Date: 201707
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: A319
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class C ZZZ
ATC / advisory: TRACON ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Human factors: Workload; Confusion; Distraction; Situational Awareness; Time Pressure; Troubleshooting
Second reporter function: First Officer; Pilot Flying
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Returned To Departure Airport
Contributing factors: Human Factors; Equipment / Tooling; Company Policy; Aircraft; Procedure
Primary problem: Equipment / Tooling

Narrative: Taking off from ZZZ; First Officer (FO) was flying. We had a Captain in the jump seat. On climb out passing approximately 5000 MSL we got a warning that the aft cargo door was not closed. After reviewing the QRH [Quick Reference Handbook] procedure we decided to return to ZZZ. Although the QRH gave the option of continuing; we decided it was more prudent to return and get it checked out. We were below our maximum landing weight. As the Captain; I made the landing. The jump seating Captain was helpful and provided information to assist us in our decision making.We completed all checklists and notifying the flight attendants and passengers; we returned to the airport. Upon landing and inspection of the aft cargo door; the latch was open and the primary latch had failed. The secondary latch kept the door from opening fully in flight.We did not have any indication of the aft cargo door being open until above 5;000 feet on climb out. The QRH did not give much advice and gave the option of continuing if the pressurization was stable; which it was at the low altitude. However; had we followed the advice of the QRH had we continued the flight; the aft cargo door latch and pressure relief door was open; we probably would have had a pressurization issue as we continued the climb to altitude. Then we would have to possibly do an emergency decent with the aft door latch actually open! (Which we did not know was the case until we landed). The QRH should be rewritten to not give the option of continuing. If you have an indication of a door open; divert and get it checked out. Only bad things can happen if you continue.Additionally; the ZZZ airport runway was about to close and only the short runway would have been available. I did consider diverting to [a different airport]; but because we were in clear skies and relatively close to ZZZ; I thought it was more prudent to land [there] than divert. This was mainly because I did not know that actual status of the aft cargo door and did not feel comfortable flying with a potential open aft cargo door; so I made the decision to put the plane on the ground as soon as possible.


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


Synopsis: A319 flight crew reported the aft cargo door opened and they returned to departure airport.