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https://www.faa.gov/news/media/attachments/19_035n-R3-8-3-20.pdf https://www.faa.gov/news/media/attachments/19_035n-R3-8-3-20... > "To address the unsafe condi

by zaroth 6y ago

https://www.faa.gov/news/media/attachments/19_035n-R3-8-3-20.pdf https://www.faa.gov/news/media/attachments/19_035n-R3-8-3-20... > "To address the unsafe condition, the FAA proposes to require four design changes: (1) installing updated flight control software (with new control laws) for the FCC operational program software (OPS), (2) installing updated MDS display processing computer (DPC) software to generate an AOA disagree alert, (3) revising certain AFM flightcrew operating procedures, and (4) changing the routing of horizontal stabilizer trim wires." > "The first design change is intended to prevent erroneous MCAS activation. The second design change alerts the pilots that the airplane’s two AOA sensors are disagreeing by a certain amount indicating a potential AOA sensor failure. The third design change is intended to ensure that the flightcrew has the means to recognize and respond to erroneous stabilizer movement and the effects of a potential AOA sensor failure. The fourth design change is intended to restore compliance with the FAA’s latest wire separation safety standards." Notably the FAA does not required a 3rd AoA sensor, but simply for software to monitor both sensors, compare the values, and if they disagree beyond a given threshold, to light an "AoA Disagree" lamp and disable MCAS for the remainder of the flight. MCAS will also only be allowed to activate one time per "High AoA" event. The AoA sensors must return to a normal range before MCAS is allowed to activate again. Finally, they must limit the maximum MCAS command authority within a set range so that manual control can always maintain altitude, whereas previously MCAS would command horizontal stabilizer adjustments without any regard to the current position. It seems that in an AOA DISAGREE situation, the flight is still permitted to take off. In my non-expert summary, they seem to be doing the absolute minimum amount of work possible to "address" the problem, and dodge completely the fundamental contradiction of why one would implement an unreliable-by-design MCAS system, hobble the control authority of that system, and further, permit flight when that system is known to be disabled. In light of these admissions, I cannot comprehend why the MCAS system exists at all, and how the added complexity (variance in airframe operation) is worth any possible benefit. In Elon-speak, the best part is no part. The best system is no system. So if you admit you can fly without it, why leave something in which is unpredictable, unreliable, and already proven to be deadly? EDIT: The total cost of compliance for applying these changes to 73 existing airframes is estimated to be ~$1 million, 70% of which is the wiring harness change. <s>The software change itself seems to amount to about 20 lines of code, so let's call it $20 million.</s> Sorry, this seems like an absolute joke, and right now I'm pretty angry that this is what they came up with after "60,000 hours of review".

henryfjordan 6y ago
> disable MCAS for the remainder of the flight. My understanding is that the MCAS or something equivalent is necessary on the 737 MAX because the engines sit lower than they really should and the forward propulsion creates torque that raises the nose of the plane. Something needs to counteract that force. If the MCAS is disabled, is the pilot able to trim the airplane manually similar to how MCAS works? Or will they have to just kinda hold the nose down with the main steering controls?
- rootusrootus 6y ago
  Basically all modern airliners with engines under the wing generate torque that tries to raise the nose of the plane. MCAS is to correct for nonintuitive behavior near the limit of the flight envelope when the wing is at a high angle of attack. Most flights never get anywhere near that, and disabling MCAS would not be noticeable.
  - deleted 6y ago
    [deleted]
- karmelapple 6y ago
  This might be oversimplification, but I'll venture it and hope someone corrects me if I'm wrong: If MCAS is disabled, then the airplane will fly very differently from the 737s that everyone with an existing 737 flight rating is used to. So the plane can fly, but it'll feel different. And that sure does seem like a problem to get back safely to the ground.
  - jacquesm 6y ago
    But not as big a problem as having MCAS misbehaving, hence the change.
  - tzs 6y ago
    MCAS only kicks in at high angle of attack with the flaps up. I believe that most flights never come near having it activate. If it disables due to faulty sensors and the pilots are told it is disabled, most flights won't have to do anything different at that point. The plane will handle just like it normally does. If there are flights that are supposed to have maneuvers that would trigger MCAS, pilots are going to need to be trained to avoid those maneuvers when MCAS is disabled.

by zaroth 6y ago

henryfjordan 6y ago

> disable MCAS for the remainder of the flight. My understanding is that the MCAS or something equivalent is necessary on the 737 MAX because the engines sit lower than they really should and the forward propulsion creates torque that raises the nose of the plane. Something needs to counteract that force. If the MCAS is disabled, is the pilot able to trim the airplane manually similar to how MCAS works? Or will they have to just kinda hold the nose down with the main steering controls?

rootusrootus 6y ago
Basically all modern airliners with engines under the wing generate torque that tries to raise the nose of the plane. MCAS is to correct for nonintuitive behavior near the limit of the flight envelope when the wing is at a high angle of attack. Most flights never get anywhere near that, and disabling MCAS would not be noticeable.
- deleted 6y ago
  [deleted]
karmelapple 6y ago
This might be oversimplification, but I'll venture it and hope someone corrects me if I'm wrong: If MCAS is disabled, then the airplane will fly very differently from the 737s that everyone with an existing 737 flight rating is used to. So the plane can fly, but it'll feel different. And that sure does seem like a problem to get back safely to the ground.
- jacquesm 6y ago
  But not as big a problem as having MCAS misbehaving, hence the change.
- tzs 6y ago
  MCAS only kicks in at high angle of attack with the flaps up. I believe that most flights never come near having it activate. If it disables due to faulty sensors and the pilots are told it is disabled, most flights won't have to do anything different at that point. The plane will handle just like it normally does. If there are flights that are supposed to have maneuvers that would trigger MCAS, pilots are going to need to be trained to avoid those maneuvers when MCAS is disabled.