Date: 201809
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Hydraulic System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Second reporter function: Pilot Not Flying; Captain
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Weight And Balance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Landed in Emergency Condition; Flight Crew Diverted
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Aircraft X did an air return to ZZZ due to a hydraulic leak in the A system after takeoff. This aircraft had just undergone maintenance to replace the #1 engine hydraulic pump and both the CA (Captain) and FO (First Officer) did a walk around paying close attention to this area. No abnormalities were found. Upon arriving at the aircraft; both hydraulic systems showed 100%. Once the hydraulic pumps were turned on during pushback; the B side remained around 100% and the A side dropped to ~85% then stabilized for the rest of the taxi. During gear retraction after takeoff; the master caution and HYD lights flickered then immediately went out. I was flying so the CA pulled up the SYS page on the CDU and saw that the A side dropped to ~40%. By the time the aircraft had climbed to 12;000; the pressure had dropped to ~34% and was steadily decreasing. The CA asked me if I agreed with returning to ZZZ and I did. I continued to fly while the CA coordinated with ATC; the flight attendants; and made a PA announcement. Once the aircraft was established on the LOC for runway XX; the CA took over the duties of PF (Pilot Flying) and made the overweight landing. The HYD and master caution lights flickered again on gear extension and on the landing rollout. At no point did the HYD annunciator remain illuminated nor illuminate during the recall. After landing; the A system indicated 20% RF (refill) and the aircraft taxied off the runway and back to the gate without any further issues.


Second narrative: During Cockpit Safety Checks upon entering the cockpit I noted both Hydraulic Systems indicating 106%. I also noted on an Inbound Defect Report that Eng 1 HYD Pump had been removed and replaced and that HYD Sys A was serviced to full. Once the airplane was powered up; and sometime after we started the APU to cool the cabin I noticed that HYD A now indicated 85% and HYD B was approximately 100%. I thought it a bit odd to apparently lose about 20% of HYD A quantity with no pumps on and mentioned it to the FO (First Officer). I went out to see if there might be a HYD leak somewhere. Due to recent [procedure] changes the 737 now leaves Hydraulic Pumps OFF during preflight inspection; and the A side was still off when I went out to look. I believe the FO had the B HYD ON to prevent winds from moving the yoke. There was no indication of any leaks on either the FO's full preflight inspection nor on my look at this time. After engine start; and during taxi out the HYD indications remained steady; HYD A 85% and HYD B about 100%. During gear retraction after liftoff the HYD Master Caution light flickered; but did not stay on long enough for us to identify the specific HYD Light on the overhead panel. The light did not re-illuminate with Master Caution Recall. As Pilot Monitoring I immediately selected the Hydraulic Quantity indication on the lower-center DU and noticed the HYD A Quantity at 44%. (HYD B was approx 85%). Following flap retraction and After Takeoff Checklist; and during initial climb out we monitored HYD indications closely and observed a slow but steady decrease in HYD A quantity. Once level at 16;000 ft the FO and I discussed the current HYD A Quantity at 35% and decreasing and I decided to return to [departure airport] before the airplane suffered a full HYD A system failure. The FO agreed. I requested a return with ATC for a Hydraulic malfunction (on VHF1); and the FO called Ops to report our return with a HYD issue and to request a gate (on VHF2). I called the flight attendants; who were all still seated as I requested during my preflight briefing due to [weather] in the area; and reported our status and my plan and expectations. I then made a PA to the passengers; informing them of a problem with a Hydraulic pump and my decision to return. With the descent we received an Off Schedule Descent light on the overhead and I (as Pilot Monitoring still) ran that QRH checklist. I ran the descent checklist; requested landing data via ACARS and we briefed the [approach]. With all checklists and briefings complete I took over Pilot Flying duties at 3;000 ft on final. Just before I called for gear down I noticed HYD A quantity at 20%. The HYD Master Caution flickered briefly again during gear extension. With gear all down and locked HYD A Quantity increased to approximately 40%. There were no HYD lights illuminated on the overhead panel; and the only light illuminated with Master Recall was the Off Schedule Descent. Overweight Landing and rollout were uneventful. Once we taxied clear we stopped to allow emergency ground crew to check the airplane. They reported a Hydraulic leak that appeared to be coming from the left main gear area. Brake pressure was normal.Due to time compression on the turn back to the field; including communication with cabin crew and passengers; approach selection in the FMC; requesting landing data for an overweight landing; and running Descent Checklist with Approach Briefing I failed to notify Dispatch with the reason for our return to the field.I feel that the First Officer and I worked exceptionally well together using CRM/TRM skills to discuss the status of our HYD system; and develop and execute a plan to avoid that status degrading into a more serious condition. Since we did not have to run an abnormal checklist for the Hydraulic system I never saw the four-block end-of-checklist symbols that many of us use to remember to [brief] the cabin crew. Thanks to the quality ofthe recurrent training I've received over the years which includes instructors 'keeping it real' and making sure we make calls to cabin crews during training sessions; I habitually called the cabin crew once the FO and I developed our plan to air return.


Synopsis: B737 flight crew reported a decrease in hydraulic quantity and pressure during climb resulted in a return to the departure airport.