Date: 201605
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: B737 Undifferentiated or Other Model
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 Main System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Not Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Anomaly: Aircraft Equipment Problem Less Severe; Flight Deck / Cabin / Aircraft Event Smoke / Fire / Fumes / Odor; Ground Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: We received a low pressure engine 1 hydraulic master caution in cruise at FL350 approximately 1 hour into a 2 hour flight. We immediately accomplished the hydraulic low pressure QRH. The hydraulic A quantity was noted at 22%. After shutting off engine 1 hydraulic pump; the quantity continued steady decline to 0%. Pressure remained 3;000 PSI on both systems. Expecting a full hydraulic A failure; the Captain gave pilot flying duties to me while he coordinated with company maintenance; Dispatch; ATC; cabin crew and passengers. We [advised ATC of the situation]. Due to heavy thunderstorms in [destination] area; flight crew made a decision to return to [departure airport] rather than continue. We continued to [departure airport] to burn fuel to get below landing weight. We reviewed the hydraulic A fail QRH in anticipation of that event and reviewed landing distance data for loss of hydraulic A. We decided on straight in [landing]; First Officer landed (flaps 40/max brake); so that the Captain could switch to alt steering on runway if necessary.On landing; fire equipment was dispatched to aircraft and we advised Tower we would be stopped on runway. We had expected to taxi to the gate momentarily. Emergency workers noted mist/smoke from #1 engine with no fire indications. Expecting it was residual engine smoke; we elected to shut down engine 1. The smoke/mist briefly stopped then restarted and we accomplished engine fire checklist; pulling fire handle but not discharging bottles. Puddles reported around left main gear and we discussed that it would probably be safer to use a super tug to bring us to gate because hydraulic fluid was likely on the hot brake rotors.Captain regularly was in contact with the flight attendants and passengers; and was very reassuring to them. Though my primary responsibility was pilot flying; I was completely in the loop on all communication; decision making and my input was sought and considered at every opportunity. I think the crew acted with superior judgement and utilized CRM strategies very well. After debriefing with Captain; I am very pleased with our decisions and actions to land the aircraft safely.




Synopsis: B737 First Officer reported returning to departure airport after experiencing loss of 'A' hydraulic system.