Date: 202010
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Weather elements / visibility: Rain
Aircraft: B737-900
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Aircraft component: Ice/Rain Protection System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter qualification: Maintenance Airframe; Maintenance Powerplant
Human factors: Distraction; Situational Awareness; Time Pressure; Workload
Second reporter function: Lead Technician
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Diverted; General Maintenance Action
Contributing factors: Aircraft; Human Factors; MEL; Weather
Primary problem: Human Factors

Narrative: Aircraft returned to gate with the right engine anti-ice valve INOP; with a history. Performed deferral procedures per MEL (Minimum Equipment List) 30-00-XX. MEL requires valve to be locked open and pressure switch connector removed. I inadvertently removed the TAI (Thermal Anti-Ice) valve connector instead of the pressure switch connector when I misinterpreted the figure in the MEL procedure. Partly due to a distraction from rain and hearing radio communication that a light extinguished when the connector was removed. Adding to the distraction and pressure to return the aircraft to service; was the requirement under this MEL 30-00-XX that an MEL and lockout of the engine HP (High Pressure) valve is necessary per MEL 36-00-XY. While performing the HP MEL; the lockout collar was stuck and took added time to resolve. The aircraft was then dispatched after maintenance was complete. After takeoff; the crew reported the right engine TAI over pressure indicator illuminated due to the pressure switch still connected. Flight Dispatch requested the aircraft divert to ZZZ to have the situation inspected by Maintenance. The proper MEL procedure was performed and corrected the mistake. The flight continued to ZZZ1. The MEL has this switch deactivated so as to not experience a nuisance light indicting an over pressure indication which is going to happen since the valve is locked open per the MEL. Having the valve connector off removed the valve position and control signal but also since the valve is locked open this did not pose an issue. I regret not taking the extra time to review the MEL to be sure I understood the procedure. Also taking extra time to discuss the situation with a colleague so we both agree with the MEL procedures. I would like to note that the MEL figure showing the anti-ice valve and pressure switch did not have a marker for the connector with a label number or reference.


Second narrative: Aircraft returned to block with right engine TAI (Thermal Anti-Ice) valve inoperative. Due to history; deferred to MEL (Minimum Equipment List) 30-00-XX. MEL requires valve locked open and pressure switch connector removed. I inadvertently removed valve position connector instead of pressure switch plug; which are right near each other. The aircraft experienced an overpressure light enroute and diverted. The problem was corrected and continued on deferral. I must take more time to carefully examine the MEL and AMM (Aircraft Maintenance Manual) pictures and plug labels. Factors were light rain and dim lighting. Need to not be in a hurry on flight delays.


Synopsis: Maintenance Technician performed incorrect Minimum Equipment List deferral procedure.