Boeing 737 MAX: MCAS Origins and Purpose

MCAS Origins & Purpose

• MCAS arose from non-compliance issues discovered post-737 MAX program launch in 2012.
• Wind tunnel data indicated shock formation at the wing-pylon junction during high-speed turns, leading to unacceptable stick force.
• The first non-compliance related to 14 CFR 25.255, requiring a positive slope on the stick force vs. G-curve up to VFC/MFC.
• The second was 14 CFR 25.143 regarding controllability and maneuverability, necessitating stick forces and gradients within satisfactory limits during maneuvering up to VFC/MFC.
• Boeing's aerodynamic fixes were unsuccessful, leading them to adapt MCAS from the KC-46 program (767 tanker).
• The original MCAS scope addressed high-speed pitch-up, triggered by both G-force and angle of attack (AOA).

Low-Speed Stall Handling Issue

• In March 2016, during flight testing, unsatisfactory low-speed, 1G handling was discovered.
• Approaching stall, the stick force back pressure gradient reduced or reversed, particularly at low weight and forward CG.
• This was caused by aerodynamic interaction between larger LEAP engine nacelles and the mid-section of the wings.
• This interaction generated extra lift forward of the CG as the stall was approached, causing a pitch-up.
• Pilots had to reduce back pressure to maintain a steady deceleration of 1 knot per second under certain conditions, which was non-compliant.

Non-Compliance with Stall Characteristics

• CFR 25.203 requires no abnormal nose pitching and positive longitudinal control force up to and throughout the stall.
• CFR 25.173 mandates a pull to maintain speeds below trim, with an average stick force gradient of at least 1 lb per 6 knots.
• Boeing tried aerodynamic solutions that did not work: Revised wing vortex generator pattern, revising the leading edge stall strip, leading edge vortal on modifications.

Reason Aerodynamic Solutions Failed

• Wing vortex generators manage flow separation, but the pitching moment linearity issue occurred before separation.
• Leading edge stall strips enhance buffet by creating a vortex, but this vortex generally doesn't affect the pitching moment.
• Vortelons primarily manage spanwise flow, not the pitching moment.
• Engine strakes reduce stall speed by approximately 5 knots but have no effect on stick force gradient.

MCAS Adaptation for Low Speed

• The adapted MCAS no longer needed a G-force input; it was triggered solely by AOA.
• Horizontal stabilizer authority was increased from 0.6 to 2.5 degrees per cycle.
• The original MCAS scope was for high speeds and it needed greater stabilizer control at low speed.
• Speed Trim System (STS) improves speed stability up to the stall warner speed.
• MCAS blends in with STS around stall warner speed, improving stick force gradient near the stall.
• STS nor MCAS changes the stall speed or characteristics; they only affect the stick force gradient.
• Elevator Feel Shift (EFS) increases stick force near stall by applying 850 psi to the elevator feel computer.
• Elevator Feel Shift acts on the elevator, while STS and MCAS act on the stabilizer.

MCAS Accident Recaps

• Lion Air (October 2018): Miscalibrated AOA probe (21 degrees too high).
• Ethiopian Airlines (March 2019): AOA probe failure on takeoff (74 degrees).
• Both aircraft crashed after repeated MCAS activations.
• Flight deck indications included stick shaker, 35-degree difference between flight director bars, differences in PLIs, airspeed/altitude disagree alerts, master caution, and AOA vane alert.

Airspeed Affects

• Pitot and AOA probes & static ports feed into Air Data Inertial Reference Units (ADIUs).
• ADIUs convert sensor data to electrical signals for the displays.
• ADIUs provide inertial position, track data, as well as attitude, altitude, and airspeed data.
• ADIU applies a static source error correction to airspeed and altitude, using static and total pressure, total air temperature, and AOA.
• An erroneous AOA will affect airspeed and altitude on the associated side due to the static source error correction.

Erroneous AOA Activation:

• ADIUs were programmed to reject AOA probe data if: the resolver output was zero volts, combined altitude was outside an acceptable range, or calculated AOA was outside the mechanical stops range.
• In both accidents, AOA was within the acceptable range, so data was accepted and passed to the flight control computer, triggering MCAS.

Flight Control Computer Role

• ADIU provides AOA data directly to the flight control computer (FCC) for MCAS.
• The speed trim system is part of the FCC, improving speed stability during low-speed, high-thrust conditions.
• During manual flight. MCAS was added to speed trim to improve stick forces near stall.
• MCAS drives the stab trim motor for up to 9.2 seconds, pausing for 5 seconds.
• Stabilizer commands are limited to 2.5 degrees at 0.27 degrees per second.

Flight Control Computers:

• The active flight control computer flip-flops between flights, starting with FCC A from aircraft power-up.
• If the left AOA probe fails on the first flight, MCAS will trigger due to FCC A being active.
• If the right AOA probe fails on the first flight, it will primarily affect the instruments without triggering MCAS.

MCAS Modifications

• In the AOA system, both attack sources are now used.
• There's now an AOA split vein monitor that looks for a difference of 5.5 degrees between the vanes.
• There's a new Mid Value Select Integrator which will pick the mid value of the two alpha probes and the previous output.
• A synthetic angle of attack will be added from the MAX-10 onwards.
• In the flight control computers, they are now dual processor monitors and cross flight control computer monitor.
• MCAS has been limited to one stabilizer trim input (High Alpha event).
• Stabilzer trim input will never exceed the authority of the elevator.
• The control column cutout function has been restored

Control Column Cutout Function

• Mechanical control column cutout functions are now replicated using the new FCC software.

Cross Flight Control Trim Monitor

• This feature provides additional protect against eronius FCC trim commands.
• Compares trim up/down outputs from both FCCs with its own trim command calculation.
• If the outputs differ, the local channel takes control of speed trim.
• The autopilot might disconnect.
• The speed trim fail light will illuminate on recall.
• Stab out of trim light will illuminate on the ground.
• Both FCCs are in use for MCAS operation, with a standby FCC checking the operational FCC signal.

Angle of Attack Processing

• There's a sense check for probe failure and this passes into the split vane monitor and looks for 5.5 degree difference.
• If there's no issues, it goes to MVS.
• The FCCs will only utilize a good AOA value.
• The split vane monitor and the MVS are bypassed if there's only one angle.
• MCAS still works on a single pro because two independent failures are thought to be remote.
• MVS is still there for monitor purposes and we'll get master caution speed trim fail.

Split Function

• Actions taken to improve procedures
• There are revisions to AFMs.
• There were 10 MMEL revisions added which ensure there is no dispatch if there issues with certain systems.
• Pilots need conversion training and there are updates to CDS software.

Timelines

• MCAS was rectified 18 months after second accident.
• Revisions that took over 370,000 engineering hours.
• Ungrounding lasted around 2 years.

Post crash changes

• MCAS reprogramming of the AOA logic.
• Updating of the FCC that could lead to runway stab.
• Delays of fix due to FCC software issue and the inability to get the FCCs to communicate.