Miami, FL/Aviation News/March 21, 2022 (www.avionicsea.com/)
Boeing developed a newer MCAS software update to provide layers of protection if the Angle of Attack (AOA) sensors provide erroneous data. First let's explain the MCAS system on the Boeing 737 max.
What is the MCAS System?
MCAS, or Maneuvering Characteristics Augmentation System, provides continuous aircraft handling characteristics in a specific set of unusual flight conditions. Prior to being upgrade the MCAS relied on information from only one Angle of Attack (AOA) sensor. The AOA sensor monitor the angle of the airplane nose. Back in the two accidents of Ethiopian airlines and Lion Air, a single AOA sensor was faulty giving incorrect data to MCAS system, which caused it to activate. In both cases, MCAS engaged repeatedly when the sensor continued to incorrectly report a high AOA.
When does MCAS activate?
The MCAS system was originally designed to activate only on the following conditions:
The pilot is flying the aircraft manually.
The aircraft nose approaches to an elevated angle of attack.
The pilot has the wing flaps in the up position.
Angle of Attack disagree alert
The Boeing design requirements for the 737 MAX included the AOA Disagree alert as a standard, standalone feature, in keeping with Boeing’s fundamental design philosophy of retaining 737 Next Generation (NG) commonality.
The software delivered to Boeing linked the AOA Disagree alert to the AOA indicator, which is an optional feature on the MAX and the NG. The software activated the AOA Disagree alert only if an airline opted for the AOA indicator.
When the discrepancy between the requirements and the software was identified, Boeing followed its standard process for determining the appropriate resolution of such issues. Boeing discussed the status of the AOA Disagree alert with the FAA in the wake of the Lion Air accident. At that time, Boeing informed the FAA that Boeing engineers had identified the software issue in 2017 and had determined per Boeing’s standard process that the issue did not adversely impact airplane safety or operation.
All customers with previously delivered MAX airplanes will have the ability to activate the AOA Disagree alert.
MCAS Enhanced protections
The 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.
The new Software
The new software with hundreds of hours of analysis, laboratory testing, verification in a flight simulator and numerous test flights. The additional layers of protection include:
Flight control system will compare inputs from both AOA sensors, If the sensors disagree by 5.5 degrees or more with the flaps retracted, (MCAS will not activate). An indicator on the flight deck display will alert the pilots.
If MCAS is activated in non-normal conditions, it will only provide one input for each elevated AOA event. There are no known or envisioned failure conditions where MCAS will provide multiple inputs.
MCAS can never command more stabilizer input than can be counteracted by the flight crew pulling back on the column. The pilots will continue to always have the ability to override MCAS and manually control the airplane.
These newer updates aids reducing the pilot's workload in non-normal flight situations and prevent erroneous data from causing MCAS activation.
Crew training
Boeing’s 737 MAX training includes a suite of computer-based training modules (CBT's), and updated documentation, and simulator training. These instructional materials are designed to provide 737-type rated pilots with an improved understanding of 737 MAX flight control systems, reinforce their technical knowledge of associated flight deck effects and operational procedures, and restore confidence in the 737 MAX.
To be certified a Boeing 737 type rating certificate, pilots must complete 21 or more days of instructor-led academics and simulator training. To earn to fly a 737 MAX, a pilots must either complete a 737 MAX specific type-rating course or, if a pilot is already certified to fly the 737NG, they must complete the NG to MAX Differences training.
Definitions
Maneuvering Characteristics Augmentation System (MCAS) – flight control law implemented on the 737 MAX to improve aircraft handling characteristics and decrease pitch-up tendency at elevated angles of attack.
Angle of Attack (AOA) – the difference between the pitch angle (nose direction) of the airplane and the angle of the oncoming wind.
Angle of Attack Sensor / Vane – hardware on the outside of the airplane that measures and provides angle of attack information to onboard computers; also referred to as an AOA vane.
Angle of Attack Disagree – a software-based information feature that alerts flight crews when data from left and right angle of attack sensors disagree. This can provide pilots insight into air data disagreements and prompts a maintenance logbook entry.
Angle of Attack Indicator – a software-based information feature that provides angle of attack data to the flight crew through the primary flight displays. It is an option that can be selected by customers.