Date: 201902
Local time of day: 0601-1200
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-800
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 System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification
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 the HYD light was displayed on FO 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 Multi Function Display SYS on Lower Display Unit and noted quantity in the Hydraulic A system read 21%. I was pilot flying at the time FO the incident. The CA told me to continue my flying duties and to take over communications while he referenced the QRH. The CA 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. The CA informed me of his intentions to [advise ATC] before the situation deteriorated and I agreed with his assessment. We [advised] ATC and flew direct to ZZZ. The CA 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. The CA asked Dispatch to also provide us with a value for the non-normal landing distance to check against his calculation and the values agreed. All of these actions were 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; the CA 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; 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 in to 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 First Officer reported Hydraulic System A quantity malfunction during cruise.