Date: 201906
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Parked
Aircraft component: Elevator
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Confusion; Training / Qualification; Communication Breakdown
Second reporter function: Technician
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Maintenance
When detected: Routine Inspection
Result: General Maintenance Action
Contributing factors: Manuals; Procedure; Chart Or Publication; Human Factors
Primary problem: Human Factors

Narrative: We were tasked with removing the Right Hand elevator assembly from Aircraft X due to a lightning strike. We removed the assembly and on the work bench transferred the trim tab and associated hardware to the new elevator assembly. We then turned over the installation of elevator to [the day shift].At the beginning of our shift [the next day] we took a turn over from [evening shift]. Evening [shift] had installed the elevator and told us that a bushing was being AOG'd (Aircraft On Ground) to install in the #2 hinge because it was missing or misplaced. After we received the bushing and installed it the Inspector asked us if we or the previous shift had verified the trim tab balance weights. We confirmed that we didn't nor did we know if the previous shifts had verified the correct weights. The Inspector then requested that we re-open the #2 balance bay panel to verify. After open up the Inspector and aircraft maintenance technicians confirmed that we were taking off the correct weights per the AMM (Aircraft Maintenance Manual) illustration.We then accessed the weights and noticed that it had 3 locations with 3 weights each. The old elevator had 3 locations with 2 weights each. So we had to remove a total of three weights from the new elevator. We then closed up the access panel and proceeded on with operational checks. It was discovered [one week later] when working a Left Hand elevator remove and replace on [another aircraft] that the incorrect weights were removed from the elevator on Aircraft X [serviced the week before]. The Inspector and aircraft maintenance technicians noticed that we had removed elevator balance weights rather than trim tab balance weights which are adjacent to them on the lower side of Aircraft X. Contributing factors are aircraft maintenance technicians working on an elevator remove and replace for the first time. The AMM illustration was misinterpreted which caused the incorrect weights to be removed. Create a [Maintenance Task] so that procedures aren't missed in the future.


Second narrative: We removed the Right Hand elevator the night previous to the incident and transferred the trim tab to the replacement elevator and turned the job over to the day shift. At the beginning of the shift I was assigned to install the right hand elevator with my coworkers. Inspector requested that we open up and check the weights for the trim tab. At this point the day shift had already installed the access panel to the #2 Balance Bay. Once we removed the panel we inspected for weights and did not see any at which point the Inspector suggested that they were on top. So we gained access to the other side of the elevator. The Inspector found that the incorrect weights were installed and that we should match the weight from the removed elevator. One weight was removed from each stack to make a total of three weights removed.At this point the Inspector was satisfied; we closed up and we carried on with the rest of the tasks. Where I made my mistake was I assumed that the people that I was working with were using the manual and that the correct procedures were being followed. I did not ask if this was the case. It turns out the wrong weights were replaced and that the proper balancing procedure was not accomplished. This was discovered the following week by the same Inspector on the same job but on a different aircraft when he observed the mechanics doing the proper balancing procedure.I believe this incident could have been avoided if a Maintenance Task existed for this task. There are a lot of steps that need to happen to remove and install an elevator; as well as a lot of people involved. This leads to confusion as to how to break down the job using [Maintenance] forms.


Synopsis: Air carrier Maintenance Technicians reported incorrectly removing wrong balance weights from elevator on B737-800 aircraft.