Either they kicked the side of the external tank and ECO sensors 3 and 4 started to work, or they admended the flight rule to allow for a launch of the shuttle with only some ECO sensors working and not all 4.
Reading this clears up some issues:
It is not yet clear whether the problem seen today, when hydrogen ECO sensor No. 3 "failed wet," involved the sensor, its wiring or an avionics box aboard the shuttle that reformats and routes the data to flight computers. But the timing of the failure indicates it more likely involves the sensor itself and not electronics aboard the shuttle.
How would they know for sure that it is the sensor itself? After all one of the possible causes identified was interference from newly installed heaters. I've been asking for a while whether this issue existed before the installation, thinking that would be the first place to start when trying to eliminate a possible cause of the ECO sensor failure.
This awnsers the question of whether the issue existed previously:
NASA's original launch commit criteria required three operational ECO sensors for a countdown to proceed. But in the wake of the 1986 Challenger disaster, the LCC was amended to four-of-four because of concerns two sensors could be knocked out by a single failure in an upstream electronic black box known as a multiplexer-demultiplexer. The single-point failure later was corrected, but the four-of-four launch rule remained on the books.
That launch rule wouldn't have existed unless this had previously been a problem I would think. That effectively rules out the new heaters as being a cause of the problem since they are only a new addition, and this issue outdates them.
The article goes on to explain that 4 of 4 rule was later admended to allow for an exeption. If the sensor "failed wet" (ie showed fuel in the ET when there was none), and they could prove the issue was not related to the multiplexer-demultiplexer, they were a go.
This rule makes complete sense. But at the same time it seems like a cop-out. We are dealing with people lives here. You would think that in between launches they would want to investigate this issue further. Just why are there sensor failures on the shuttle? Just what is different between ECO sensors 3 and 4 then 1 and 2 that never fail? All ECO sensors seem to be designed exactly the same, so why the location specific nature of the problem?
To me it still indicates that the ECO sensor failure cause must be tied to something inherent in the the shuttle. If the ECO sensors are all identical, it makes only sense that something on the shuttle, or shuttle assembly is interfering with normal operation at those zones. It could also be a software glitch, but I'm assuming they would have looked at that potential cause first.
And you would figure that NASA would be particularly interested in resolving this issue since it has caused the delay of so many launches since 1986.
It's really suprising to see this from an organization like NASA really when you think about it. What are they making? Cars? If your tailight fails on your car, who notices the shotty engineering? On the Shuttle, unresolved faults are a whole other ball game.
The Trouble With Sensors VII
The Trouble With Sensors VI
The Trouble With Sensors V
The Trouble With Sensors IV
The Trouble With Sensors III
The Trouble With Sensors II
The Trouble With Sensors I