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Comment by devit | Hacker News Reader

How expensive are these sensors compared to the cost of the plane?

Any reason to not just use at least 9 of them, pick the median value and forget about it?

Yes there is. If you put too many sensors then there is always one in need of repair. And then it becomes dangerous: the pilot and maintainer ignores the alarm, delay replacement, leading slowly to a failure.

This is why the redundancy choices and availability calculations should always be made pondering the maintenance burden.

Well, I've been working in mass mobility and they had 4x redundancy with different sensor types. The more sensors went bad, the more restricted the operation regime became as it went through degraded modes. And the on-board systems were of course aware of the status of the sensors.

I think that created the right incentives: you could ignore a faulty sensor or two, but the results were increased travel times, so costs to the operator.

I always wonder why we don't have massive arrays of micro-sensors on planes spread out over the plane. If a handful your 65k sensors start to fail, you can delay repair for quite some time - otoh, if sensors start to fail all over the plane, you know you have to wake up the pilot hard, since it's a systemic problem like icing.

Wiring harnesses are a major weight savings. If you wired them all in serial, then that wire is a single point of failure. Otherwise you’d have 65k wires going to each one.

No, you put them on a bus like so many other things in modern vehicles today. They don't need their own separate wires or even their own bus. With lots of sensors, you're going to have several independent buses anyway, so a failure on that bus would only make you lose a fraction of your sensors.

It's incredibly unlikely, but not entirely unforseeable that enough sensors would fail at the same time for the same reason. All four engines on the 747 BA9 failed at the same time due to volcanic ash [0]. If you just take the median of nine, it's possible that a mere five failed sensors could break MCAS. And then you are back to the issue of handing over to the pilots.

[0] https://en.m.wikipedia.org/wiki/British_Airways_Flight_9

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