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Comment by kds3 | Hacker News Reader

> It’s very unlikely that both would fail simultaneously. If they did, it’s very unlikely that both would provide the same erroneous readings.

They don't have to fail simultaneously in a flight. And they don't have to fail by internal sensor problems. There are many cases in which they can simultaneously fail and give same readings, article even mentioned such types of events:

>> That probability may have underestimated the risk of so-called external events that have damaged sensors in the past, such as collisions with birds, bumps from ramp stairs or mechanics’ stepping on them.

And AF447 gives an example when such erroneous readings combined with pilot errors may lead to.

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If they’re damaged on the ground, surely it’ll be noticed that the sensors are claiming an extreme AoA while just sitting there, and they’ll be fixed.

AF447 is an example of a fly by wire system that has to keep working no matter what happens, thus a bunch of redundant systems and a series of alternate modes the system can fall back on to operate in a degraded state.

MCAS, in contrast, is not a critical system. It could shut down with no problems at all. These crashes happened only because it didn’t shut down when faced with a failed sensor, because it couldn’t detect the failure.

> If they’re damaged on the ground, surely it’ll be noticed that the sensors are claiming an extreme AoA while just sitting there, and they’ll be fixed.

AoA sensors by design don't work reliable in low speed and they don't work at all on the ground.

> AF447 is an example of a fly by wire system that has to keep working no matter what happens, thus a bunch of redundant systems and a series of alternate modes the system can fall back on to operate in a degraded state.

AF447 is a good example that two AoA sensors can simultaneously have same erroneous readings. It's not that unlikely.

Can you elaborate on AF447? I’ve never heard of the AoA sensors being connected with that crash, and a quick search indicates that they were working fine.

I've confused AF447's Pitot tubes with AoA sensors. But I think point is still valid: two sensors _can_ simultaneously have same erroneous readings and we have to be sure pilots can handle such situations.

First: I said it’s very unlikely, not impossible. Second: the failure of AF447’s pitot tubes was detected immediately and the system switched to an alternate law as a result; this contributed to the crash because the pilots were less familiar with how the system operated in that mode. Third: AoA sensors operate by a completely different mechanism so even if this was what happened (it wasn’t) this demonstrates nothing relevant.

You’re just badly arguing details while ignoring the actual point here.

> With two sensors, you can detect failure. It’s very unlikely that both would fail simultaneously. If they did, it’s very unlikely that both would provide the same erroneous readings.

> First: I said it’s very unlikely, not impossible.

> You’re just badly arguing details while ignoring the actual point here.

My point here was that with two AoA sensors you can't reliably detect failure. They can both fail simultaneously and provide the same erroneous readings. And because it's not that unlikely we have to be sure that pilots can handle MCAS problems when two AoA sensors fail.

> Second: the failure of AF447’s pitot tubes was detected immediately

Because A330 have three pitot tubes, right? Not two out of two sensors?

I'm not arguing about AF447 case, I gave it as an example that two sensors _can_ have same erroneous readings. Airbus engineers were not sure that can happen in real life, so stall warning issue was a real surprise for them.

You’re just badly arguing details while ignoring the actual point here.

What was the point? Two sensors (in this case alpha vanes) can fail at the same time and in the same way on an Airbus:

https://avherald.com/h?article=47d74074

AF447 had two working aoa sensors that never disagreed. It didn’t have an aoa indicator in the cockpit though. You can infer what the aoa is by the vertical airspeed and engine output though. The inputs from left and right stick canceled each other and it power on stalled out of the sky landing belly first. This was pilot error because they had unreliable airspeed indication due to pitot tube icing.

Airspeed is a critical system while aoa is not. (Unless aoa is tied to mcas like in the max jets)

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