Date: 202004
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: FCU (Flight Control Unit)
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Powerplant; Maintenance Airframe
Human factors: Time Pressure; Training / Qualification; Workload
Second reporter function: Technician
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Published Material / Policy; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural FAR
Detector: Person Maintenance
When detected: Other Post Service
Result: General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Chart Or Publication; Manuals; Procedure; Aircraft
Primary problem: Chart Or Publication

Narrative: I was assigned to work on Aircraft X at ZZZ (hangar) with my co-worker. We were performing ECO. Coworker printed out the ECO [Engineering Change Order] work card; we gathered required equipment; went to the aircraft and performed ELAC software update. The second portion of the ECO was to disconnect 2 wires from the back of the rack of each ELAC. We read the procedure from ECO together; accomplished the task; performed tests; upgraded the cards and after every test passed closed it all up and went to production to sign the work card. We realized that the second portion of the work card (wiring disconnecting) is not printed out for us to sign. We asked for it to be printed. Promptly signed it and went home. Next day my coworker performed identical work card again and found on the last page of the ECO that effectivity for his aircraft is not applicable for wiring removal from the ELAC's. He re-checked the effectivity for Aircraft X we worked on the day before and realized that the wire removal procedure (second part of the ECO) was not applicable for Aircraft X either. He promptly informed [Maintenance Control] and Aircraft X was taken out of service.  The event occurred because the effectivity page was found towards the end on the last page of the ECO after all the work steps were already performed and we both missed it. Usually effectivity is applied before printing work cards and it is found in the beginning of the work steps. The work-card has many pages and none of us ever performed such procedure before. Raining conditions and being short on time didn't benefit us either.I suggest to move the effectivity page towards the front of this ECO. We will make sure to double or triple check every step of the work card in the future even if it takes much longer. Safety is my priority and there is no room for unsafe events like this to be repeated.


Second narrative: Working on ECO [Engineering Change Order] on Aircraft X; the ECO have 2 parts ; one part is to install software on ELAC 1&2 and the second part of the ECO is to remove ELAC wiring associated with the VMO/MMO overshoot; but Aircraft X is not one of the aircraft effectivity to perform part 2 of the ECO. On the ECO paper work the last page shown the effectivity of the aircraft and but did not show ..aircraft effectivity on the front page of work card 0002 for removing the wire.  Was overlook by mistake. Did not pay close attention every page of the paper work. Need to put the effectivity of aircraft on the front page of the ECO paper work where it will be notice.


Synopsis: Technicians reported misreading an ECO; resulting in non compliance with established procedures.