On Thu, 30 Nov 2006 01:00:34 -0500, in a place far, far away, Jim
Kingdon made the phosphor on my monitor glow in
such a way as to indicate that:

So over the last year or two there have been a lot of dicsussion of
ECO (engine cutoff) sensors, which figure out whether the shuttle's
external tank has run dry.

The shuttle was originally designed so it could launch with 3 good
sensors (out of 4). This was upgraded to 4 of 4 due to issues with
the electronics upstream from the sensors.

That may have been the excuse, but the real reason was that they just
went through and (over)tightened all the ground rules after Columbia,
to demonstrate how "serious" they were about flight safety.

Then the electronics were
redesigned to solve said issues. But the flight rule remained at 4 of
4, but with a certain amount of waffling about waivers and such. I've
often wondered why they didn't just go back to the original 3 of 4
flight rule. Apparently they now have:

Shuttle Program Manager Wayne Hale said senior managers today agreed
to press ahead with a launch attempt even if one of four engine cutoff
- ECO - sensors in the ship's external fuel tank fails during the
final hours of the countdown.
http://www.spaceflightnow.com/shuttle/sts116/061129frr/

Yes, they were saying after the last flight that they were finally
going to change that dumb rule. There was no reason they couldn't
have launched on the Friday before.