B737 MCAS INVESTIGATION

What is MCAS?

How does it work? Did MCAS brings on aviation industry into disaster? Did MCAS impact the aircraft technical system? So many questions you wonder how this system works and how does it implemented to ensure the safety. Let's go through it!!

Please take note, all the information are based on current investigation and application which been introduced to B737-Max. This article is just a first phase on determining the error and root cause. The second phase of investigation analyst will be updated soon. Thanks and Enjoy your reading!

MCAS was introduced to counteract the pitch up effect of the LEAP-1B engines at high AoA. The engines were both larger and relocated slightly up and forward from the previous NG CFM56-7 engines to accomodate their larger diameter. This new location and size of the nacelle causes it to produce lift at high AoA; as the nacelle is ahead of the CofG this causes a pitch-up effect which could in turn further increase the AoA and send the aircraft closer towards the stall. MCAS was therefore introduced to give an automatic nose down stabilizer input during steep turns with elevated load factors (high AoA) and during flaps up flight at airspeeds approaching stall.

The MCAS is designed to only activate when the wing flaps are retracted after take-off and the aircraft has gained altitude.

However, it is speculated this system may have played a part in both accidents, with the climbing and diving behaviour of the flight the result of the pilot's struggle to overcome the automated system.

On the face of it this seems like a sensible, beneficial system. However following the accident to Lion Air MAX-8 PK-LQP on 29 October 2018, shortly after take-off, in which it appears that the Captains AoA sensor was faulty, it is believed that the MCAS used this erroneous AoA data to command nose down stabiliser which was not counteracted sucessfully by the crew until the aircraft impacted the water.

Boeing have been working on a software modification to MCAS since the Lion Air accident. Unfortunately although originally due for release in January it has still not been released due to both engineering challenges and differences of opinion among some federal and company safety experts over how extensive the changes should be. Apparently there have been discussions about potentially adding enhanced pilot training and possibly mandatory cockpit alerts to the package. There also has been consideration of more-sweeping design changes that would prevent faulty signals from a single sensor from touching off the automated stall-prevention system.

MCAS was introduced by Boeing on the 737 Max 8 because its heavier, more fuel-efficient engines changed the aerodynamic qualities of the workhorse aircraft and can cause the plane's nose to pitch up in certain conditions during manual flight.

Angle of attack sensors on the aircraft tell the MCAS to automatically point the nose of the plane down if it is in danger of going into a stall.

This is done through horizontal stabilizers on the plane's tail which are activated by the aircraft's flight control computer.

According to Boeing, MCAS does not control the plane during normal flight but "improves the behavior of the airplane" during "non-normal" situations.

These could be steep turns or after takeoff when a plane is climbing with flaps up at speeds that are close to stall speed.

According to the flight data recorder, the pilots of Lion Air Flight 610 struggled to control the aircraft as the automated MCAS system repeatedly pushed the plane's nose down following takeoff.

The pilots of the Ethiopian Airlines plane reported similar difficulty before the aircraft plunged into the ground shortly after takeoff.

Boeing has faced accusations that it failed to properly inform pilots and airlines of the MCAS anti-stall controls, which are new to the 737 MAX, added to deal with the risk that its larger engines, mounted higher up and further forward on the wings than on the previous version of the plane, could cause the nose of the plane to point too high in certain maneuvers, leading to a loss of lift. Boeing has said that pilot manuals already contained instructions on how to override other automatic pitch-trim systems that also could push the aircraft’s nose down, steps that also turn off the MCAS.

(Updates)

The U.S. Federal Aviation Administration said Monday that there was no evidence yet that would lead it to ground the 737 MAX 8 and that it expected to mandate software updates to the MCAS by the end of April that would include changes that reduce reliance on pilots’ memory of procedures to deal with pitch-trim problems.

A preliminary report on the Lion Air Flight 610 accident blamed it in part on a faulty angle of attack sensor that triggered the MCAS system and automatically forced the plane's nose down.

Pilots flying the same Lion Air plane the previous day had managed to override the automated flight control system.

Boeing came in for some criticism after the Lion Air crash for allegedly failing to adequately inform 737 pilots about the functioning of MCAS or provide training about the system.

Following the Lion Air crash, Boeing issued a bulletin to airlines operating the 737 Max 8 advising pilots how to override the MCAS system.

The US aircraft manufacturer issued a statement on Monday saying it was too early to understand the cause of the Ethiopian Airlines accident.

Boeing also said it was working on software updates to the MCAS system which would be deployed across the 737 Max fleet.

It said procedures already exist to "safely handle the unlikely event of erroneous data coming from an angle of attack (AOA) sensor," the suspected cause of the Lion Air crash.

"The pilot will always be able to override the flight control law (MCAS) using electric trim or manual trim," the aircraft manufacturer said.

A growing list of countries have grounded their 737 Max aircraft after the two deadly crashes in just five months.

Boeing has described the Max series as its fastest-selling family of planes, with more than 5,000 orders placed to date from about 100 customers.

Incident reports (Lion Air)

* INFORMATION RETRIEVED FROM FLIGHT DATA RECORDER OF LION AIR JT610*

- Pilots were battling multiple malfunctions almost as soon as the flight began

- Faced a cacophony of warnings that started seconds after takeoff and continued for the remaining 11 minutes before the crash

- Alerts included a stick shaker warning, and instruments that registered different readings for the captain and copilot

- Pilots were pulling back on the control column with a force of as much as 100 pounds of pressure, in final moments of dive

- Data indicated that the plane was controllable, however, with the pilots having kept it under control for about 10 minutes before the final dive

- Records from previous flight of the same plane showed another set of pilots had an identical set of failures and landed safely

- Plane’s angle-of-attack sensor on the captain’s side was providing dramatically different readings than the same device feeding the copilot’s instruments, with the captain’s sensor indicating the nose was more than 20 degrees higher than its counterpart ; As a result, stick shaker was activated on the captain’s side of the plane, but not the copilot’s

- New safety feature called the Maneuvering Characteristics Augmentation System (MCAS) activated as a result of erroneous reading, and pitched the plane up & down more than 30 times during the flight

- Pilots attempted to counter MCAS by trimming the plane in opposite direction & pulling back on control column, but in last 30 seconds of flight, counter measures seemed to lose aggressiveness, resulting in loss of 5000 feet of altitude in just 20 seconds ; Also became much harder to pull back the control column & lift the nose during fatal dive

- Questions remain over why the pilots didn’t shut off the trim system during the roughly 10 minutes they were assessing the plane’s behavior, and why the plane exhibited the same malfunctions it had on the previous flight after it was supposedly repaired overnight?

**NOTE

“A high level of competency in hand-flying (both the physical and the cognitive aspects) is necessary for safe flight operations, regardless of the level of autoflight equipment installed…[italics added for emphasis].”

But the challenge to pilots becomes a lot more acute if they are unaware of a critical new system, as in the case of MCAS in the 737 MAX-8

The MCAS lowers the nose automatically to prevent a stall, or the loss of lift, if it detects that the angle of the plane’s nose is too high relative to the ground. A malfunctioning sensor may have led the MCAS to engage repeatedly, countering the pilots’ maneuvers.

A pitch augmentation system function called 'Maneuvering Characteristics Augmentation System’ (MCAS) is implemented on the 737-8, -9 (MAX) to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and dunng flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column tnm switch or stabilizer aisle stand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using Input data from sensors and other airplane systems.

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer Incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer Input is lower at high Mach number and greater at low Mach numbers. The function Is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.