Date: 201208
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: Commercial Fixed Wing
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: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Critical
Detector: Automation Aircraft Other Automation; Person Flight Crew
When detected: In-flight
Result: Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: During climb out at 12;000 FT MSL; the #1 engine failed to respond out of idle after a deceleration to avoid approaching weather. When a left climbing turn and power were added to continue departure; the left engine stayed in idle; airspeed/climb performance ceased and a return to [departure airport] was accomplished (single engine; full flap approach; landing). Remainder of flight [was] uneventful.


Second narrative: On departure climbing through about 11;500 FT MSL and left turn to ATC assigned heading we encountered a significant loss of thrust on the #1 (left) engine. During [a] climbing left turn the #1 engine did not respond to thrust inputs. There was only a very slight feeling of yaw (more like a slight wiggle) as the #2 engine responded normally by providing MCT thrust. The aircraft was at 12;100 FT MSL and about 230 KIAS but only climbing at about 200 FPM and maintained about 220 KIAS which was slightly below Green Dot for the bank/altitude etc. I made a comment to the Captain to watch airspeed and wondered why we were only climbing at 200 FPM. At the exact same moment I made the comment about speed/climb rate; the Captain was verifying the speed brakes were in and the flaps were up. At that same time I looked at the engine indications on the Engine Warning Display. I called out to the Captain the #1 Engine is showing abnormal thrust. It indicated EPR of .986 and N1 of about 34% (#2 engine EPR was about 1.9 and N1 of about 89%). There were no ECAM warnings or cautions and no sounds associated with the loss of thrust. It appeared the #1 engine remained in flight idle even when MCT thrust was selected by the Captain. As I called out the abnormal #1 engine indications; the Captain disconnected the Auto Thrust System; reduced the #1 engine thrust lever to idle; leveled the aircraft and rolled out of the left bank. He attempted to increase thrust on the #1 engine and the engine indications showed no response. The Captain and I conferred; quickly agreed we had a loss of thrust but not a total failure of the #1 engine. There was no ECAM; no obvious cause for the problem and so the Captain and I agreed to use the Stalled Engine Checklist from the Flight Manual as a guide to assess and work the problem. The Captain had already completed the ENG SURGE/STALL QRC memory step of Thrust Lever - Idle. I completed the Stall Checklist as directed by QRC. The #1 engine was providing normal flight idle thrust so we did not shut down the engine. Engine Anti-Ice and Wing Anti-Ice was selected in accordance with the Stall Checklist. The Captain notified the Lead Flight Attendant of the situation. I took over ATC calls after coordination with Dispatch. The Captain briefed the approach and overweight landing considerations. I completed the Approach Descent Checklist and Final Descent Checklist. The approach and landing were well flown and uneventful. We cleared [the] runway and had the fire trucks/emergency vehicles follow us to gate. Other than hot brakes; all else was normal.


Synopsis: Airline flight crew reported #1 engine locked in idle thrust during climbout. Crew declared an emergency and returned to departure airport.