The hardest thing at Halton, I found, was the way they **** with your head. So really the entire course
If I had to put it down to a single moment, it would be CBRN week. The group I was put in to go for the "initial gassing" was interesting... somehow a mix of characters that together made the tension quite large and we all really just wanted to get it over and done with. Sat outside waiting to go in to the "respirator testing facility" (gas chamber) for about 2 and a half hours on cold, wet muddy woodland floor was bad, but then being told "Right it's too late to fit you in today, it'll have to be tomorrow" was just awful. Also I had a major panic later in the week after I failed my first of two attempts at the test for the "initial action" drill. This is where you hold your breath, close your eyes, turn around, helmet off and between your legs, mask out of pouch and on your face, with no twists in the rubber harness that keeps it on your head, blowing out and shouting GAS GAS GAS within 9 seconds. It's not actually that hard, but knowing that you've messed it up once already and you only have ONE chance to get it right again is pretty ****ing horrible in my experience. I practiced it over and over and over and over and over that night, made the backs of my ears bleed from pulling the mask off. I remember feeling actually sick about it, after totally nailing it about 20 times in a row, then epicly failing and not being able to do it again. It's things like this that make you think "So... no matter how well I CAN do it a hundred times in a row, if I make this same sort of mistake tomorrow randomly when I happen to be doing the test, I'm ****ed! ARRGGGH!" Seeing other people that were previously better than me at the drill taking the test before me and failing their second attempt (meaning they were reflighted) made things even worse, the whole "oh my god, look at their faces :| that's going to be me
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" kicked in majorly.
It might sound a bit daft that I got all panicky over that, but that's what the place does to you. Just general annoyance and total lack of time to complete tasks constantly. I've heard it's changed a bit now and the kit prep stuff isn't so bad, but I expect they've replaced it with some bull****.
As for Cosford, it depends what you've already got experience with. Some people had lots of experience with hands on stuff and soldering so flew through that part of the course, and had major trouble with the Cisco networking stuff. Others had lots of experience with computers/networks and so flew through that part of it, but struggled with the soldering stuff. I didn't find any of the course particularly tough if I'm honest - but then again I got the highest average score so that makes sense! /shameless trumpet blowing.