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Comment by bdonlan | Hacker News Reader

All flight controls on the A330 are fly-by-wire - they're connected via computers. The most critical surfaces have secondary, dumb controllers that can take over if the primary flight computers fail, but less critical systems may not be connected to those backup systems - it's expected that a triple fault of the primary flight computers should be very rare, and manual braking is available as a backup after all.

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Right. In this case we have a completely-unprecedented triple failure of the primary flight control system causing loss of auto-brake, spoilers and thrust reversers; a wet runway with less than expected braking action; a tailwind landing and still a happy outcome.

Honestly defense-in-depth seems to be working OK here.

And note that for all aircraft the landing distance is calculated _without_ thrust reversers or spoilers, and thus is based on braking performance alone.

Some of the challenge here was the seconds spent at near landing speed without any braking being performed, as that'll eat up runway distance fast, following the auto brake failure prior to manual braking commencing.

It's also worth stating (for the OP) that thrust reversers are potentially really dangerous -- if they deploy in flight without being commanded to, the aircraft can -- or, more likely if the engine computer does not detect it and shut itself down, will become barely controllable, with previous fatal outcomes. [1]

Two or more FCPCs failing is sufficient evidence that a "faecal fan incident may be occurring" that the risk of deploying them is just not worth it, especially as (pointed out by this parent above) they are effectively optional equipment and the FAA requires you to expect them to be inop to be safe.

What they do save is fuel and time -- taxi time to the terminal, permit the use of high-speed runway turnoffs, etc.

[1] https://en.wikipedia.org/wiki/Lauda_Air_Flight_004

Yes, it worked because there as a direct controller to fall back to. But worryingly because two things, it wasn’t a triple failure of the redundant flight computers, is seems to point to a singular failure of the monitoring logic.

The other thing is that the report notes that the aircraft came perilously close to a disaster because it calls out other specific factors could have easily eaten up runway margin leading to a disaster.

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