Flightdeck door is generally always open on the ground because the dispatcher and cabin crew have to have access.
The strongest part of the aircraft is over the wings that is where the main spar is.
If you hit something at 600 mph you are unlikely to survice regardless of where you sit or adopt the brace postion. Brace postion is only of any use if you hit something at a shallow angle at low speeds or if you go off the runway and hit the fence at the end.
The training of pilots and standards of safety in this country are amongst the highest if not the highest in the world, so it's extremely unlikely you will have a problem.
Here is a statistic I read once in a aviation article. If you were to board a jet plane every day it would take around 26000 years before you would be involved in a accident and even then the odds are you would probably surive it.
Anyway this is the last post from me for a few days, I have to drive to Gatwick now to do some work for a couple of days. Sunny Amsterdam, Copenhagen, Edinburgh and Asturias here I come! (well for 35 minutes that I'll be on the ground for)
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