maneuvering Characteristics Augmentation System (MCAS)

 
MCAS, or Maneuvering Characteristics Augmentation System, provides consistent airplane handling characteristics in a very specific set of unusual flight conditions. MCAS now contains multiple enhanced protections:

• Measurements from two Angle of Attack (AOA) sensors will be compared.

• Each sensor will submit its own data to the airplane’s flight control computer.

• MCAS will only be activated if both sensors agree.

• MCAS will only be activated once.

• MCAS will never override the pilot’s ability to control the airplane using the control column alone.

MCAS ON 737 Max
The Maneuvering Characteristics Augmentation System (MCAS) flight control law was implemented on the 737 MAX to mitigate the aircraft's tendency to pitch up because of the aerodynamic effect of its larger, heavier, and more powerful CFM LEAP-1B engines and nacelles. The stated goal of MCAS, according to Boeing, was to provide consistent aircraft handling characteristics at elevated angles of attack in certain unusual flight conditions only and hence make the 737 MAX perform similarly to its immediate predecessor, the 737NG

MCAS role on Accidents

Investigators determined that MCAS was triggered by falsely high angle of attack (AoA) inputs, as if the plane had pitched up excessively. On both flights, shortly after takeoff, MCAS repeatedly actuated the horizontal stabilizer trim motor to push down the airplane nose. Satellite data for the flights, ET 302 and JT 610, showed that the planes struggled to gain altitude. Pilots reported difficulty controlling the airplane and asked to return to the airport.

On March 11, 2019, after China had grounded the aircraft, Boeing published some details of new system requirements for the MCAS software and for the cockpit displays, which it began implementing in the wake of the prior accident five months earlier:

• If the two AoA sensors disagree with the flaps retracted, MCAS will not activate and an indicator will alert the pilots.
• If MCAS is activated in non-normal conditions, it will only "provide one input for each elevated AoA event."
• Flight crew will be able to counteract MCAS by pulling back on the column.

On March 27, Daniel Elwell, the acting administrator of the FAA, testified before the Senate Committee on Commerce, Science, and Transportation, saying that on January 21, "Boeing submitted a proposed MCAS software enhancement to the FAA for certification. the FAA has tested this enhancement to the 737 MAX flight control system in both the simulator and the aircraft. The testing, which was conducted by FAA flight test engineers and flight test pilots, included aerodynamic stall situations and recovery procedures." After a series of delays, the updated MCAS software was released to the FAA in May 2019. On May 16, Boeing announced that the completed software update was awaiting approval from the FAA.The flight software underwent 360 hours of testing on 207 flights. Boeing also updated existing crew procedures. The implementation of MCAS has been found to disrupt autopilot operations.

On April 4, 2019 Boeing publicly acknowledged that MCAS played a role in both accidents

Why do newer 737s need MCAS?
U.S. Federal Aviation Administration (FAA) regulations require that all commercial jetliners have smooth handling characteristics in all flight conditions. This includes rare instances where the nose of the airplane is unusually high, such as when it is climbing aggressively or performing excessively tight turns with high bank angles.

When does MCAS activate?
MCAS was designed to activate only when all three of the following conditions occur at the same time:

1. The pilot is flying the airplane manually.
2. The airplane nose approaches a higher-than-usual angle.
3. The pilot has the wing flaps up.

How did MCAS initially work?
Prior to being enhanced, MCAS relied on information from a single Angle of Attack (AOA) sensor to monitor the angle of the airplane. In the two accidents, a single AOA sensor gave incorrect information to MCAS, which caused it to activate. In both cases, MCAS engaged repeatedly when the sensor continued to incorrectly report a high AOA.