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Comment by linuxftw | Hacker News Reader

> the MCAS system itself failed

The MCAS system didn't fail. It operated exactly as it was designed, it received faulty sensor data which in turn flew the plane into the ground.

> Bugs in software happen because situations where they arise are sometimes hard to predict.

Everything that cause MCAS to drive the plane into the ground was 100% predictable and testable.

There are two basic problems. The first, is that the people that thought MCAS was a good idea in the first place are incompetent. The second problem is the people that designed and implemented the MCAS system were even more incompetent.

Idiots managing idiots, and if I was the CEO of that company, that system would have never been allowed in the first place.

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> The MCAS system didn't fail. It operated exactly as it was designed, it received faulty sensor data which in turn flew the plane into the ground.

You're technically right, but I'd say that incorrect sensor data is something that should be taken into account. And failing to take that into account is a failure on part of the MCAS.

It did take it into account, but the incorrect angle reporting was not severe enough to result in the sensor being marked as faulty. From the sound of things it was designed to deal with large bit flips, sensor jamming the the home position or sensor misalignment (which is at fixed angles because of the mounting holes) rather than the sensor jamming in other positions or being horribly miscalibrated.

The company that repaired the sensor used in the Lion Air Flight did loose their license to repair any and all certified avionics parts and is probably out of business at this point. Donno about the technician that forged the post-install tests of that sensor.

The architecture of the system was such that no cross-checking could occur. There was a feature to implement an AoA disagree light, but that only came into play if an optional package was purchased.

The Lion Air crash data had the computer registering an angle off by a ridiculous degree that should not have even been attainable by flying one of these planes in the manner for which it was designed.

The Ethiopian FDR showed even more ludicrous measurements as I recall. It wasn't just a case of "It wasn't bad enough", it was a case of the computer believing the plane was belly flopping into the air stream (75 degrees AoA!).

Note the Space Shuttle only hits around 40 degrees on it's reentry in order to keep the wind hitting the thermally insulated bottom, and that only at Mach 10+.

There is no excuse for any software engineer/computer scientist to ignore the basic physics of the domain they are coding for to the point they cannot identify when a critical measurement is so far out of whack as to be instantly dismissed.

FDR graph: https://visualapproach.io/wp-content/uploads/2019/04/Data_Sh...

> I'd say that incorrect sensor data is something that should be taken into account

I think that is the GP's point: MCAS operated exactly as it was designed, and it killed people because it was designed incompetently, not taking into account things that obviously should have been taken into account.

Is the fact that MCAS only relied on data from a single AOA sensor (when two existed) not a defect?

Multi-sensor inputs are not required for systems whose failure impact isn't rated as catastrophic, which Boeing specifically avoided in their safety analysis, ostensibly due to the fact they knew the FAA would require sim training for a multi-sensor system. So if you accept Boeing's original classification it is not a defect.

Given that upon further testing after these disasters, the results required them to reclassify MCAS and the Flight computer it runs on as a single, potentially catastrophic point of failure, it is now a defect.

Just wanted to point out that regulationwise, that situation evolved over time due to withholding or material omission of critical information with regards to the nature of the system to regulators.

Whether or not a jury will agree is yet to be seen.

> So if you accept Boeing's original classification it is not a defect.

Then it's a design defect. The FAA even seems to think so. It seems like hair splitting.

I think saying the system didn't fail relies an overly tight notion of the MCAS system boundary.

> The MCAS system didn't fail. It operated exactly as it was designed, it received faulty sensor data which in turn flew the plane into the ground.

"It worked on my airplane"

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