What is Boeing MCAS, and how does it work?
Boeing’s Maneuvering Characteristics Augmentation System (MCAS) is a 737 MAX flight-control function that automatically commands nose-down stabiliser trim during manual, flaps-up flight when angle of attack becomes high. It was designed to provide consistent handling and control-column forces, not to act as an autopilot or a complete stall-prevention system.
In real-world aviation, MCAS is a software function within the 737 MAX Speed Trim System and flight-control computers; it is not a sensor, trim motor or stand-alone box. Under ordinary operations, it should do nothing noticeable.
Why was MCAS added to the Boeing 737 MAX?
MCAS was added to address a change in pitch characteristics at elevated angles of attack. The MAX’s larger LEAP-1B engines sit differently from those on earlier 737 generations, and their nacelles can generate additional lift ahead of the aircraft’s centre of gravity when the nose is raised substantially.
That aerodynamic effect can increase the tendency to pitch up and alter the expected control-column force gradient. Boeing used automatic stabiliser trim to make the MAX meet handling requirements and feel more consistent with earlier 737s. MCAS was not needed continuously to keep the aircraft airborne.
How does Boeing MCAS work?
On a modified, in-service 737 MAX, MCAS follows a tightly restricted sequence:
- Check the activation conditions: The aircraft must be in manual flight with the autopilot disengaged, the flaps retracted and the calculated angle of attack above a threshold that varies with flight conditions, including Mach.
- Compare both angle-of-attack sensors: The revised flight-control system checks the left and right vane readings. If they disagree by 5.5 degrees or more under the relevant conditions, MCAS is inhibited and an
AOA DISAGREEindication is generated. - Command stabiliser trim: When its conditions are met, MCAS uses the electric trim system to move the horizontal stabiliser in the nose-down direction. It changes the incidence of the stabiliser rather than directly moving the elevators.
- Limit the command: Revised MCAS activates only once for a single elevated-angle-of-attack event. Another activation requires the angle of attack to return below the activation region and then rise again.
- Preserve pilot authority: The command is limited so that the crew can counter it using the control column. Pilots can also command electric trim themselves or remove electrical stabiliser drive using the trim cut-out system.
The stabiliser affects the force needed to hold a given pitch attitude, so an unwanted nose-down command may initially feel like increasing back-pressure on the control column. Continued mistrim can produce very high forces even though the elevators remain connected to the pilots’ controls.
Is MCAS a stall-protection system?
No. MCAS is handling augmentation triggered by calculated angle of attack; it does not impose an angle-of-attack limit or guarantee that the aircraft cannot stall. Stick shaker and other stall-warning functions are separate, although faulty sensor data can affect more than one system.
This distinction makes more sense alongside our side-by-side explanation of A320 and 737 control philosophy, particularly the difference between conventional controls and fly-by-wire envelope protection.
How did the original and revised MCAS differ?
The original design could accept one faulty angle-of-attack input and command stabiliser movement repeatedly. The post-grounding redesign added sensor comparison, restricted repeat activation and limited command authority.
| Feature | Original MCAS | Revised MCAS |
|---|---|---|
| Angle-of-attack data | Used one sensor selected through the active flight-control computer | Compares both angle-of-attack sensors before activation |
| Sensor disagreement | A single erroneous reading could trigger MCAS | Significant disagreement inhibits MCAS and produces an alert |
| Repeated commands | Could reactivate after the pilot used electric trim if the high reading remained | Commands once per elevated-angle-of-attack event |
| Command authority | Repeated activations could accumulate severe nose-down stabiliser trim | Authority is limited so the pilot can counter the command with the control column |
What went wrong in the two 737 MAX accidents?
Faulty angle-of-attack information triggered the original MCAS logic on Lion Air Flight 610 and Ethiopian Airlines Flight 302. Because the system trusted one sensor and could reactivate after opposing pilot trim, it repeatedly commanded nose-down stabiliser movement while the crews were also dealing with stick shaker and conflicting indications.
The two accidents killed 346 people and led to the worldwide MAX grounding and redesign. MCAS was central to the accident sequences, but the investigations also examined certification, system architecture, assumptions about pilot response, crew information and training, maintenance, and operational decisions.
Can pilots switch off or override MCAS?
There is no dedicated MCAS OFF switch. If stabiliser movement is uncommanded, crews use the established runaway-stabiliser procedure rather than trying to diagnose MCAS in the moment.
That procedure centres on maintaining aircraft control, managing automation and electric trim, and using the stabiliser trim cut-out switches when directed. Cutting out the electric trim system also removes MCAS’s ability to drive the stabiliser, after which manual trim remains available. Exact memory items and switch terminology in the approved aircraft checklist and training material take precedence over any summary.
How should MCAS behave in a flight simulator?
A faithful MCAS simulation should remain dormant throughout normal airline-style flying. If a simulated MAX pitches down while the flaps are extended or the autopilot is engaged, that is not valid MCAS behaviour; we would first check autopilot modes, centre of gravity, Speed Trim System behaviour, duplicate trim bindings, noisy control hardware and add-on failures.
Simulator products vary widely. Some represent the original logic, some model the revised system, and others provide only a simplified trim response or no functional MCAS at all. Sensor-disagreement inhibition and the one-command-per-event restriction are useful indicators of which logic has been modelled.
A downloadable 737 MAX 8 simulator counterpart is available in our library, but a visual or freeware aircraft should not be assumed to reproduce the certified MAX flight-control software unless its documentation explicitly says so.