Date: 201811
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Hydraulic Main System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not 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: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On climbout; while leaving approximately 12;000 feet and climbing to 14;000 feet per ATC; we received a master caution HYD light. We looked up and on the overhead panel saw the HYD A Elec 2 Low Pressure light illuminated. At that time; ATC called to give us a higher altitude; but we requested an altitude of 14;000 feet to work on an issue. They granted our request. We then went to the HYDRAULIC PUMP LOW PRESSURE QRH (we agreed it was the correct checklist and switch) which simply stated:1 HYD PUMP switch (affected side) ...........OFF...so the First Officer turned off the appropriate switch while the Captain continued to fly.Next; we pressed the MFD SYS button and noticed the HYD A QTY % around a guesstimate of 73%. (Often times normal HYD A press in flight is 77-78 %.) We would soon realize the A HYD quantity was decreasing approximately 2-3% per minute.After discussion between the pilots; we decided to head back to [departure airport]. I'm guessing we were approximately 50nm from [departure airport] at that time. ATC gave us direct to [an enroute waypoint] but then we decided the best and quicker course of action was to ask for a more direct route via a heading to [departure airport]. ATC complied giving us a heading closer to [departure airport].We called and spoke with Maintenance and told them we turned off the HYD A ELEC 2 switch. They did not add any suggestions but they told Operations that we were returning so a gate would be ready for us. Our next move was to talk to the flight attendants and then speak to the passengers. We received a message from Dispatch at that time asking if we were diverting. This was a very high workload environment so we simply hit the divert button in ACARS and said 'yes.'ATC had us at 270 knots originally so we asked to speed up to 300 knots to shorten the time. Approach cleared us for the visual and then told us to contact Tower. We called that frequency but they never answered. This was beyond frustrating as we were on Final Approach in an extremely high workload environment. Soon after; they called and we were cleared to land. The landing and taxi was uneventful. Maintenance came to the cockpit and said they found an internal leak and although we were at about 30% HYD A quantity % when we blocked in; we soon after had zero and an RF next to the quantity.




Synopsis: B737NG First Officer reported returning to departure airport after loss of hydraulic system A.