Re: @Ledswinger
There was actually a fire a couple of years ago (fortunately on the ground) in the water-activated emergency beacon on the roof of a plane at Stansted or Gatwick. So even these need to be shut down sometimes.
I don't know what the failure rates on components on modern aircraft are, but I wouldn't be surprised if every 2 or 3 flights globally take off with some system powered down until they can get the plane to overnight maintenance. I know that all the manufacturers provide a list of components that are either non-essential, or have so many back-ups, that you can fly with them not working.
Biting the hand that feeds IT
