Lemurian,
I've done this numerous times in this thread, but I'll be glad to do it again:
So what do I think are contributing factors to this accident:
a) reduced sensory feedback channels in cockpit
neither the throttles nor the spoilers lever actually move to feed back current status. All status on thrust and spoilers has to be obtained from the displays, leaving us with just optical input which requires quite a few brain cycles to process. Look at how much brain tissue is related to the different sensory channels, and you'll get an idea how quickly information from those channels is available to us (the more brain, the more processing is needed, the longer it takes).
This is no problem at any time in normal ops and probably not even in most emergency situations, but as has been said earlier by the gentleman who had the bad fortune to have to eject and the good fortune to survive, your brain goes into weird modes. (btw, that's part of my grief with some of the postings here, of course everybody, pilot or just enthusiast, can look at the books in bright daylight and will immediately know what needed to be done to save those souls, and I'm 100% sure if the pilots of that flight were among us, they would know just as well. It's the way our brain processes information that sometimes gets us into trouble.)
b) a logical "dead end street"
The ground spoilers/autobrakes logic is very stringent in what it requires to be true before it will itself allow the retardation devices to do their job.
I don't have the logic diagrams in front of me, but from memory, it's ((wheel spin up) AND (one or both squat switch depressed) AND ((both thrust levers at or near idle) OR ((one thrust lever at or near idle) AND (one thrust lever in reverse)) OR (both thrust levers in reverse))
Oh, and of course AND GS armed but let's take that for a given.
Heck, most laymen would require a minute or two to understand those conditions in plain English!
Back in Warsaw, it was concluded that one of the reasons for the accident was the fact that one of the squat switches was merely millimeters away from it's switching point, but it was still signalling "FALSE" that is, not on the ground. The whole equation was therefore FALSE. Nothing the pilots could have done.
Now this time: After the original omission, the TR left in CLB, was a fact, do you think they'd slap on their foreheads and go "oh buddy, look we've forgotten the thrust lever up there, let's take it to IDLE!"? No!
They have not merely forgotten about it, it was pushed off their list! Now, on the runway, no Spoilers, what did they probably have on their mind? What's the chances the TL they had forgotten about in a stressful situation just seconds ago was on top of their minds now? Speeding down a short, slippery runway with no means of braking? I bet the stress they were in now made what they experienced just a few seconds ago look like an afternoon walk.
I'm sure they had their mind set on exactly two things: brakes and spoilers. Who knows in what order.
Now, this is where I think they've been let down by the system: you can hit the brakes, and as PBL explained, they did so with tolerable timing (intolerable by the runway, but tolerable by human standards). They moved the spoilers lever. Do you want to try to guess why? Because it's the only damn thing directly relating to the spoilers! Tunnel vision! No room to think about the logic equation from up there! No time to rethink your approach and discover your omission.
Yet the logic (and not the computer!) had no way out for them but to answer the 200 souls question: what precious lever have you missed, gentlemen? The clock is ticking! I want your answer!
Wasn't there a chance that the plane might have been hydroplaning? That the main wheels have not spun up? We know why the spoilers didn't come up, they did not! Who knows what they thought the reasons might have been.
Now, I don't want to hear how this or other things could have happened on a Boeing, an MD, an Embraer or a Canadair. They are different Airplanes. They probably have their own design flaws. Heck, the fact that the Lauda Air 767 crashed years ago clearly had a good Boeing contribution. This however is about this Airbus. This is about a system that only takes one answer. And about brains that get into modes where they just can't come up with that one answer.
So it's back to my three requirements to any automation system:
- prevent error
- forgive error
- fail gracefully
pj
ps: sorry that this might be a little less "dry" in tone than what might be considered adequate for this forum.