1.   If the valve has jammed shut, causing the feed-and-expansion tank to run dry, again turn off the water supply.

2.   Boeing disputed the finding, saying that a valve could only jam under extreme lab conditions that would not be seen during every day commercial flights.

3.   In its report, the NTSB said valves could jam and then reverse, causing the rudder to move in the opposite direction commanded by the pilot.

4.   The safety board said the valve could jam and then reverse, causing the rudder to move in the opposite direction commanded by the pilot.