Based on all the posts so far, it sounds like generally, all failures except for the first one tend to be brought about by a human error. I am led to the conclusion that failures in a cascade are potentially highly correlated, and what makes them correlated is the effect they have on the diver, by causing stress, and increasing the risk of an inadequate reaction. The more the diver has experienced complex scenarios, however, the better the diver handles stress, the less their performance is impaired, and the lower the effect one failure has on the other. The purpose of drilling cascading failure scenarios in training is to expose the diver to these sorts of complex situations. I suppose one could say that the degree, to which the failures are correlated decreases with experience and the number of complex scenarios practiced, and that's what makes the cascade increasingly unlikely. Makes sense.
(bold above added by me)
It's not so much that it causes stress. It's that it increases task loading. Think of the tasks you have to do as a diver as a chain. When something happens, potentially urgent tasks get inserted into the chain and start to take priority. That means that tasks further down the chain may not get the attention they need, or any attention at all.
Example. Suppose you get a free flow. It becomes urgent to have to shut down that regulator. Other tasks that you were managing fine up to that point, like keeping in trim and keeping track of your buddy, get overruled by this task. Many people in this situation become very focused on the most urgent task and fail to give much if any attention to other tasks at that moment. What can happen then is that the urgent task is taking up all of the attention and less urgent tasks start to "bunch up" in the task chain. So going back to our example, you may succeed in shutting down the post but because you have delayed giving attention to less urgent tasks your depth may have changed and you may have lost track of your buddy. These in turn become urgent issues of their own which can lead to delaying other tasks which can start to snowball.
So when we talk about a "cascade failure" what we're talking about is the human element. The inability to keep our task chain from "bunching up". Eventually this will lead to stress but stress is the symptom, not usually the cause.
The big question here is how to back out of that situation. I feel that training is an important element in this because, for example, if it takes 10 seconds to shut down a post you have delayed other tasks for less time than if it took you 20 seconds to shut down the post. So training and practice are important.
Secondly, in my opinion, you need to be aware of this "task chain" thing as a diver so you can understand what is happening in the situation. Confusion causes delays. Understanding what is happening will help you get things back on the rails faster.
Third, I think it's really important to have a couple of "rules of thumb" that help you organize your thinking. What I teach my Open Water students is what I call my ABC rule. A for "air", B for "buoyancy" and C for "communication". When your task chain is bunching up then you run through the ABC in your mind. Does everyone have an air source? Yes? ok, is everyone in control of Buoyancy and not acending/decending unaware? .... Yes? ok.... now we communicate. We make a plan and we execute it.
This allows the diver to organise thinking and get the most urgent tasks to the top of the list in the right order. By the time we get to "B" we are getting back to diving in a normal diving mode (perhaps with a little Macgyvering to get us that point) and everything else after that becomes easier.
Finally, what I tell my Open Water students is that multitasking is done like a computer does it. A computer never really does anything simultaneously, it switches between tasks (very quickly) and does one thing at a time. So I tell them to go through ABC but not to get tunnel vision on one particular task. It's completely feasible during the hand-off of the octopus, for example, (the A from ABC) to switch your attention for a short time to your surroundings (Task B from ABC) and then back again. We practice this in the pool with task loading exercises so they can get a sense of managing multiple tasks by consciously switching attention from one task to another and then back again. The ability to do this is innate in some people but in my experience most benefit from being trained to do this.
R..
P.S. If you think ABC doesn't cut it as a technical diver then you're wrong. Substitute Gas for Air and you're there. Last December I made a dive where a diver had a problem with his drysuit and made an uncontrolled ascent through a number of stops before he got it sorted out. He started descending again (again uncontrolled) but still had 50% in his mouth. We "caught" him at about 15m and held him up until he switched back to back gas (the A -or G- from ABC), at which point we let him go so he could get his buoyancy back in control (the B from ABC). After he was back to diving we communicated about what to do to adjust the deco plan for the unplanned ascent (the C from ABC). To me this was a textbook example of how to apply ABC even in a technical context.
R..
