> the software workarounds don't behave like a hardware-engineered mitigation
You probably should never get on an Airbus, because they won't fly without computers.
And, yes, I'm aware of the two philosophical camps regarding ultimate authority in command (pilot vs. software). That doesn't negate my point. Software, in either case, shouldn't be a workaround solely because it's easier to implement than a hardware change. Using it as an engineered mitigation is ok, but you have to actually implement the mitigations properly. For example, the hazard analysis listed MCAS as "critical". With that classification, it required redundant sensors, yet Boeing didn't make that the default and opted instead to sell it as an option. (Never mind the fact that the classification should have been higher, they didn't even follow through with their own processes based on the mis-classification).
Mechanical systems aren't inherently better or more resistant to hidden flaws. Remember it was a failure of the mechanical AOA sensor that initiated the MCAS failure.
> Software, in either case, shouldn't be a workaround solely because it's easier to implement than a hardware change.
Software runs our world now. Mechanical computers on airplanes were around for decades before software, and they were hardly free of fault and problems.
Besides the fact that failure modes of mechanical systems are generally more understood than software, you're again conflating my point and having an entirely different conversation. This isn't about some "mechanical vs. software" dichotomy. I literally said there is nothing about software that makes it inherently dangerous and that using software as an engineered mitigation is fine if the mitigations are implemented properly.
What isn't fine is all the process and design gaps that occurred. Things like using software as a mitigation, not because it was the best alternative, but simply because it was cheaper/faster. Things like not following your own hazard analysis when it comes to mitigation. Or not characterizing the risk accurately or the failure modes because you didn't understand the system interactions.
>Remember it was a failure of the mechanical AOA sensor that initiated the MCAS failure.
This is exactly why, had they followed their own procedures and hazard analysis, redundant sensors would have been a default. A "critical" item (like MCAS in the hazard analysis) is supposed to get redundant input as a default.
My post isn't about mechanical vs. software. It's about the the allure of using software as a workaround that leads to process gaps and bad design philosophy. Like removing sensors because of a supplier/cost issue and assuming "we'll fix it with software" instead of doing the hard work to understand, characterize, and mitigate the risk effectively.
There were more than one AOA sensors. Not hooking the other one up to the MCAS system could hardly be a cost issue. Nor is it an issue of software vs hardware. It being implemented in software had nothing to do with MCAS's problems. The software was not buggy, nor was it a workaround. What was wrong was the specification of how the software should work.
You can note that I acknowledged there were multiple AOA sensors in other replies. Further, they were already “hooked up” to MCAS, but Boeing made this safety critical redundancy requirement an option within the software. That’s bad practice, full stop.
I still maintain that the software was 100% a mitigation to a hardware change and I think there’s plenty of other evidence supporting that. E.g., has they not updated their engines, would MCAS be installed? If the answer is no, then it was a mitigation to a risk introduced by a hardware change.
I’ll say it one more time just to be clear: I’m not saying the concept was bad. I’m saying their design philosophy and implementation was bad. They could have used software mitigation within the right process/philosophical framework just fine. What doesn’t work is using software as an “easy” risk mitigation strategy when you don’t understand the risk or the processes necessary to fully mitigate that risk. The problem arises because software is a seductive fix because it’s relatively easy and cheap on its surface, but if your design philosophy and processes are equipped to implement it effectively, that “easy” fix is rolling the dice.
No disagreement, there.
> it was a mitigation to a risk introduced by a hardware change
It wasn't really a risk. It was to make it behave the same.
Allow me to explain something. A jet airliner is full of hardware and software adjustments to the flying characteristics. For example, look at the wing. What do you think the flaps and slats are for? They are to completely change the shape of the wing, because a low speed wing is very very different from a high speed wing. There are also systems to prevent asymmetric flaps, as that would tear the wings off.
The very existence of the stab trim is to adjust the flying characteristics. The stab trim has an automatic travel limiter to constrain the travel as the speed increases because, you guessed it, full travel at high speed will rip the tail off.
The control columns are connected to a "feel computer" which pushes back on the stick to make the airplane feel in a consistent way from low speed to high speed. This feel computer can be mechanical or software. Pilots fly by the force feedback on the stick, not the travel of it. The idea is to make the airplane "feel" like a completely different airplane. Without the feel computer, they'd promptly rip the airplane apart.
There are plenty more of these. The MCAS concept is no different in any substantive way.
Your thesis that using software to run it as some unusual risk is simply dead wrong. What was wrong with the MCAS system was:
1. reliance on only one sensor
2. too much travel authority
3. it should have shut itself off if the pilot countermanded it
What was also wrong was:
a. Pilots did not use the stab trim cutoff switch like they were trained to
b. The EA pilots did not follow the Emergency Airworthiness Directive sent to them which described the two step process to counter MCAS runaway
There weren't any software bugs in MCAS. The software was implemented according to the specification. The specification for it was wrong, in points 1..3 above.
P.S. Mechanical/hydraulic computers have their own problems. Component wear, dirt, water getting in it and freezing, jamming, poor maintenance, temperature effects on their behavior, vibration affecting it, leaks, etc. Software does not have those problems. The rudder PCU valve on the 737 had a very weird hardover problem that took years to figure out. It turned out to be caused by thermal shock.
>It wasn't really a risk. It was to make it behave the same.
Hard disagree here. The fact that it did not behave the same demonstrates there is a real risk. That risk could have been mitigated in various ways (e.g., engineering via hardware or software, administrative via training etc.) but they did not. You downplaying the risk as not credible is the same mistake.
>Your thesis that using software to run it as some unusual risk is simply dead wrong.
My thesis is that they didn't know how to effectively characterize the software risk because, as you point out, software risks are different than the risks of mechanical failure. Software doesn't wear-out or display time-variant hazard rates like mechanical systems. Rather, it incurs "interaction failures." The prevalence of these failures tends to grow exponentially as the number of systems that software touches increases. It's a network effect of using software to control and coordinate more and more processes. Which is why we need to shift our thinking away from the mechanical reliability paradigm when dealing with software risk. Nancy Leveson has some very accessible write-ups on this idea.
So what does that mean in the context of Boeing and, in a broader sense, Tesla? Boeing did not capture these interaction risks because they had an overly simplified idea of the risk and mitigations. They did not capture the total system interactions because they were myopically focused on the software/controls interface. They did not capture the software/sensor interface risk (even though their HA identified that risk). They did not capture the software/human interface risk. They thought it was a "simple fix". Tesla, likewise, is trying to mitigate one risk (supplier/cost risk) with software. FTA seems to implicate them in not appropriately characterizing the new risks they introduced. I'm saying that is a result of a faulty design philosophy that downplays (or is ignorant of) those risks.
The 757 has a stellar safety record.
MCAS did not make the airframe operate the same in practice, especially from a human factors perspective. It confused pilots about why it was acting the way it was and that was a major factor in the accidents.
MCAS had a built in delay; it wasn't a continuous command. It would push the nose down and periodically disengage and allow pilots to bring the nose back up. This type of intermittent feedback is difficult to resolve in real-time, especially for an untrained pilot under stress.
But to the larger point, what you're relating is actually bolstering my point. The design philosophy was poor; they didn't fully understand the interaction effects of the system (to include software, hardware, people, and the environment). In that simplified mental model, they thought software was an easy fix to their problem and they didn't follow through with the necessary risk mitigation. This includes having a redundant AOA sensor feed into MCAS (which their hazard analysis already required), characterizing MCAS properly as having the potential for causing a 'catastrophic' mishap, training for their pilots (which they didn't think was necessary because it was the 'same' airframe, despite different handling characteristics), and an appropriate understanding the human factors that govern its use.
If we erroneously simplify our mental model and claim it's "just software" and an "easy fix," we miss all of that.