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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.

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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.

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  • $\begingroup$ Are you saying the the handling characteristics issue that the Leeham News article discusses is not the reason for MCAS? Is the pitch-down behavior of the NG prior to the stall? $\endgroup$ – sdenham Apr 5 at 18:55
  • $\begingroup$ He’s not saying that the article is wrong. The handling characteristics at high AoA is precisely what MCAS is for. The FCU already increased aft stick forces when the stall warning was active in the NG. The difference is that, at a certain AoA the lift created by the engine nacelles became a factor, making a stronger nose up moment than the NG created at the same AoA. One selling point of the MAX is that it required very little transitional training for pilots currently flying NGs. They used the MCAS to more closely match the performance of the NG. $\endgroup$ – TomMcW Apr 6 at 22:52
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    $\begingroup$ @TomMcW I don't disagree with the points you are making, but my question is not "what is MCAS for?" $\endgroup$ – sdenham Apr 8 at 1:43
  • $\begingroup$ I suppose the reason for the instability of pushing nose at high angle of attack is the center of lift is in front of the center of mass. But why does shifting the engine forward shift the center of lift? Without considering the secondary effect, the thrust of the engine is directed parallel to the fuselage and thus the wing. So there should not be a discernible difference in the torque with respect to the center of mass of the whole plane. $\endgroup$ – Hans Apr 17 at 2:18
  • $\begingroup$ @Hans at high AOA the engines are producing aerodynamic lift. Thus the further forward (or larger) they are, the further that moves the total center of lift forward. $\endgroup$ – fooot Apr 17 at 2:26
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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.

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  • $\begingroup$ This answer does not mention the feel/centering unit, but implicitly you seem to be saying that it would not be a candidate solution because, contrary to the Leehams News article, satisfying the handling characteristics requirement was not the problem that MCAS was developed to solve. If so, could you modify your answer to make this point explicitly, preferably with references to an authoritative source on what made some sort of mitigation necessary? $\endgroup$ – sdenham Apr 5 at 20:10
  • $\begingroup$ @sdenham, the problem with the centering unit is that it is designed to pull the yoke towards the center, but to augment stability you need to move the center around, which is what trim is for. $\endgroup$ – Jan Hudec Apr 6 at 8:00
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    $\begingroup$ @JanHudec I am not suggesting that the Feel and Centering Unit, as currently implemented, solves the problem - if it did, there would be no need for any sort of modification to compensate for the effect of the new engines. $\endgroup$ – sdenham Apr 6 at 13:48
  • $\begingroup$ @sdenham, and I am not talking about current implementation, but overall purpose. The problem that MCAS was developed to solve is poor handling characteristics, but modifying handling characteristics is a problem completely unrelated to the purpose of the feel and centering unit. $\endgroup$ – Jan Hudec Apr 6 at 19:02
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    $\begingroup$ @JanHudec According to 737ng.co.uk/B_NG-Flight_Controls.pdf "The elevator feel computer provides simulated aerodynamic forces... Feel is transmitted to the control columns by the elevator feel and centering unit." This (and the name) suggests that it does more than centering, and modifying the control forces is what the Leeham News article says is the purpose of MCAS. $\endgroup$ – sdenham Apr 6 at 20:02
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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

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  • $\begingroup$ Thank you for replying, but could you expand on why it would inevitably result in a sudden effect? Would it be impossible for the unit to progressively increase the force, relative to that produced by the NG's unit, as the airspeed decays towards the stall? (I say speed rather than AofA, because, AFAIK, the unit does not get that data, but it does have a pair of dedicated pitots.) $\endgroup$ – sdenham Jun 4 at 14:06
  • $\begingroup$ @sdenham, Increasing the force makes a difference with the NG thus the need for training but Boeing didn’t want extra simulator training, thus the need of MCAS and the most transparent possible. It is sudden because it occurs unexpectedly at flap retraction once speed is above 230 knots. Please refer to the following website. reuters.com/article/us-ethiopia-airplane-regulator-insight/… $\endgroup$ – user40476 Jun 4 at 17:15
  • $\begingroup$ How would using the feel and centering unit be seen as a fault by pilots, but using trim wouldn't be? $\endgroup$ – fooot Jun 26 at 16:29
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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.

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  • $\begingroup$ But near the wing you have side effects from the flow around the wing. There are plenty of alpha vanes on fuselages, e.g. on all airbus aircraft that I know of. It would also be possible to correct that error mathematically with the pitch rate and airspeed. If you move the angle of attack sensor to the wing you'd have to calibrate it for all different flap settings, with and without failures of either flaps or slats. Much easier to just calibrate it on the fuselage where at least the geometry doesn't change. $\endgroup$ – Jan Jun 26 at 16:22

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