[azernik]

Sensors break. They get gunk on them. They ice over.

Generally a sensor with such a critical failure mode would be triple-redundant - if one fails, the discrepancy between sensors is flagged and the aircraft runs on the other two until the broken one is fixed. In this case, the aircraft had two sensors of the type (Angle-of-Attack), but MCAS was only listening to one of them.

[pas]

How does this AoA sensor works? I'm guessing it has to sense the direction of the wind and then calculate the vector of the wings compared to that?

I've seen pictures of it ( https://aviation.stackexchange.com/questions/2317/how-does-a... ) and seems to be a simple mechanical component.

Why isn't this compounded with something that works by estimating the AoA from other factors? A few simple gravity based sensors would be able to tell the vector of the plane, and simply assuming that wind (airflow) is parallel to the ground would go a long way. Or is the vertical component of the local airflow so variable?

[inferiorhuman]

Why isn't this compounded with something that works by estimating the AoA from other factors?

Cost, probably. The 777 and 787 still only use two alpha vanes, but they calculate a synthetic angle-of-attack value as well.

[azernik]

When in flight, accelerometers only tell you what forces are being exerted by the wings and the engines, not which way is down. Consider, for example, the way the drinks in a cup don't spill when a plane turns.

[pas]

I know that an airplane can make a roll that keeps the fluid in the cup, but when usual commercial planes fly, they don't do stunts, so usually there's no centrifugal force to mimic gravity. When a big jet points its nose down or up people and fluids feel it pretty much as if they were on the ground on a slanted flat surface as one of its edge is being raised or lowered.

Sure, this would probably worth next to nothing in turbulence, but in a simple take off and landing (where MCAS is already active and depends on AoA) it might help.

And of course I might be completely ignorant of most of the relevant problems with using any kind of gyroscopic or acceleration based sensor.

[mannykannot]

I think general relativity rules out the possibility of making a gravity detector that can distinguish it from acceleration.

[tropo]

No, but cost probably does. There have been a number of satellites flown (GOCE, GRACE, SLATS) with the sort of equipment you'd need. With that equipment, simply measure the strength of gravity in multiple parts of the plane. This gets you altitude, just as GPS or air pressure would, and then you can determine the angle of the plane.

An edit as response to the followup mentioning Einstein, due to HN throttle:

Yes, yes... and it doesn't matter for this purpose, because you can measure gravity at multiple points within the aircraft and because gravity falls off with distance.

https://en.wikipedia.org/wiki/Inverse-square_law#Gravitation

We have built equipment sensitive enough to measure this difference and we have flown it in satellites.

[mannykannot]

"Einstein’s ground-breaking realization (which he called “the happiest thought of my life”) was that gravity is in reality not a force at all, but is indistinguishable from, and in fact the same thing as, acceleration, an idea he called the “principle of equivalence”.

https://www.physicsoftheuniverse.com/topics_relativity_gravi...

[gtr32x]

Thanks for the elaboration. Could you help me further understand one more thing? When you say MCAS only listens to one, does that mean during the time when one AoA sensor fails? Or it always listens to one during normal operation?

Also, it does seem like Boeing dropped the ball here to not build further redundancy here.

[inferiorhuman]

MCAS alternates between the left and right sensor each time the plane lands. With the Lion Air flight I think cycling the electrical power for diagnostic work caused MCAS to pick up the faulty sensor for two flights in a row.

[dTal]

> MCAS alternates between the left and right sensor each time the plane lands

This sounds like spectacularly bad design that manages to extract negative value from having two sensors. What is the logic behind this?

[cjbprime]

Pilots and first officers tend to switch who has flight control on each leg of their flights (which is the Pilot Flying vs Pilot Assisting), and the MCAS system uses the AoA vane associated with the side of the cockpit that currently has flight control.

There is no good reason for only listening to one sensor.

There is a sort of good reason for having a split between pilot/copilot side: the instruments are redundant (both physically and electrically), so in the event of malfunction you can failover to the other side.

[gtr32x]

That actually sounds awful, sorry for my naivety if this is just industry standard. But for such a mission critical piece to have no redundancy build over it is just poor. Especially that it's prone to failure since it's situated on the outside of the plane.

It just seems to be that this is some terrible engineering done on Boeing's end of not fully understanding the critical situation here.

Generally two failures: 1. a lack of redundancy in a mission critical sensor 2. a blind trust on MCAS's priority over pilots

[inferiorhuman]

a lack of redundancy in a mission critical sensor

There is redundancy in the sensors, but the sensors are not being used in a redundant manner. There are whispers that the 767 fuel tanker (KC-46/KC-767) has a system similar to MCAS that will look at both alpha vanes for disagreement, which is a bit damning to say the least.

a blind trust on MCAS's priority over pilots

The entire purpose of MCAS is to engage only when the pilot is flying to prevent the pilot from doing something dangerous. Previous generations of 737 had the same problem but the MAX is more delicate and compounds it with nacelles that generate lift.

[azernik]

Part of the problem was that MCAS was originally designed with very little control authority, and so wasn't considered safety-critical. However, during testing they realized they needed to up the gain, and made pretty major retuning without reexamining their safety assumptions.

Plus the bug with the resets on the limiter.

[6nf]

I believe the MCAS uses the pilot-side sensor.

[azernik]

Boeing dropped the ball in many, many ways.

[kjar]

The lack of redundancy by default coupled with extreme profit seeking by Boeing for the 'upgrade' is inexcusable. TL;DR They shit out a faulty product hundreds died as a result