Date: 201902
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Hydraulic Main System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Not Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Overcame Equipment Problem; Flight Crew Landed in Emergency Condition
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: At cruise altitude approximately 100 NM from top of descent we received a Master Caution Warning and noted HYD was displayed on First Officer's annunciator panel. We looked at the Hydraulic Panel on the Forward Overhead Panel and noted ENG 1 LOW PRESSURE light was illuminated. We then looked at our MFD (Multi Functional Display) System on Lower Display Unit and noted quantity in the Hydraulic A system read 21%. I was pilot monitoring and told the First Officer to continue to fly and take over communications while I referenced the QRH. I followed checklist for Hydraulic Pump Low Pressure; which led us to turning off the ENG 1 Hydraulic Pump switch. We continued to monitor the quantity and noted that it was continuing to decrease at a decent rate. I told the First Officer of my intentions to [advise ATC] before the situation deteriorated and he agreed with my assessment. We [advised] ATC and flew direct to ZZZ. I notified Dispatch of our situation and briefed flight attendants. In anticipation of possibly losing System A we reviewed the 'LOSS OF SYSTEM A' QRH checklist. I referenced the Non-Normal Configuration Landing Distance table and asked Dispatch to also provide me a value to check against my calculation; and the values agreed. Again; this was just preparation in case we lost System A.Eventually; our quantity indicated 0%; but we still had good pressure in the system with no other warning or caution lights. Once communications had been completed and QRH checklists run and reviewed; I again took over ATC communications. We briefed the approach and again reviewed our plan if we lost system A. Except for indicating no quantity in system A; but still with good pressure; we had a normal descent and approach. We lowered gear and flaps earlier than we would normally; considering the possibility that the increased demand on the system could lead to a loss of pressure. When we lowered the gear; we noted quantity increased to 20%; where it stayed for the entirety of flight. We had an uneventful landing and taxi into the gate. We debriefed with local Maintenance. We also debriefed our performance. We felt we maintained safety; managed the emergency well; followed standard procedures; communicated and considered possibilities if the situation deteriorated and how we would handle those situations.




Synopsis: B737 Captain reported main hydraulic system malfunction during cruise.