Maintenance Airframe; Maintenance Powerplant
Aircraft Equipment Problem Critical; Deviation / Discrepancy - Procedural MEL / CDL; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Departure Airport
Human Factors
I was assigned to research/troubleshoot/fix an overboard cool fail caution message. I un-deferred [reactivated] the auto-pressurization system per MEL 21-24-6b. But in the time I had available; I could not duplicate the original discrepancy or determine what would have caused it. I re-deferred the system; described the troubleshooting steps I had done; and let the crew-leader carry the discrepancy forward to Maintenance Control. The next morning the pilots could not pressurize the aircraft in-flight and had to return to the airport. Day-shift mechanics found the overboard exhaust shutoff valve safety-wired in the OPEN position instead of closed as the MEL requires. Cause: Other than the research I had done that night; I was not otherwise familiar with the system. So before touching the airplane; I read the MEL 21-24-6b and the referenced Aircraft Maintenance Manual (AMM) sections (tasks 21-24-00-040-806 and 21-24-00-040-808). On my notepad; I wrote that deactivating the overboard shutoff valve required it to be safety-wired in the OPEN position; and that deactivating the inboard shutoff valve also required the OPEN position. It is very clear on the MEL in the remarks section and the maintenance procedure section that the overboard exhaust shutoff valve must be CLOSED; so I am not entirely sure why I wrote it down in my notepad incorrectly. I do clearly remember reading the AMM references. Looking at it now; I see the AMM mentions both; but the 'closed' wording is on the second page. I must have missed that in my quick reading of the AMM and just seen the 'open' on page 1. At the time; though; I believed my notes were correct; since I had written them right out of the AMM. Since I had all the information I needed from the AMM; I did not take note of what position the valves were in before cutting the safety wire and un-deferring the system. After troubleshooting; I was unable to find the original cause of the discrepancy; but instead found a similar message: INBD COOL FAIL status message came on when passenger door closed and service door open. Later; I found that this message was by design; so I should have signed off the discrepancy as could not duplicate. But at the time; I thought there must be something wrong with the passenger door micro switches. We were out of time; so I pulled and tagged circuit breakers and returned the valves to the deferred positions. I had my notepad with me in the avionics compartment and was diligent to check the valves in the mirror and verify they were both in the OPEN position as my notes required before safety-wiring them there. As I was finishing this; I learned that the airplane had to be back at the terminal. I chose to not do the pressurization check; as I felt I was only putting the airplane back in the same configuration it came in with. This is where I did not fully understand the deferral process. I did not understand that I was doing an entirely new deferral process; separate from the previously deferred status. So I thought of the pressurization check as something that was good to do; not required. As we were already late going to the gate; I chose to skip it. But if I had done the pressurization check; it would have been obvious that the valve was in the wrong position. Apparently; there was a miscommunication between the Crew Leader and I over whether I was done with the aircraft; since as I was closing up the avionics bay door; mechanics were already there to take the airplane back to the terminal. We were behind schedule. The discrepancy had already been signed by the Crew Leader and carried forward to Maintenance Control before I could double-check the corrective action I had written previously. This is why at the beginning there is a name and a random part number. This was a note from a fellow mechanic to me; which I was going to remove before I signed off the discrepancy. But before I could clean up and double-check the text; the discrepancy was already signed off; and the aircraft released. I was out of routine. This incident could have been easily avoided; had I done the pressurization check. I have learned that every step in the AMM and MEL are important. And those steps that seem unnecessary are usually necessary as a cross-check; even if for no other reason. Double-checking my notes would also have helped; since regardless of my misreading the AMM; it would have been obvious from other available paperwork that my notes were wrong. This would have been less likely to happen if I had been working with a trainer or another mechanic who knew the system better. But I was overconfident in my ability to figure out a problem on my own. This was partly an overreaction to my 3-month probationary evaluation where I was told verbally that I was not accomplishing enough work each day; and was asking too many questions. Ever since then; I have been attempting to only ask the trainer for help once I have explored all other options. Incidents are usually a chain of events; and this one certainly was. I recognize the gravity of the situation that occurred; have researched how I believe I made the mistake; and have taken steps to prevent this from happening again.
Following troubleshooting; the AMT deactivated the system again; but safety wired the valve in the incorrect position.