Date: 201703
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Aircraft: A300
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical
Detector: 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: At FL200 Number 2 engine rolled back suddenly. All fuel pumps had been on since preflight. A lot of yaw and bank. Disconnected auto flight and throttles. Started descent. Checked engine rotating and turned on continuous relight. Left ECAM disappeared or diminished. Was it at idle or sub idle? I don't know. Lights but no horn. Checked that the engine responded to throttle movement then left it at idle and flew with auto throttles disconnected. Only used the engine for thrust reverse on landing and to taxi.Did the long checklist. Contacted ATC. They asked if we wanted the trucks ready or rolled. While I was thinking they made the decision. I would have rolled the trucks also. I think they could have gotten in place sooner as I had a lot of localizer wobble and had to disengage the autopilot. I wonder if the trucks crossed the beam? I forgot to ask the fire chief.Had to stay high longer due to TCAS alert. I slowed rate of descent to avoid RCAS. After the traffic passed I used boards to get down.MEL for bad pack on the side of the good engine. Made airflow path in case we lost the engine with the good pack. I could have used APU for pressurization and I would have if we were really single engine.I planned on using both engines in the event of a go around. And certainly would have used number two if anything happened to number one.Didn't turn off stuff in the checklist since the engine was still running. I probably should have started the APU a little earlier (before we got to it in the checklist) although it was backup only.Did a 20 flap landing at 151 knots. Very smooth just under max landing gross weight. Thrust reversers on both engines. Min auto brakes. Cleared runway. Stopped as requested then continued taxi to hangar gate and transloaded.Great backup and initiative by the FO. For example; when we shut down; he told me the stairs were on the wrong side for the MEL slide inop. And I was busier than normal with manual throttles and he assumed some of my duties in addition to his own. Very nicely done on his part.Talked to fire chief. They have three levels of alert. I think it would be good info to incorporate in training. Cause: Engine failed or went to idle; so quickly that I thought it had failed.


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


Synopsis: A300 flight crew reported returning to departure airport after Number 2 engine abruptly rolled back to idle.