On March 1, 2002, approximately 1840 mountain standard time, a Horizon Airlines Bombardier DHC-8-401, N410QX, experienced a failure of the number two hydraulic system during en route cruise to Boise, Idaho. The airline transport pilot, his first officer, the two flight attendants, and the 69 passengers were not injured, and the aircraft did not sustain any damage. The 14 CFR Part 121 scheduled passenger flight, which departed Seattle-Tacoma International Airport, Seattle, Washington, about two hours earlier, was being operated in visual meteorological conditions. The flight, which was on an IFR flight plan, eventually landed at its destination around 1900. There was no emergency evacuation. According to the flight crew, during en route cruise, they noticed the number two hydraulic system quantity beginning to decrease. As they began to run the appropriate checklist, they noticed the illumination of several hydraulic system caution lights. Eventually, the number two system lost pressurization, and the rudder became dependent upon the sole remaining system (hydraulic system number one) for its movement. After the landing gear was extended manually, the aircraft landed without further incident, but it had to be towed from the runway, as the failure of the number two system rendered the nose wheel steering inoperable. It was later discovered that the number two system hydraulic line to the elevator had failed, leading to a loss of pressure in the number two system. The reason for the failure could not be positively determined.