AETS-Sofema provides a status overview regarding the Manoeuvring Characteristics Augmentation System (MCAS) system.
Introduction – What does MCAS do?
The 737 MAX was designed to handle and feel the same to the pilot as the 737 NG.
Without the MCAS function, in some small areas of the flight envelope:
- When approaching a stall and during higher g-force manoeuvring (The new engines contribute to the control column feeling lighter in the 737 MAX than the regulations allow.)
Note – These are not areas of the flight envelope in which the airplane normally operates. FAA regulations, specifically 14 CFR 25.143, 25.201, 25.203, 25.251, and 25.255, still require the control column to have a higher pull-force feel in these flight regimes than would exist on the 737 MAX (without the added stability from the STS and MCAS function.)
- For the FAA to certify the 737 MAX, the original design had to meet FAA requirements for control force feel when manoeuvering or deviating significantly from trimmed equilibrium.
- The aircraft manufacturer is responsible for making design decisions and showing compliance with applicable regulations. In this case, Boeing elected to include an additional flight control law in the Speed Trim System (STS), which is part of the flight control software that provides the required control force feel to the pilot.
- MCAS enhances the feel of the column forces in manual flight and is only operative with flaps up. MCAS provides signals to move the horizontal stabilizer at elevated angles of attack to compensate for the aerodynamic effects of the 737 MAX’s larger and more forward-located engines, resulting in the required column feel to the pilot.
- Duration of the MCAS command is a function of AOA and airspeed. At higher airspeeds, flight control surfaces are more effective than at lower airspeeds. Therefore, a smaller MCAS command at higher airspeed has the same effect as a larger MCAS command at low speeds.
- Without MCAS, the 737 MAX would not meet FAA’s regulatory requirements.
Proposed Mitigations
Boeing proposed multiple updates to the MCAS function. The MCAS activation software now includes a maximum limit of one nose down stabilizer command during a single elevated AOA event and cannot be reset by pilot activation of the electric trim switches.
An AOA sensor monitor was added to prevent MCAS from using an AOA input if it differs from the other AOA input by more than 5.5 degrees.
Boeing incorporated a maximum command limit to disable the MCAS and speed trim operations if the stabilizer position exceeds a reference position.
This limit ensures sufficient elevator control is available to provide manoeuvring capability using control column inputs alone.
For more information please email team@aets-sofema.com
Tags:
Boeing 737 MAX, Boeing 737 MAX design, Boeing 737 MAX Practical training, Boeing 737 MAX Safety, Boeing 737 MAX Theoretical training, EASA Part 147 compliant training, FAA Approval, Manoeuvring Characteristics Augmentation System, MCAS
