Narrative:

At 2;500 ft cleared for visual. Gear selected down. 'Hydraulic master caution' light/chime and 'gear unsafe' light -- no green lights. Captain called for 'hydraulic light on' memory items. First officer accomplished memory items. Main accumulator pressure was zero. Emergency accumulator was fully pressurized. Hydraulic quantity indicator was at the bottom of the green arc and was not decreasing further. Captain called for 'hydraulic light on' abnormal checklist and assumed ATC radio duties. Notified ZZZ that a go-around was necessary due to a hydraulic problem and declared an emergency. Instructed to climb to 3;000 ft and given a heading northbound. Instructed to contact approach. 'Hydraulic light on' abnormal checklist was completed. At this point; the gear was down with 3 green lights and flaps were set to 15 degrees. Hydraulic quantity was still holding steady at the bottom of the green arc. Notified approach ready to fly the approach and vectored for a right base to runway xxl. On final; flaps selected 20 degrees. Cleared to land; notified ATC a full stop would be made on the runway to assess hydraulics. Approach and touchdown were normal. After touchdown; first officer selected hydraulic pump switch to override. On rollout; the left propeller automatic-coarsened when power levers were brought below flight idle and into reverse. The aircraft veered slightly and was kept on the centerline with rudder; brakes; and nosewheel steering. Left propeller recovered when the power levers were brought out of reverse. After coming to a full stop; all 4 hydraulic accumulators were fully charged and the hydraulic quantity was stable and in the green arc. No further quantity loss was noted between when the hydraulic master caution initially went off on approach and coming to a full stop on the runway. Captain and first officer agreed that the hydraulics system was safe for taxi with close monitoring. Cleared runway at taxiway and then cleared to gate by ground. Taxi was uneventful. Unknown to crew why hydraulic malfunction occurred. Unknown to crew why left propeller automatic-coarsened on rollout. Crew determined these 2 events were unrelated.

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Original NASA ASRS Text

Title: SF340 Captain reports hydraulic master caution and unsafe gear during approach. After complying with checklist procedures flight returns for landing. During rollout left engine auto coarsens momentarily when reverse is selected. Normal taxi to gate.

Narrative: At 2;500 FT cleared for visual. Gear selected down. 'Hydraulic Master Caution' light/chime and 'Gear Unsafe' light -- no green lights. Captain called for 'Hydraulic Light On' memory items. First Officer accomplished memory items. Main accumulator pressure was zero. Emergency accumulator was fully pressurized. Hydraulic quantity indicator was at the bottom of the green arc and was not decreasing further. Captain called for 'Hydraulic Light On' abnormal checklist and assumed ATC radio duties. Notified ZZZ that a go-around was necessary due to a hydraulic problem and declared an emergency. Instructed to climb to 3;000 FT and given a heading northbound. Instructed to contact Approach. 'Hydraulic Light On' abnormal checklist was completed. At this point; the gear was down with 3 green lights and flaps were set to 15 degrees. Hydraulic quantity was still holding steady at the bottom of the green arc. Notified Approach ready to fly the approach and vectored for a right base to Runway XXL. On final; flaps selected 20 degrees. Cleared to land; notified ATC a full stop would be made on the runway to assess hydraulics. Approach and touchdown were normal. After touchdown; First Officer selected hydraulic pump switch to override. On rollout; the left propeller automatic-coarsened when power levers were brought below flight idle and into reverse. The aircraft veered slightly and was kept on the centerline with rudder; brakes; and nosewheel steering. Left propeller recovered when the power levers were brought out of reverse. After coming to a full stop; all 4 hydraulic accumulators were fully charged and the hydraulic quantity was stable and in the green arc. No further quantity loss was noted between when the hydraulic master caution initially went off on approach and coming to a full stop on the runway. Captain and First Officer agreed that the hydraulics system was safe for taxi with close monitoring. Cleared runway at taxiway and then cleared to gate by Ground. Taxi was uneventful. Unknown to crew why hydraulic malfunction occurred. Unknown to crew why left propeller automatic-coarsened on rollout. Crew determined these 2 events were unrelated.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.