Date: 201809
Local time of day: 0601-1200
Location: ZZZZ.ARTCC, FO
Flight conditions: VMC
Light: Daylight
Aircraft: B757 Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
ATC / advisory: Center ZZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Flying; Captain
Reporter qualification: Flight Crew Multiengine; Flight Crew Instrument; Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: Just prior to the top of descent the left engine thrust degraded slightly; rolling back from approximately 1.6 EPR to 1.4 EPR. The other engine parameters were similarly degraded. No vibration was present. We ran the Engine Indication Fluctuation checklist and this seemed to stabilize the engine somewhat. We were well into the descent now and discussed the issue with the crew. We briefed the single engine approach and missed approach procedures and performance in case they were needed. When we leveled briefly at 3000 feet on the downwind and increased power the left engine did not respond normally. It seemed to be slightly more degraded than at cruise. I chose to keep it running since we were getting thrust from the engine. The approach to Runway XX was under VMC conditions and we continued to a normal landing. We did not receive any special handling from ATC. After block in I made a maintenance logbook entry and spoke with [Maintenance control].


Second narrative: I was the Pilot Monitoring and had just come back from Third Break about 10-15 minutes prior to this event. We were right around the TOD (Top of Descent) when I noticed that the auto throttles had let the airspeed slip about 10 knots. Upon looking at the engine indications; we noticed that the left engine indications were not stable with the right side and had rolled back to about 50% of its normal power. The trend command on the gauges still showed the FADEC commanding more power; but the engine was not producing it. The rest of the engine indications on the left side also indicated that the left engine was indeed operating at a reduced thrust setting. We had no additional EICAS messages. Moving the left throttle back would reduce thrust on the engine; but we could not gain thrust higher than the amount that it had rolled back to and seemed to be responding erratically. The right engine was responding completely normal.The Captain flew the aircraft while I began looking at checklists. The checklist that we found was the Engine Indication Fluctuations checklist. After discussing it with the crew; we determined that this checklist would have us shut down the EEC (Electronic Engine Control) units to both engines and retain manual thrust control. We agreed that this was the closest checklist to our situation. After completing the checklist and having both EEC controls OFF; it appeared that we had regained manual thrust control of the left engine and the right engine. Since we were in a descent; we could not bring the power of the left engine up too far; but it seemed that the engine was responding to our inputs.At this point; we discussed the possibility of this potentially turning into a single engine event if the engine had any additional issues down lower. As a precautionary measure; I pulled up landing data for a single engine go around obstacle procedure. The Relief First Officer Pulled up Flaps 20 landing distance in the event that this led to an engine shutdown in flight.At this point we decided that a Flaps 25 landing would be prudent for reduced drag just in case the Engine began to stop responding again. However; we thought that the engine would continue to operate normally in the EEC off mode.As we leveled off and were vectored onto the localizer on a 10 mile final; the Captain pushed both thrust levers up and the left engine once again began responding with only partial power. I asked both crewmembers if they would like a delay vector to discuss anything else. The Captain responded that we were stabilized and we could see the airport and he would prefer to continue the approach and land.At this point I asked if he would like for us to advise ATC of what was going on. He said that we could just continue the approach and landing without [advising ATC].We continued our approach uneventfully with the left engine producing partial power; and the right engine producing normal thrust. The Captain flew a very nice approach and was stabilized the entire way with a nice landing in the touchdown zone.We taxied in and shut down and then debriefed the event amongst ourselves.


Synopsis: B757 flight crew reported the left engine performed at a degraded rate during descent and landing.