daedelus,
I work in the Tower at Manchester, and I agree that in fog, we are "sometimes" in clear blue sky
However, imagine you driving a car at 240kph on a perfectly straight road.....then somebody suddenly puts a hood over your head, and says that you HAVE to slow down to 50kph inside 2.5km AND turn 30degrees left...then slow to 20kph to go to your stand.
Now, sit on the end of the runway. You have still got the hood over your head.....advance the throttles..you are moving...you get to [approx] 220kph and pull back on the stick...10 seconds later you are in clear blue sky... not a problem is it...but an engine has just failed... Put the hood back on....
Simplistc, YES but it is an "approach" [sorry] to the problem
watp,iktch