Date: 201311
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; First Officer
Human factors: Other / Unknown
Second reporter function: Captain
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
Result: General Declared Emergency
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Captain was Pilot Flying. I was the Pilot Monitoring. We were climbing through FL180 on a Standard Instrument Departure. With full climb power on the engines; we heard a loud bang accompanied by vibration and a yaw to the left. The EGT was flashing red and indicated approximately 998 degrees. The Captain announced; 'Left engine failure;' and retarded the throttle to idle; disconnected the autopilot; and leveled the aircraft. We both assessed the engine instruments; determined it was a normal engine failure; declared an emergency; and requested an immediate return to ZZZ for landing. We proceeded with the Engine Failure/Shutdown Checklist in the QRH as we started a slow turn and descent back to ZZZ. We assessed the engine and determined a restart was possible. We ran the In-flight Engine Start Checklist but could not get the N2 to rise above 25%. We then aborted the in-flight restart and proceeded with the One Engine Inoperative Landing Checklist. We were cleared the ILS XXL at ZZZ and flew the 15 flap approach and landing to an uneventful full stop landing. With no smoke; fire; or braking issues we taxied to Gate XX; shut down the aircraft; and deplaned the passengers.  Periodically throughout the descent to landing; we communicated with the flight attendants and passengers to anticipate an uneventful landing. We also informed ATC of our souls on board and fuel remaining when questioned about this. We told them to inform crash; fire; and rescue personnel which engine had failed and that a normal landing and rollout was anticipated.


Second narrative: We departed ZZZ on the Departure. While climbing through 18;000 feet; there was a loud bang from the left engine area. The left engine N1 and N2 were rolling back; and the EGT was flashing RED and indicated approximately 998 degrees. I turned off the autopilot; reduced the left engine thrust lever; leveled the aircraft; and corrected the yaw. I announced a left engine failure and the First Officer pulled out the QRH. We assessed the engine and ran the Engine Failure/Shutdown Checklist. I told Center we had lost an engine; and declared an emergency and we needed to return to ZZZ. Center gave us a turn back to ZZZ and a lower altitude. The First Officer ran the checklist and; due to the parameters; we attempted an engine restart. The N2 never exceeded 25% so we aborted the restart. We notified the flight attendants and passengers of the problem and prepared for a one engine landing visual approach to XXL with an ILS backup. The First Officer computed the numbers for a Flaps 15; one engine landing; under max landing weight. We had an uneventful landing and rollout. The fire trucks reported no smoke or fire observed so we taxied single engine to Gate XX and shut down the aircraft. Contract Maintenance confirmed N2 section failure from initial inspection.


Synopsis: Air carrier departure described an engine failure during the climb to flight levels and the actions taken by the crew and ATC during the return to the departure airport.