Narrative:

This flight was scheduled for ZZZ-ZZZ1-ZZZ. I recently finished training as a first officer in june; and I am working on finishing consolidation. There was a MEL on the aircraft for an elevator PCU. The MEL required a maintenance procedure to check the flight controls with each of the hydraulic systems. The aircraft was loaded when we arrived. The captain confirmed that the logbook was not on the aircraft when we arrived. He asked the mechanic to check on the aircraft status and let us know if we could start preflight. The mechanic came back to tell us the aircraft was good for preflight; so the captain and I began our preflight. The mechanic brought the logbook and told us the repetitive inspection still needed to be done for the flight controls. He asked the captain to help with the flight control check and inspection. The MEL didn't mention which status message would be expected. There were 2 status messages; and the captain asked the mechanic if they were both associated with the MEL. He agreed that they were associated with it. We should have looked both of them up at this time; rather than assume this was correct. One status message was for right elev PCU and one was for left ggu.the mechanic and captain accomplished the dmp; flight control check with each hydraulic system. The check went well. The logbook was all signed off with repetitive inspection. The mechanic seemed rushed and was concerned about getting a late departure. We completed the preflight inspections and were ready to go close to our scheduled block time; but there was no aircraft tug or marshaller. In the CRM safety and departure brief; we discussed the possibility of windshear on takeoff since there were thunderstorms south and east of airport. There were also windshear advisories on ATIS; so we selected LLWS precautions on aero data. This ensured max thrust and flaps 20 setting for takeoff. We discussed the threat of remembering to ask for flaps 20; and doing a max thrust takeoff in lightweight 757. We also discussed the new procedure of asking for flaps 20 with after start checklist. I advised ramp we needed tug and marshaller; and still needed someone to pickup a copy of the [dangerous goods form]. The mechanic asked us if he could remove chocks and stairs. The captain confirmed all the doors were closed; the brakes were set; and all the checklists were complete. The captain told the mechanic he was clear to remove the chocks and stairs; but he forgot about waiting for someone to pickup a copy of the [dangerous goods form].the taxi out was normal. The captain gave a normal takeoff brief; and pointed out that the flaps were set at 20 and that max thrust was set. He also emphasized the delayed rotation speed and calling for flaps 5 instead of flaps 1. The captain forgot to call for the before takeoff checklist; and I missed that he didn't call for it as well. Both of us did visually check to confirm flaps were at 20. The takeoff was uneventful with the delayed rotation. It wasn't long after departure when we had to ask for weather deviations with a very busy controller. The rest of the flight to ZZZ1 was uneventful. After landing at ZZZ1; we taxied in and had to do an 180 on ramp to park on the extra parking spot.when we did the shutdown/secure checklist with FMC clearing procedure; the captain's AFD went blank. It normally still works with just battery power. After completing checklists; the captain checked the status messages again and saw a left ggu status message. He realized what it was; and confirmed it by looking it up in the 757 MEL. When he looked it up he discovered that it was a no takeoff item. It should have been written up and fixed before leaving ZZZ. We should have been more diligent and looked up each status message to confirm whether it was associated with our flight control PCU MEL item. I will make sure I trust but verify all status messages in the MEL rather than taking the mechanic's word about it. I will also make sure to look for items or events that take me out of my flow in order to trap errors such as the before takeoff checklist. The captain wrote up the left ggu status message at ZZZ1 and told the mechanic about it. He was able to troubleshoot the problem; and eventually fixed the ggu problem.

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Original NASA ASRS Text

Title: B757 First Officer reported misidentifying an EICAS message; which lead to the aircraft flying in a non-airworthy status.

Narrative: This flight was scheduled for ZZZ-ZZZ1-ZZZ. I recently finished training as a first officer in June; and I am working on finishing consolidation. There was a MEL on the aircraft for an Elevator PCU. The MEL required a maintenance procedure to check the flight controls with each of the hydraulic systems. The aircraft was loaded when we arrived. The Captain confirmed that the logbook was not on the aircraft when we arrived. He asked the mechanic to check on the aircraft status and let us know if we could start preflight. The mechanic came back to tell us the aircraft was good for preflight; so the Captain and I began our preflight. The mechanic brought the logbook and told us the repetitive inspection still needed to be done for the flight controls. He asked the Captain to help with the flight control check and inspection. The MEL didn't mention which status message would be expected. There were 2 status messages; and the Captain asked the mechanic if they were both associated with the MEL. He agreed that they were associated with it. We should have looked both of them up at this time; rather than assume this was correct. One status message was for R ELEV PCU and one was for L GGU.The mechanic and Captain accomplished the DMP; flight control check with each hydraulic system. The check went well. The logbook was all signed off with repetitive inspection. The mechanic seemed rushed and was concerned about getting a late departure. We completed the preflight inspections and were ready to go close to our scheduled block time; but there was no aircraft tug or marshaller. In the CRM safety and departure brief; we discussed the possibility of windshear on takeoff since there were thunderstorms south and east of airport. There were also windshear advisories on ATIS; so we selected LLWS precautions on aero data. This ensured max thrust and flaps 20 setting for takeoff. We discussed the threat of remembering to ask for flaps 20; and doing a max thrust takeoff in lightweight 757. We also discussed the new procedure of asking for flaps 20 with after start checklist. I advised ramp we needed tug and marshaller; and still needed someone to pickup a copy of the [Dangerous Goods Form]. The mechanic asked us if he could remove chocks and stairs. The Captain confirmed all the doors were closed; the brakes were set; and all the checklists were complete. The Captain told the mechanic he was clear to remove the chocks and stairs; but he forgot about waiting for someone to pickup a copy of the [Dangerous Goods Form].The taxi out was normal. The Captain gave a normal takeoff brief; and pointed out that the flaps were set at 20 and that max thrust was set. He also emphasized the delayed rotation speed and calling for flaps 5 instead of flaps 1. The Captain forgot to call for the Before takeoff checklist; and I missed that he didn't call for it as well. Both of us did visually check to confirm flaps were at 20. The takeoff was uneventful with the delayed rotation. It wasn't long after departure when we had to ask for weather deviations with a very busy controller. The rest of the flight to ZZZ1 was uneventful. After landing at ZZZ1; we taxied in and had to do an 180 on ramp to park on the extra parking spot.When we did the Shutdown/Secure checklist with FMC clearing procedure; the Captain's AFD went blank. It normally still works with just battery power. After completing checklists; the Captain checked the status messages again and saw a L GGU status message. He realized what it was; and confirmed it by looking it up in the 757 MEL. When he looked it up he discovered that it was a no takeoff item. It should have been written up and fixed before leaving ZZZ. We should have been more diligent and looked up each status message to confirm whether it was associated with our flight control PCU MEL item. I will make sure I trust but verify all status messages in the MEL rather than taking the mechanic's word about it. I will also make sure to look for items or events that take me out of my flow in order to trap errors such as the Before Takeoff Checklist. The Captain wrote up the L GGU status message at ZZZ1 and told the mechanic about it. He was able to troubleshoot the problem; and eventually fixed the GGU problem.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.