Date: 201605
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Flight phase: Cruise
Aircraft component: Hydraulic Lines; Connectors; Fittings
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Returned To Departure Airport
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Intermittent Master Caution and Low Pressure on Engine 1 Hydraulic Pump Completed QRH items and turned pump off. Checked Hydraulic page and noted System A pressure around 3;000 psi but Quantity had dropped to 22%. Assigned First Officer (FO) Flying pilot duties and I contacted Dispatch and Maintenance Control through ARINC. Maintenance Control recommended continuing to destination as long as QTY was constant. Upon evaluating our route (over water) and weather (at and near destination) and consulting the diversion guide in the FOM we decided a return was the safest course of action. Flight attendants and passengers briefed after we began our return and ATC notified.HYD A QTY slowly declined to read 0%. We thought that we would lose the entire A HYD system at that point. We reviewed the Loss of System A checklist and independently checked the abnormal landing distance chart for loss of A system HYD and found we had 6;000 ft or so to spare. The QRH had a very helpful breakdown of the items we could expect to become inoperative if we lost HDY SYS A. It allowed us to discuss; plan and brief things such as:Change to use of autopilot B prior to loss of A System. When to drop the gear (considering we would not be able to retract it); increased fuel burn and time associated with manual gear extension procedure. Changes to Go Around if we used manual extension. Effect on steering and decision to stop on runway prior to taxiing. Effect on Thrust Reversers which could compound a steering issue.Dispatch sent message to land at ZZZ. We had discussion via ARINC but we were at FL300 over ZZZ and we chose to continue to the departure airport. This gave me time to check on the status of Flight Attendants (FA); passengers and First Officer at various points without rushing. Another benefit was a further reduction in aircraft weight.First Officer flew nice approach and landing. We stopped on the runway without incident.This is when unplanned things began to happen. The Air Rescue Firefighter (ARFF) reported mist or light smoke coming from ENG 1. I shut it down immediately and they reported the mist had stopped. I informed the firefighters that we were starting the APU. At this time we still had good pressure on the A HYD system and ALT steering available so I was considering taxiing to the gate. While talking with the FAs We received report that the mist appeared again near ENG 1. We thought it might be from a normal shutdown but we ran the Engine Fire checklist through pulling the fire handle to cut off all hydraulics to ENG 1. No sign of fire so no bottles were discharged. ARFF got out of truck and took a closer look to confirm that. As we were discussing taxiing to the gate ARFF reported a puddle under our aircraft. The First Officer interjected that hydraulic fluid could leak on our hot brakes while taxiing and cause some more problems for us. Taking his input and upon conferring with firefighters I decided to shutdown engines; depressurize all hydraulics and get towed to gate. We confirmed all passengers seated and all exits closed prior to beginning tow.Once at the gate we briefed the fire fighters and maintenance. HYD A system pressure remained at 3000 psi through touchdown and QTY jumped to 16% after landing then back to 0% after hydraulics depressurized. Maintenance found leak and a cracked fitting. I then went over to the gate to quickly debrief the FAs before they boarded their next flight. The First Officer and I debriefed at a later time and it was then that I realized I had not provided ATC with the Souls on Board (SOB) count or fuel in minutes at the time I informed ATC. We did give it to them prior to landing when they prompted us for it. We both agreed that all employees had done their job well in handling this situation. We discussed some of the issues and duties of the Captain as manager of the situation and FO as flying pilot.




Synopsis: B737 Captain reported returning to the departure airport when a master caution light indicated a slow loss of hydraulic fluid from the A system.