B757-200
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe
Air Traffic Control Issued New Clearance; Flight Crew Landed in Emergency Condition
Aircraft
Approximately 25 minutes into the flight at FL370; the left engine vibration indication began climbing into the 'yellow band'; and remained there. Cockpit crew performed 'memory items' in order to return and maintain engine vibration within normal limits. The left engine was near idle and power to the right engine was added in order to maintain airspeed. Airspeed was slowly decaying anyway; so a descent and return was requested. A few attempts to return the left engine to normal power were made. That was a step that the engine vibration checklist called for in the event that possible engine ice might be the cause of the vibration and power 'cycling' might remedy. It did not. A return to [the departure airport] was accomplished followed by an uneventful overweight landing.
Flying at FL370; B-757-200 crew experienced vibration and power loss in the left engine. Crew executed an uneventful landing back at the departure airport.