B757 Undifferentiated or Other Model
Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 148; Flight Crew Type 6517
Workload; Troubleshooting
Aircraft Equipment Problem Less Severe
Air Traffic Control Provided Assistance; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport
Aircraft
This flight was a ferry flight from ZZZ to ZZZ1 to return an aircraft from a [maintenance] check. This was our second ferry segment today and planned to be an ETOPS verification flight. Prior to departure we experienced a tarmac delay due to various maintenance items unrelated to our event and one additional on taxi out. We were also briefed by the maintenance team that all four (4) pitot tubes had been replaced but only for reliability reasons and we noted this information. On taxi out; we recalled that a 'full rated' take-off was due per the maintenance release document and re-programmed the FMC with max thrust speeds after selecting the TOGA button on the TMC panel. On take-off from ZZZ we experienced an engine anomaly. During the take-off roll and as the Captain called 'rotate'; I noticed that the engine parameters on both #1 & #2 engines did not match and pointed this out to the Captain. I noted that the #1 engine was still at 'max' thrust EPR & N1 (103.1%) but the #2 engine was showing 95.3% N1 with the other engine parameters reflecting the same reduction from 'max' thrust. I did not feel any adverse yaw nor aircraft instability. However; just prior to the Captain's rotation callout; I did feel a subtle acceleration change but nothing that was explainable at that moment. As we continued to climb out; I called for VNAV at 800 feet AGL per the profile. The VNAV armed but did not engage per the FMA. At that time; I decided to continue manually flying the aircraft and both engines responded normally to throttle inputs. We choose to select FLCH which allowed the aircraft to change the thrust setting and pitch command bars but we were unable to receive the CLB thrust indication on the EICAS. We selected CLB on the TMC but it immediately reverted to CON. As we continued to climb out to a safe altitude and diagnose our situation; we noticed on the VNAV climb page that we had an 'all engines' prompt and the page header was 'VREF + 80 knots'. From rotation until this point; both engines were operating normally; aside from the thrust difference between #1 & #2; so we choose to select the 'all engines' prompt. It was my thinking that the #2 engine thrust reduction may have triggered the FMC and TMC to provide us with single engine information. Once we selected and executed the 'all engines' prompt; we regained VNAV capabilities and continued our climb with both engines stable and no abnormalities. Once stable and above 10;000 feet; I remained the flying pilot and managed ATC while the Captain contacted Dispatch and [Maintenance Control]. During the call; the 'auto throttle disc' EICAS message displayed and the auto throttle annunciation on the FMA disappeared. We ran the QRH which; to our surprise; did not have us turn off the A/T ARM switch on the MCP. Shortly thereafter; I changed our pitch mode and the auto throttles reengaged but failed shortly thereafter. Both failures occurred while in level flight after being level for a period time. We then ran the QRH again and decided this time to turn off the A/T ARM switch on the MCP to prevent any additional undesired distractions while briefing the loss of the auto throttle system. A return to ZZZ was then determined. A normal descent and approach was accomplished where multiple level offs allowed us to gauge the engine response in the event of a go-around. Our throttle movements were matched normally with the expected engine response. We both determined that our anomaly on take-off was such and had a go-around been necessary; both engines would respond normally. A normal landing was executed with ZZZ CFR (Crash Fire Rescue) positioned to assist. Our only other engine anomaly during the course of this flight was with the same engine. The engine was unable to fully attain commanded thrust and maintained a lag of .05 EPR in all thrust positions. During this flight; we never that safety of flight nor the aircraft was compromised.
[Report narrative contained no additional information.]
B757 flight crew reported abnormal engine indications on climb and returning to the departure airport.