Date: 201808
Local time of day: 1201-1800
Location: TJZS.ARTCC, PR
Flight conditions: VMC
Aircraft: B737 Next Generation Undifferentiated
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
ATC / advisory: Center TJZS
Aircraft component: Hydraulic System
Component problem: Failed
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument
Human factors: Workload; Troubleshooting
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: At FL370 during cruise; approximately 10-15 minutes prior to the top of descent; the FO (First Officer)) and I noticed a brief flicker of the Master Caution light. Upon investigation; we saw the System A hydraulic quantity reading 22%. We continued to monitor the hydraulic quantity; and reviewed the QRH procedure for LOSS OF SYSTEM A. As a precaution; I notified Dispatch of a potential emergency landing; and requested they notify SJU operations to have personnel standing by to install the landing gear pins. We also notified SJU ARTCC that we had a hydraulic loss; and might need emergency handling.The FO and I considered the impact of manual gear extension. Once lowering the gear; we would be committed to landing at SJU; and would not have the fuel to make our alternate of STX. Although there was a large storm over the island of Puerto Rico; it did not significantly impact a visual landing at SJU; nor a 2-engine missed approach and second landing attempt at SJU. The airfield was CAVOK with a scattered deck at 4;800 feet.Planned aircraft weight on landing was 139;500 LBS. According to the QRH; flaps 15 landing distance with loss of system A was 5;030 feet on a dry runway at 144;000 LBS. This was sufficient landing distance for either Runway 8 or Runway 10. We elected to do a flaps 30 landing on Runway 8 with autobrakes set to 3. QRH (144K) landing distance was 4;800 feet. Upon our request; San Juan ARTCC cleared us direct to the SJU airfield; and we started a descent. The FO and I discussed a plan to allow sufficient time to configure with manual gear extension procedures. Shortly afterward; the System A quantity dropped to 21%; along with another flicker of the Master Caution light. At this point I [advised ATC]; and requested ARFF support. Dispatch had asked us to advise if we were [advising ATC]; however I do not recall if I advised them of the actual declaration. Shortly afterward the Eng1 LOW PRESSURE light illuminated steady.Since I did not know where the fluid leak was; I elected to run the LOSS OF SYSTEM A checklist. We ran the first 4 steps; and began discussing manual gear extension. We decided to lower the gear passing 10;000 feet; and were configured by 8;000 feet. We had a jumpseater onboard; who was a RJ Captain. Since the flight deck was crowded; we elected to have the jumpseater pull the manual gear extension handles. The gear extended exactly the way I remember it did in the simulator the last time I practiced the procedure. I ran the Non-Routine Landing Considerations checklist; with no HAZMAT; and no emergency evacuation planned. I gave a precautionary landing briefing to the FAs (flight attendants); and made a PA to the passengers. The Number 1 FA told me the PA was difficult to hear. Since I had spoken loudly into the pedestal handset; I didn't think a second PA would be effective. I asked the Number 1 to relay my intentions to the passengers: that we would be stopping on the runway; and we would have safety vehicles meeting us there.We flew a visual approach to Runway 8; backed up by the ILS. The approach and landing were without incident; and we stopped just prior to S6 taxiway. I made a PA for the passengers to remain seated. Maintenance asked us to shut down our engines; and requested we pass the gear pins out the cockpit window. They pinned our gear and connected a tug. While this was happening; I made a PA to further explain to the passengers what the station personnel were doing to the aircraft. We were then towed to the gate. This terminated the emergency. Post flight maintenance inspection revealed a blown seal on a right wing spoiler actuator.


Second narrative: [Report narrative contained no additional information.]


Synopsis: B737 flight crew reported that the aircraft lost hydraulic system on descent.