Why would Boeing's MCAS be preferable to a modified Feel and Centering Unit?

Question

According to some sources, the purpose of MCAS on Boeing's 737 MAX variants is to increase the back-force needed to further raise the nose when flying manually at high angles of attack, in order to give the airplane acceptable handling characteristics when approaching a stall.

Both the MAX variants and their predecessors, however, already have an elevator Feel and Centering Unit (page 8), the purpose of which appears to be the generation of appropriate stick force feedback in all stages of flight. If so, then, at first sight, this would be the appropriate unit in which to implement the function of MCAS, raising the question of why MCAS would be the preferred solution.

A few possibilities have occurred to me, but they are just guesses:

1. MCAS functionality needs angle-of-attack input, which may not be available where the Feel and Centering Unit is located (in the tail), and it would be complicated to get that information to it.

2. This answer states the the Feel and Centering Unit is a mechanical computer; given that, it therefore might not be easily modified.

3. Modifying the Feel and Centering Unit would require re-certifying it in toto, not just its new feature.

4. In addition to modifying the handling characteristics, MCAS is also seen as contributing to stall prevention directly, by reducing the angle of attack.

NB: Recently, Dominic Gates wrote an informative article in the Seattle Times about the origins of MCAS.

Answer 1

On the 737 NG, at high angles of attack, the nose of the plane would naturally pitch down, helping to recover from a stall and to increase airspeed.

The larger engine nacelles on the 737 MAX are located forward of the center of gravity, which means that at high angles of attack they are pushing the nose up. MCAS helps to push the nose down in this situation, similar to how the 737 NG would behave.

The added nose down trim has the side effect of requiring more elevator input at high angles of attack, but that was not the primary purpose.

There are several reasons that the Feel and Centering Unit would have a difficult time providing similar functionality:

1. The airplane should pitch down even the absence of control input, so it could not be done by simply changing the way the elevators respond to pilot input.
2. MCAS takes input from angle of attack, altitude, flap position, and airspeed. The Feel and Centering Unit currently just senses airspeed, so it would somehow need to get the other inputs.
3. The Feel and Centering Unit is a mechanical computer, so adding new inputs and changing the behavior could be very complex to design and certify.

Another possibility would be to use the Mach Trim system to adjust the Feel and Centering Unit. The Mach Trim system uses the flight computer to adjust the elevator neutral position to provide stability at higher speeds. While the flight computer should have all of the information needed, stabilizer trim provides much more control authority.

Answer 2

A change in the pitch feel system wouldn't solve the problem. It's the MAX's natural behaviour separate from the flight control system (that is, behaviour when you aren't touching the controls). As Fooot says, the MAX's engines have the effect of moving the overall center of lift forward somewhat, which is more or less the same thing as moving the center of gravity aft.

The airplane, in certain regimes (flaps up), ends up being neutrally or almost neutrally stable in pitch, especially at higher power settings where the thrust is contributing to the nose-up moment - bad enough that the airplane's pitch attitude would drift up when it should be rock solid, and worse, the natural pitch down you should get with a speed decrease wasn't there or was very weak. It could be countered by the pilot, but the hand flying workload goes way up when you have to constantly intervene with an airplane that has a bit of a mind of its own. Flying just about any airplane with an excessively aft CG is like that.

The proper fix would be to move the operating center of gravity range forward to cancel out the influence of the more forward engines and make the horizontal tail larger to compensate so that the tail power required to rotate at takeoff (which usually the tail's hardest job) is still there. They didn't want to go that route and decided to use software to run the stab in the background to "mask" the stability problem from the pilot so that they could keep the C of G range where it was. It's basically an artificial stability system with a narrow operating requirement added on as a band-aid to avoid a far more expensive modification.

It's not the first time it's been done. I recall something similar was done on another type, namely the MD-11, that allowed the airplane to be operated with a farther aft CG than normal, to reduce tail down force in cruise, reducing trim drag. I dimly remember an incident from long ago where the system was disconnected while in cruise and the pilot took over hand flying his neutrally stable airliner cruising at mach point whatever, and a Pilot Induced Oscillation got started that rattled people in the back around pretty good, like shaking a tube of Pringles potato chips.

Answer 3

Boeing wanted the MCAS effect to be transparent to the pilot, as a proof nothing about it was mentioned in the FCOM.

Acting on the feel and centering mechanism would require a sudden effect on the column which would have been noticed and declared as a fault by the pilots since to get the same effect as does the trim, considering the elevators area compared to the THS area, you would need a sudden tremendous visible displacement of the column centering while a single trim shot of 2.5° ( .6° in the original design) is less disturbing, mainly because short trim movements are possible for other reasons even though in manual flight, such as Mach trim.

Who will bother much about a single trim shot, was it not a faulty AOA producing repetitive trim shots? As a matter of fact it remained transparent till the faulty AOA producing the crashes

Answer 4

The root of the problem with the MCAS is that the Angle of Attack sensor is near the nose instead of on the leading edge of the wing, where it belongs. It's supposed to be measuring the Angle of Attack OF THE WING. Instead, it measures the Angle of Attack OF THE NOSE. These measurements are not the same while the aircraft changes pitch.

With the long fuselage, bringing the nose down from climb to cruise will falsely indicate a nose-up attitude because the fuselage is rotating around the wing. This is why both crashes occurred from problems that arose when changing from climb to cruise. Whether the MCAS malfunctions or not depends completely on the rate at which the pilot changes pitch. Push forward too far on the stick, and you are doomed, when the MCAS takes over pitch control and will not give it back.

The false Nose Up reading moves the elevator trim to further lower the nose, again triggering a false nose-up reading. It's a feedback loop that will not stop until MCAS hits its limit, which gets reset every time the pilot follows instructions and presses the reset button. Press it more than three times, and trim is set to the mechanical limits of the elevator trim. In the second crash, the pilot hit the reset button more than 20 times.