What is MCAS?

That gives the plane its low-to-the-ground design. Back in the 1960s, this worked well with the smaller, “cigarette” engines, but with the introduction of increasingly powerful engines, requiring increasingly wide fan diameters, it left Boeing trying to come up with fixes for the new variants of the jet, yet still keep that low-to-ground fuselage.

For example, on the 737 Next Generation (the -600, -700, -800, -900 and -900ER, and known as the “NG”) the bottoms of the engine nacelles are flattened out, rather than being circular. This is to try and fit the bigger engines under the wing in the limited ground clearance space available, giving it the characteristic “chipmunk” cheeks, as if it were a rodent with its mouth stuffed full.

The LEAP engines on the 737 MAX are the largest yet for the 737 and have to be mounted higher and further forward to give the necessary ground clearance. During testing and analysis during the MAX’s design stage, Boeing found that the engine and its larger nacelles would pitch the aircraft up when the plane was operating at high angles of attack and mid-range Mach numbers.

That pitch-up problem, exacerbated by the aerodynamic issues, is one that the Maneuvring Characteristics Augmentation System (MCAS) is trying to fix.

As a rule, jet aircraft with engines under the wing will tend to pitch nose-up as the thrust is increased. This is because the thrust line, the direction in which the thrust travels and propels the aircraft, is below the jet’s centre of gravity. Similarly, as thrust is decreased, such aircraft will pitch down. This phenomenon is known as the pitch-power couple.

This tendency to go nose-up is a well known characteristic of the newer variants of the 737 and it has increased as the engines have got bigger and more powerful.

It means that if pilots need to go-around upon approach to landing, they have to be aware the plane may pitch up too far and so counter this by pushing the nose down slightly.

Thus, MCAS, a system that briefly pushes the nose down automatically if the onboard computers determine that it is pointing too far up, was developed to help reduce this pitch-up tendency. It was later extended to improve flaps-up, low-Mach stall characteristics.

As Boeing puts it, MCAS is designed to enhance the stability of the plane “so that it feels and flies like other 737s.” For Boeing, this was important for several reasons.

Applying for certification as a new variant of an existing aircraft is a much shorter process than certifying an entirely new type. In addition, less additional training is needed for pilots switching to the MAX from the previous version, the 737 NG.

Indeed, Southwest’s contracts for the 737 MAX even included a rebate should simulator training, which is more costly than the short computer-based training detailing the differences between the MAX and the NG, be required.

MCAS was designed to work when three conditions are met simultaneously: in manual flight, when the plane’s flaps are up and at higher angles of attack. The angle of attack is the angle between the relative air flow and the wing. As Skybrary puts it succinctly, it’s the difference between where a wing is pointing and where it is going.

If these three conditions occur, MCAS will activate to push the aircraft’s stabiliser to give a nose down movement. The main aim of pushing an aircraft’s nose down is to prevent the plane from stalling, a dangerous condition where not enough lift is produced over the wings to keep it flying.

But one of the key failings of MCAS as installed on the 737 MAX is that it gets its inputs from a single angle-of-attack sensor. This is almost unheard of in the aviation industry, where redundancy, or having backup, is key to ensure safety. In the accident flights, faulty readings from one sensor set off the MCAS system.

In addition, MCAS doesn’t just push the nose down once, but repeatedly, each time the AOA is at an elevated figure for more than five seconds. While the crew can interrupt the nose-down movement by using their trim switches on the control column, MCAS reactivates after 5 seconds if the high angle of attack continues.

In the flight prior to Lion Air 610, the crew used the stabiliser cutout to halt the repeated nose-down movements. Boeing had assumed that such action would help in the event of erroneous MCAS activation but not that it would be necessary, an assumption Indonesian investigators found to be incorrect in their final accident report published on 25 October 2019.

Furthermore, the repeated MCAS activations in the accident flights meant that eventually the forces became too much for pilots to override manually by pulling the stick back. The forces recorded on the Lion Air flight reached 103lbs, or approximately 46 kilograms — the equivalent of two fully laden suitcases at the 23kg commercial airline weight limit.

Perhaps even more crucially, pilots were not aware of the existence of MCAS. In a lawsuit filed against Boeing, the union representing Southwest pilots, SWAPA, alleges that Boeing rushed the certification and introduction of the plane and says Boeing made “systematic and concerted efforts” to minimise the differences between the 737 NG and the MAX.

“In short, Boeing had wrongfully persuaded SWAPA and Southwest that the 737 MAX was just another 737. That was not the case,” states the suit, arguing that pilots the world over were outraged by Boeing’s failure to disclose the presence of MCAS before the Lion Air crash. It adds: “As SWAPA and the public later learned after the Ethiopian Airlines flight 302 accident on 10 March 2019, Boeing’s deliberate 737 MAX design shortcuts and questionable decisions resulted in a different aircraft that was not safe and should not have been certified as airworthy.”

Boeing has so far promised to fix all the problems with MCAS with a software update. According to information provided by Boeing as of December 2019, the system will compare inputs from both AOA sensors. If they disagree by more than 5.5 degrees, MCAS will not activate.

If MCAS does activate, it will provide one input for each event where the angle of attack is too high. In addition, it will not command more stabiliser input than can be counteracted by the pilots, Boeing promises.

Boeing will also likely update flight crew and engineers manuals to include more information on MCAS. It remains to be seen what additional training will be mandated for pilots. Boeing so far has said it has updated the computer-based training for 737 MAX pilots, but there have been calls for additional simulator-based training — and even, most recently from Canadian regulatory representatives, for MCAS to be removed from the MAX entirely.

The future of the MAX is intrinsically linked with MCAS. The question is whether Boeing can persuade airlines, pilots, regulators and passengers that the aircraft can be operated safely, with or without it.

Boeing had a problem with the 737 MAX, and it was not a new problem