Date: 201509
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Aircraft: A300
Operator: Air Carrier
Operating under: Part 121
Mission: Cargo / Freight / Delivery
Flight phase: Parked
Aircraft component: Fuel Trim System
Component problem: Improperly Operated
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Powerplant; Maintenance Airframe
Human factors: Distraction; Situational Awareness; Training / Qualification
Second reporter function: Technician
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Maintenance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Maintenance
When detected: Aircraft In Service At Gate
Result: General Maintenance Action
Contributing factors: Human Factors; MEL; Procedure
Primary problem: Human Factors

Narrative: I went down to aircraft to troubleshoot the Captain EFB inoperative. During the troubleshooting of that issue; the trim tank isolation valve pushbutton switch was flashing. I had this same issue only weeks before; and that problem was fixed with a relay. After suspecting the EFB was a bad CPU; I quickly troubleshot the flashing trim tank isolation valve light. I got the relays I had replaced before; and it did not fix the problem. More troubleshooting would be needed. At approximately XA:00; [another technician] and I were doing more detailed troubleshooting and tracked down/replaced several other relays. With no fix yet; I got the lift truck and went up to the area just forward of the APU. [The other technician] cycled the trim tank ISO valve switch several times; and no valve moved; or made noise. I had pulled up to MEL 28-22-54 on an iPad. We decided to MEL the trim tank valve. At approximately XB:30 I began the lock out procedure of the valve. Somewhere in this time; I had focused on one valve that I thought was the correct one. This valve was actually the APU low pressure valve; and 2 feet away with the same actuator on top is the trim tank ISO valve. So; I actually locked out the APU from fuel. The trim tank ISO valve was not locked out; but the circuit breaker was pulled and the system was not operable. At the time; I thought this was the correct valve and I was doing everything right. Looking at the area again today; I see where I made my mistakes. I mistakenly identified and was focused on the wrong valve. Many factors contributed to this event; including my self-imposed time limit of end of my shift; even though the aircraft would sit for another 7 hours; heat; MEL procedures; first time doing job; but mostly frustration. I think there were many factors that contributed to this event. Time constraints were self-imposed by end of my shift. Frustration level was high; heat and humidity were high; simple problem turned into hours of troubleshooting. To prevent this from happening again; if nothing is working; slow down and step back. Take a break and re-assess the problem again. I did not!


Second narrative: Reference MEL trim tank isolation valve inoperative. My part of this procedure included entering the discrepancy. After verbally confirming with the mechanic that all the maintenance procedures for the MEL 'trim tank isolation valve - inoperative' were completed; I contacted Maintenance Control to process the MEL and acquire a control number. I entered all the data and ordered an actuator. I 'did not' physically perform any part of the MEL procedure. This MEL did not have a video attached that may assisted in the mechanic selecting the correct valve.


Synopsis: Maintenance technicians described the conditions and situation that contributed to locking out the incorrect fuel valve while deferring it per the MEL.