Narrative:

At cruise; we noted the left hydraulic system fluid quantity was at four quarts with normal pressure. Referencing the QRH; we turned the left hydraulic pump switch to 'off'. The left hydraulic pump control circuit breaker popped and the pump went to high pressure. Resetting the circuit breaker; the circuit breaker again popped with pump at high pressure. In an effort to minimize the pressure on the system; to slow rate of leakage; and noting that the circuit breaker stayed in with the pump in 'low'; we reset the circuit breaker with the hydraulic pump switch in the 'low' position. The system seemed to stabilize at three quarts with the pump in 'low'. We contacted maintenance control and apprised them of our situation. It was decided to continue and monitor the system. The fluid quantity slowly dropped to two quarts. We were preparing to re-contact maintenance control; when dispatch contacted us and recommended a diversion; which we did. The passengers were briefed about the diversion. Approaching our divert airport; we declared an emergency and made a normal visual approach. We did not receive traffic priority. During the descent/approach; the left system quantity fell to one quart. We selected 'hi' pressure for the approach; and the pressure remained normal throughout the approach; landing; and roll-out. Thrust reverser's; brakes; and nose wheel steering were operating normally; so we turned-off the landing runway near its end. With the fluid quantity still indicating one quart; we shut-down the left engine. The fire chief and a contract mechanic arrived to inspect the airplane. We attempted to keep the cabin cool with APU air conditioning; but; apparently; it was ingesting some hydraulic fluid fumes; as an odor and haze developed the cabin. We stopped the APU air conditioning; and the cabin soon cleared. After his inspection; the fire chief cleared us to proceed to the gate. The mechanic informed us that we did have a hydraulic leak in the tail area. As a precaution; we installed the landing gear pins and had the airplane towed to the gate. The passengers were deplaned to the terminal; to await the replacement airplane. Some hours later a replacement airplane arrived. All but four passengers were re-boarded. (Some passengers told us that they overheard the 'no-show' passengers talking about getting another flight out.) our [airport] hosts did a thorough search of the gate area and could not find the missing passengers. Customer service recommended that we leave with out them; which we did.

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Original NASA ASRS Text

Title: An MD83 Captain discovered low hydraulic fluid quantity in the left system during cruise. Attempts to preserve fluid by turning of the pump were unsuccessful due to the pump control circuit breaker tripping. Fluid quantity continued to decrease and the decision was made to divert.

Narrative: At cruise; we noted the left hydraulic system fluid quantity was at four quarts with normal pressure. Referencing the QRH; we turned the left hydraulic pump switch to 'OFF'. The Left Hydraulic Pump Control circuit breaker popped and the pump went to high pressure. Resetting the circuit breaker; the circuit breaker again popped with pump at high pressure. In an effort to minimize the pressure on the system; to slow rate of leakage; and noting that the circuit breaker stayed in with the pump in 'LOW'; we reset the circuit breaker with the hydraulic pump switch in the 'LOW' position. The system seemed to stabilize at three quarts with the pump in 'LOW'. We contacted Maintenance Control and apprised them of our situation. It was decided to continue and monitor the system. The fluid quantity slowly dropped to two quarts. We were preparing to re-contact Maintenance Control; when Dispatch contacted us and recommended a diversion; which we did. The passengers were briefed about the diversion. Approaching our divert airport; we declared an emergency and made a normal visual approach. We did not receive traffic priority. During the descent/approach; the left system quantity fell to one quart. We selected 'HI' pressure for the approach; and the pressure remained normal throughout the approach; landing; and roll-out. Thrust reverser's; brakes; and nose wheel steering were operating normally; so we turned-off the landing runway near its end. With the fluid quantity still indicating one quart; we shut-down the left engine. The Fire Chief and a contract mechanic arrived to inspect the airplane. We attempted to keep the cabin cool with APU air conditioning; but; apparently; it was ingesting some hydraulic fluid fumes; as an odor and haze developed the cabin. We stopped the APU air conditioning; and the cabin soon cleared. After his inspection; the Fire Chief cleared us to proceed to the gate. The mechanic informed us that we did have a hydraulic leak in the tail area. As a precaution; we installed the landing gear pins and had the airplane towed to the gate. The passengers were deplaned to the terminal; to await the replacement airplane. Some hours later a replacement airplane arrived. All but four passengers were re-boarded. (Some passengers told us that they overheard the 'no-show' passengers talking about getting another flight out.) Our [airport] hosts did a thorough search of the gate area and could not find the missing passengers. Customer Service recommended that we leave with out them; which we did.

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.