On April 11, 2004, at 1435 mountain daylight time, a Canadair (Bombardier) CL-600-2C10, N609QX, operated by Horizon Air as Horizon Flight 4339, experienced a complete failure of the airplane's number 1 system hydraulics, while on approach into the Denver International Airport (DEN), Denver, Colorado. The flight crew shut off the system and made an uneventful landing. The captain, first officer, 2 flight attendants and 56 passengers on board were not injured. The scheduled domestic passenger flight from Ontario, California, to DEN was being conducted on an instrument flight rules flight plan under the provisions of Title 14 CFR Part 121. Visual meteorological conditions prevailed at the time of the incident. The captain reported that while in cruise flight at flight level 330, the crew received a master caution light and a hydraulic system 1 low pressure along with warning lights for the outboard ground spoilers, outboard spoilerons, and the outboard spoilers. The hydraulic fluid quantity and fluid temperature readings were good, but the system pressure was dropping. The crew ran the emergency checklist and continued their flight to Denver, using the backup (1B) system. On final approach and landing, the captain reported that they lost number 1 system pressure and quantity. The crew shut off the 1B system and continued to land. After landing, the crew shut down the number one engine and taxied to the gate. A post-incident examination of the airplane's number 1 hydraulic system showed that the hydraulic pump had come apart internally and seized. Metal fragments were found in the hydraulic filters and lines. Company maintenance replaced the pump and filters and flushed out the lines several times to remove all debris. The number 1 system hydraulic pump, the filters, metal fragments, cockpit voice recorder, and flight data recorder were retained for further examination. An examination of the data obtained from the flight data recorder showed that the first loss of system 1 occurred approximately 29 minutes after takeoff. The data shows that the second loss of system 1 occurs 1.5 hours after the first loss event. The data then shows the airplane touching down approximately 10 seconds after the second loss event. The hydraulic pump was examined at Parker Hannifin Corporation's Abex Division, Kalamazoo, Michigan, on May 11, 2004. The examination showed that several piston shoes in the pump's hanger and piston subassembly had come apart. The resulting metal fragments subsequently caused damage to the hold-down plate, the hanger, and other internal components of the pump, resulting in its seizure. An Abex Division representative reported that over the past two years, 29 other hydraulic pumps had been returned to the factory with seal leaks and piston shoes damage. The representative said that because of this, several pump improvements were developed including a return to a previously qualified design of the pump's piston shoes. At the time of the incident pump's examination, Abex was in the process of issuing retrofit engineering orders that incorporated the improvements.