[ghshephard]

You only need two sensors to determine if one is faulty. You need three if you need the correct measurement. For this application, all that is required is a warning that the AoA sensors aren’t providing correct information.

[mantap]

That's true but it puts the aircraft at a greater risk of stalling, because MCAS is supposed to be a safety feature to prevent stalls.

Two sensors are sufficient to prevent false positives, but three or more are necessary to mitigate false negatives.

If the MAX is really so easy to stall that it needs a computer system to prevent stalls, then that system should be robust enough to survive a sensor out.

[missosoup]

There is no risk of stalling.

There was never any risk of stalling.

MCAS was added solely so that the 737 MAX 8 could share a type rating with the previous members of the 737 family. This is what allowed Airlines to put pilots on this plane without any significant new training.

Please stop perpetuating urban myths.

Here, listen to a guy who actually know what he's talking about describing the overall situation: https://www.youtube.com/watch?v=btZXVPfh-pE

[mantap]

I don't think I'm perpetuating an urban myth. My understanding is that MCAS was necessary to correct a deficiency in the controls - that's the controls violated an FAA rule that the pitch up should not be superlinear relative to the control input (which clearly increases the risk of stall). And indeed after testing they had to increase the strength of MCAS because the tendency to pitch up was worse than previously anticipated.

The desire for a common type rating is an explanation for why the FAA rule was violated without MCAS.

[missosoup]

This is incorrect. And this has been dispelled by multiple commercial pilots including Juan from the link above who is one of the most experienced pilots in the country.

The origin of this noise is sensationalist news coverage and misinterpretation of FAA rules. I expect a higher standard on HN than perpetuating myths. You guys are like the flat earthers of aviation.

https://en.wikipedia.org/wiki/Maneuvering_Characteristics_Au...

As Juan points out in either that or another video, the 767 has a stronger pitch-up characteristic and yet doesn't have MCAS and doesn't fall out of the sky from stalls.

Another myth and piece of sensationalist news bullshit is that Boeing made a safety feature that would have prevented the accidents optional. The safety feature in question was AOE Disagree Indicator. The reason it was optional is because AOA doesn't mean anything to civilian pilots and the option only made sense for airlines who hire ex-militaray pilots who can actually read the AOA indicator and make sense of it.

[roelschroeven]

From the Wikipedia article you linked: "The 737 MAX's larger CFM LEAP-1B engines are fitted further forward and higher up than in previous models. The aerodynamic effect of its nacelles contributes to the aircraft's tendency to pitch up at high angles of attack (AOA). The MCAS is intended to compensate in such cases, modeling the pitching behavior of previous models, and meet a certain certification requirement"

Note that last piece: "intended to [...] meet a certain certification requirement". So yes, part of the intention of MCAS is to meet the certification requirements. It's a bit downplayed in the text, but is very important nevertheless: the certification requirements MUST be met (MUST in the sense of the Internet RFCs). Modelling the behavior of previous models is a sufficient condition for that, and is what Boeing was trying to do anyways, but is not critical. Complying to the certification requirements is critical.

[piva00]

So the whole point of MCAS is to basically create an emulated flight envelope more similar to how previous generations 737s flew just to avoid a type rating re-certification?

People died because Boeing was playing with emulators in real life?

[missosoup]

The shared type rating was a huge draw for airlines. Without it, the 737 MAX 8 would not have been nearly as popular in orders as it was.

Boeing got very greedy and multiple failures have occurred along the way. MCAS on passenger planes is not unheard of, and has been implemented for various reasons before including shared type ratings. It was just never botched across the board this badly before.

The biggest failure as parent comment correctly notes, is MCAS was given significantly more control authority than originally planned after flight testing showed it was ineffective as per the original design. But this change in control authority didn't trigger a review process which would have reclassified it from non-critical to critical system.

[VBprogrammer]

> The reason it was optional is because AOA doesn't mean anything to civilian pilots and the option only made sense for airlines who hire ex-militaray pilots who can actually read the AOA indicator and make sense of it.

Do you have a source for that because as far is I can see it is rubbish.

Military pilots are probably more familiar with AoA as early swept wing aircraft needed to be flown with careful attention to it. However several civilian accident investigations have suggested an AoA indicator as a potential mitigation. It's not exactly like reading tea leaves, if you keep the value below 15 or so degrees the wing is flying. Much above that it won't be.

They are also used to good effect by backcountry private pilots who need to extract maximum performance from their aircraft.

[missosoup]

Commercial pilots are trained to work with airspeed and glideslope not AOA. I don't know what else to tell you. If you want to hear the same thing from an authority figure, the same youtube channel I linked earlier discusses this. Pick up a sample manual if you want to check something that's a basic fact in the industry.

[7952]

I always wondered if it makes sense to base anything like this on AOA alone. The aircraft has a ground speed of several hundred knots, a sensible air speed, is at a normal attitude, in normal wind conditions, no wind sheer warnings and with normal engine function. In that situation AOA sensor failure is far more likely than a stall.

[CydeWeys]

But if they aren't providing correct information then MCAS can't work and now your plane has different flying characteristics than the older 737s, which is all your pilots are trained on because the whole point of MCAS is to prevent requiring recertification.