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Comment by programmer_dude | Hacker News Reader

What I don't understand whenever I read anything related to the MCAS system is why does it not check if the plane is in a dangerous nose dive using other sensors the plane might have? Does the average plane not have a gyroscope? My sub 500 dollar phone has both a gyroscope and an inertial measurement unit (accelerometer).

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Pitch (q) is not the same as angle of attack (alpha). You can detect q with a gyro, but not alpha. I use this helpful graphic with my students: https://i.stack.imgur.com/A62jW.pngyQ.jpg

Note that AoA and the angle of the airframe body (relative to the horizon) may not be the same. Even if the airframe is pointing downwards (below the horizon) or really high up doesn't mean the wings have a sufficient AoA to generate lift.

Can't the IMU sense if you are generating lift (or not)?

No, because you can stall a wing at any attitude realtive to the horizon. It isn't about the pitch of the airplane, it's about the relative airflow over the wings.

So if you're falling quickly the airflow comes from "below" which causes a high angle of attack even if the nose is pointing level compared to the horizon.

I'm doubtful. Consider this: the aircraft can experience a downward acceleration (eg. due to turbulences) without being in a stall. Measuring the AoA is still the most reliable way (assuming the sensor itself is reliable) to detect impeding stall.

That's a very interesting thought. How would you do it? You can only sense the acceleration - if it's lower than 1G you're falling. The angular momentum doesn't actually tell you anything in an aircraft.

I wonder if it's possible to estimate the local air speed based on ground speed (GPS) and local weather data (can we measure wind speed at altitude?).

Probably because entering a "dangerous nose dive" is the correct way to avoid an impending stall, which is what MCAS is supposed to do.

Gyroscopes by themselves can not sense the orientation of a body moving through three dimensions.

Why not? I thought this is precisely what gyroscopes did.

Electronic gyroscopes measure rotational speed. In order to measure orientation you need to integrate over time and align it with a reference frame, typically by estimating gravity using the accelerometer. An inertial measurement unit has both of these.

Cheap phones often don't have gyroscopes but still are able to rotate your screen orientation based on the accelerometer.

The spinning-disk ones kind of do, they aim to keep their orientation, but they also need gravity or so for initial alignment/realignment (otherwise things would get funky when you make a trip around the earth, since technically your plane is upside down if it would fly halfway around the earth ..)

What is that[0] then?

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

Attitude != Angle of Attack.

Orientation = attitude ≠ AoA

Good clarification, but there’s no rule saying that the gyroscope has to be used all by itself.

False. A gyroscope can be used to determine the orientation of a body.

Because MCAS never assumes the input sensors could be faulty.

Sounds like a shitty and incompetent design. Grounding the plane was the right decision.

Totally agree.

I agree, but I see the reasoning. MCAS failure leads to runaway trim for which there is an established and well known procedure to fix--in the checklist almost 50 years. For a problem with trim you flip the trim cutout switch. Doesn't matter if the problem stems from MCAS, autopilot, crossed wires, the magentic pole flipping, jammed electric motor, whatever.

It's like explaining to a computer user that rebooting will fix aproblem--they don't need to know the exact cause but just how to recover from it.

It's worse than this. MCAS is designed to only look at one of the two AoA sensors. If this happens to be faulty, it doesn't know, because it's not comparing the sensor to the other one. Instead of a system which can notice and account for disagreeing sensors and fail gracefully, we have a system which is a designed single point of failure and fails... lethally.

You can't handle faults with only two sensors since you won't know which one to trust. At least three would be needed for a minimal level of redundancy.

You can handle faults by disabling the MCAS system which is a lot better than the current solution of diving the nose into the ground.

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