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How is MCAS in 737 MAX 8 different from the stick pusher in MD80 or DHC8 and from the Airbus alpha protection, in terms of overall safety?

All these systems will push the nose down at high AoA and all of them rely on the AoA data, which can become corrupted?

If all of those systems perform a similar function, why is only MCAS currently under scrutiny?

(this is an operation safety question, not related to Boeing's apparent lack of proper disclosure and training)

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  • $\begingroup$ They activate under different scenarios. Only MCAS is under scrutiny because it's the newest of those therefore it may require some initial updates like all software once released. $\endgroup$ – Afe Apr 28 at 16:16
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    $\begingroup$ Just being pedantic here, but an important distinction: MCAS is not a stall protection system. $\endgroup$ – Radu094 Apr 28 at 19:12
  • $\begingroup$ @Radu094 It's an excellent point, not pedantic at all. The purpose of MCAS is not stall protection, and comparing it to "other stall protection systems" is like comparing Valencia oranges "to other apples." VTC. $\endgroup$ – Ralph J Apr 28 at 21:21
  • $\begingroup$ I did not imply that MCAS is a stall-protection system; my original question was changed by en editor. Regardless of its purpose, the outcome of MCAS is a nose-down attitude change, similar to the other two systems. I was wondering why MCAS is currently considered evil, but the other two are not. $\endgroup$ – mike_KS Apr 28 at 22:53
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    $\begingroup$ What MAY have happened (currently under investigation) is the MCAS kept pushing the nose down beyond the ability of the elevator to compensate, especially after it was turned back on. Sadly, at this point, the aircraft may have been actually stalled, but machines can only do what they are programmed to do and it kept firing. There was some discussion of programming logic on this site, hopefully MCAS "brain" can read other (elevator) inputs and abort/reset. It needs to be tied to other systems (including pilots) to better determine if stall is "real". $\endgroup$ – Robert DiGiovanni Apr 29 at 1:36
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MCAS is currently under scrutiny because it has apparently caused two crashes resulting in 346 fatalities. Other similar systems have undergone scrutiny in the past. For example, the flight control system in the Airbus A330 was under the microscope after suffering a number of very similar failures. On Qantas flight 72 the computer received faulty AoA data causing a sudden nose-down. They were fortunate enough to have enough altitude to recover, but it resulted in some serious injuries. It was determined that the incident was not completely isolated.

As to why the A330 was not grounded where the 737 Max was, that is probably due to the fatalities involved. Airbus just dodged a bullet in that the crews had sufficient time to recover. Timing was everything.

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    $\begingroup$ The OP seems to ask on technical differences, not on the manufactures or regulators response or the impact on carriers. A look at ATSBs final report on QF72 reveals a significant different approach regarding fault tolerance. $\endgroup$ – Andreas Apr 28 at 21:24
  • $\begingroup$ @Andreas The question in the title is slightly different from the one in the body. I was answering “If all of those systems perform a similar function, why is only MCAS currently under scrutiny?“ That seems to be what he’s getting at according to his comments. $\endgroup$ – TomMcW Apr 29 at 0:22
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The main difference is that the pilots can overcome "stick pusher" systems used in other aircraft by pulling back on the yoke/stick, though that may be difficult depending on how exactly the system works. (Some use the standard auto-pilot servos, which are relatively easy to overpower, but others make it more difficult.)

MCAS operates by adjusting the horizontal stabilizer trim, which cannot be overcome by the pilot pulling back on the yoke, no matter how much force is applied. In the two noted accidents, the pilots did try countering with opposite trim several times, but as soon as they stopped, MCAS kicked back in to undo their corrections. They did not follow the emergency checklist for runaway stabilizer trim as Boeing/FAA expected, which would have disabled MCAS as a side effect.

There's also a separate problem where MCAS normally uses a single AOA sensor, which goes against aviation engineering principles of having redundant everything. Boeing did offer an optional feature to compare multiple AOA sensors and alert the pilots if they disagreed, but neither of the two accident airlines purchased that option. One of the proposed fixes is to make that a standard feature, both for all new production and mandatory retrofit of all existing planes without it.

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    $\begingroup$ Sorry, but this is inaccurate. The MD-80 has a dedicated hydraulic stick pusher that requires significantly more force than the autopilot servos. The Dash 8 uses pressure from a nitrogen tank. Both do not use the AP disconnect but a separate switch. They are intentionally difficult to override due to the risk of a deep stall. Airbus FBW uses an completely unoverridable software command. $\endgroup$ – user71659 Apr 29 at 17:16
  • $\begingroup$ Updated. Thanks for the info! $\endgroup$ – StephenS Apr 29 at 17:29
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How does MCAS compare to stall protection systems installed in other aircraft?

Compared to Airbus A320

  1. Boeing didn't initially inform pilots or provide training materials for pilots explaining the existence, purpose, effects and means to control MCAS. AFAIK Airbus have always provided pilots with training materials and other information about flight envelope protections and flight control laws (normal, alternate, direct etc).

  2. MCAS used input from only one AoA sensor of the two available. AFAIK Airbus A320's flight envelope protections use inputs from three AoA sensors and can detect when a single AoA sensor is giving erroneous readings and disregard the faulty sensor.

  3. MCAS will repeatedly attempt to make corrections even after it has initially been temporarily disabled by a pilot's control inputs. I don't think this applies to Airbus A320s - If flight envelope protections are disabled I believe that means transitioning to a different flight control law without any possibility of automatic and unannounced reversion to normal law.


Related questions on Airbus stall-protection or flight envelope protection

Other

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