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Comment by phkahler | Hacker News Reader

>> What he's probably saying is there is nothing wrong with the plane's design without the MCAS.

That is a common IMHO misconception. Since the new version with "fixed" MCAS requires additional training, it does not follow that MCAS exists simply to avoid retraining and non certifying it the same. In that case, Boeing should have simply dropped MCAS and did the certification and training. Instead they opted for the near financial disaster and disruption to their supply chain AND failure to avoid additional training.

I suspect MCAS is covering for a very bad corner case in the flight dynamics.

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Your suspicion is correct.

FAA rules require on all planes that the control stick takes more and more pressure to pull back as the plane approaches stall.

The 737 MAX doesn't meet this requirement, apparently it gets easier to pull the stick back as it approaches stall due to engine nacelles. This is what MCAS fixes, as the plane approaches stall, it adjusts trim to provide that extra back-pressure on the stick.

No amount of additional pilot training will allow the plane to fly without MCAS, because this is a hard rule and it's not certifiable without it.

The idea this deficiency can be masked by software is perverse. These parts of the regulations need to be true without electrical power. They're supposed to be a function of aerodynamics.

This is only superseded in absurdity by the fact there are two switches to disable the stabilizer trim motors, thereby making the airplane no longer airworthy while in flight.

> These parts of the regulations need to be true without electrical power.

How would you make that happen in a fly-by-wire airplane? In fact, how would you implement it at all in a digital fly-by-wire system without software?

If you feel that using software to control an airplane is perverse, you might want to avoid flying.

The 737 is not fly by wire.

Airbus fly-by-wire have an alternate law mode where yoke input has matching control surface deflection, just like cable or hydraulic planes. I did not say no software anywhere ever. I said using it to mask aerodynamic deficiency is wrong.

These are transport category aircraft. Not fighter jets. Positive static and dynamic stability is required. All of this is reviewable. Is the software?

Thanks! That's the first I've heard what the actual problem was. Reducing pressure instead of increasing would make the controls inherently unstable.

It's only right at the edge a stall condition, under most normal situations the controls are stable.

> I suspect MCAS is covering for a very bad corner case in the flight dynamics.

yeah, much larger, stronger, and more forward positioned engines so the thrust leads to pitch up stalls in worst case scenarios.

> I suspect MCAS is covering for a very bad corner case in the flight dynamics.

I cannot judge the severity, but it covers for rule 14CFR §25.203(a) "Stall characteristics", see http://www.b737.org.uk/mcas.htm#background

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