Date: 201212
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Light: Night
Aircraft: A321
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Aircraft component: Engine Air Anti-Ice
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Lead Technician
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Training / Qualification; Confusion; Communication Breakdown; Situational Awareness
Second reporter function: Technician
Anomaly: Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: Pre-flight
Result: General Maintenance Action
Contributing factors: Aircraft; Chart Or Publication; Human Factors; MEL; Procedure
Primary problem: Human Factors

Narrative: Aircraft X is the aircraft involved in event. Mechanic Y and Z were the Mechanics assigned to the aircraft that night. I was the Lead Mechanic assisting with the troubleshooting. This event occurred at night; December 2013 and into the following morning. The aircraft was parked in the remote parking location adjacent to Gate 38. Aircraft X had an MEL on the Number 2 Engine Anti-Ice Valve which was to be locked 'Open' for revenue service. During troubleshooting of problem; Valve was unlocked and allowed to close to the normally 'Closed' position. We performed the troubleshooting without issue. The troubleshooting consisted of an operational check of the valve and wiring checks. After Troubleshooting had ended; Mechanics Y and Z were instructed to lock valve 'Open' and close Fan cowls in preparation for service. The aircraft flew revenue service for four legs when the problem was discovered. The Anti-Ice valve was not locked 'Open' as prescribed per the MEL.  After landing in ZZZ1; the pilot wrote in the Logbook his observations of the aircraft's performance and questioned it in comparison to the MEL procedure. Maintenance personnel in ZZZ1 looked into the issue as the aircraft was scheduled for Maintenance that night at ZZZ1. It was at this time that ZZZ1 Maintenance discovered the valve not locked 'Open' per MEL requirement. I believe that a lack of experience and knowledge on behalf of Mechanics Y and Z played a role in the incident. Another cause was my trust in Mechanics Y and Z to accomplish the work properly. I did not check the work of less experienced mechanics to ensure the MEL was properly restored. To my knowledge; ZZZ1 Maintenance locked the Anti-Ice Valve in the 'open' position to comply with MEL procedure and signed-off the Logbook write-up concerning the Anti-Ice valve. Would [recommend] to [do] better to oversee Maintenance as a Lead Mechanic to ensure proper maintenance is performed. Also; make sure the mechanics understand better what procedures must take place when working with MELs.


Second narrative: Aircraft X had an MEL 30-21-01 deferral we were troubleshooting. Upon completion of troubleshooting I; Mechanic Y; was task with returning the aircraft to its MEL configuration; per MEL 30-21-01. The Anti-Ice Valve was to be locked out in the 'Closed' position and safety pinned. This is the procedure I followed. There was only myself and Mechanic Z outside performing the closeup.The aircraft flew several short legs. The outgoing flight crew did not notice an issue; but the oncoming crew noticed and made a Logbook entry. There was no other Technician present who had Airworthiness [sign-off experience] on the aircraft. The next night; the aircraft was a [scheduled] Routine Overnight (RON) and they were able to further troubleshoot and repair the issue. [Recommend] having another Technician who has the Airworthiness [qualifications] who can also verify the work that was performed.


Synopsis: A Lead Mechanic and Line Mechanic report about a Number 2 Engine Anti-Ice Valve that was not locked-out in the 'Open' position as required for an MEL deferral on an Airbus aircraft.