3 ms·

I agree with the concern for cascading failures but I'm not sure I agree with the comment re: Air France. There was no error cascade there as far as I can see.

by 51Cards 14y ago

I agree with the concern for cascading failures but I'm not sure I agree with the comment re: Air France. There was no error cascade there as far as I can see. The pitot tubes froze and the when the aircraft lost airspeed data it did what it was designed to do, turned off auto-pilot and gave control back to the pilot. The pilots were the ones who failed to understand and manage the situation from there on, relying on a computer that was unable to provide data. Perhaps (said with a little tongue in cheek) it wouldn't be a bad thing if the computers flashed up a big "I don't know!" on the screens so the humans would know to look elsewhere for their information. Just a thought. Then again the humans need to know how to get the data in other ways too. As my auto racing instructor used to say, "It was a failure of the squishy bit between the steering wheel and the seat".

epochwolf 14y ago
There is a big "I don't know" warning. It's called alternate law. And there is another mode called direct law for when the computer really doesn't know.
barrkel 14y ago
There were also inconsistent stall warnings. If the pitot tubes freeze over, I expect windspeed will be underreported, which will make it look like a stall. IIRC, the pitot tubes were only out of commission for a minute or two, but the co-pilots ignored the (now correct) stall warning all the way into the ocean (until the pilot woke up and came back, but then it was too late). I guess the real problem here is having only a single modality for detecting windspeed. It doesn't matter how many pitot tubes you have if they all have the same failure mode; quantitative redundancy isn't as good as qualitative.
genwin 14y ago
Do you know why the plane doesn't use GPS as a backup method for airspeed measuring?
- whiskers 14y ago
  Because airspeed is relative to the speed of the air around you (not relative to your position on the earth). The pitot tubes (which caused the initial problem by icing over) are there to determine exactly that.
  - genwin 14y ago
    Doh! Finally, it makes sense.

by 51Cards 14y ago

epochwolf 14y ago

There is a big "I don't know" warning. It's called alternate law. And there is another mode called direct law for when the computer really doesn't know.

barrkel 14y ago

There were also inconsistent stall warnings. If the pitot tubes freeze over, I expect windspeed will be underreported, which will make it look like a stall. IIRC, the pitot tubes were only out of commission for a minute or two, but the co-pilots ignored the (now correct) stall warning all the way into the ocean (until the pilot woke up and came back, but then it was too late). I guess the real problem here is having only a single modality for detecting windspeed. It doesn't matter how many pitot tubes you have if they all have the same failure mode; quantitative redundancy isn't as good as qualitative.

genwin 14y ago

Do you know why the plane doesn't use GPS as a backup method for airspeed measuring?

whiskers 14y ago
Because airspeed is relative to the speed of the air around you (not relative to your position on the earth). The pitot tubes (which caused the initial problem by icing over) are there to determine exactly that.
- genwin 14y ago
  Doh! Finally, it makes sense.