Narrative:

That day may 2012; I worked alone. I catch one inbound and push out the layover. There are usually work assignments on the layover. In this case it was an A300 aircraft. After pushing my outbound; I did a service check and four other [task] cards that were assigned. I received a nacelle (nac) cooling valve in the [onboard aircraft parts] box for the MEL's also assigned. It was a valve for the spray bar type cooling system. I looked up the aircraft maintenance manual (amm) and looked at the MEL's. The MEL's confused me about which side was inoperative I went out to the aircraft and opened the #1 engine inboard thrust reverser. This side has the poppet system; and doesn't have the same type of valve I had in the box I received. I looked around at the components and did not see anything wrong with them. The valve for this type is all the way on top; and I guess I just didn't look hard enough at it. This is where I messed up! I closed this side and opened the outboard thrust reverser. This side has the spray bar type with the same valve I received. This one was locked 'open' and I replaced it. I closed up the engine and ran it. The ECAM message 'nac cooling valve open' did not come on before; during or after shutdown. I thought I had fixed the problem. The same aircraft come back to ZZZ three more times and took a total of ten flights over the following six days. I do not understand how the aircraft flew that many flights and the ECAM message never came back? That message came on the ECAM that day; after the engines [were] shut down upon arrival here. How come it didn't come back for six days?I would think that my lack of experience with the MEL's had a lot to do with it. I've only been here about three months and coming from the hangar; it's a very different world. I understand now what I did. I did not understand that (both) valves; one on each side were locked 'open'. I did not notice the inboard valve was locked 'open'; but only saw the outboard side was. Plain and simple; I just didn't understand the MEL.the actual MEL's 75-46-01-xx-B1 and 75-46-01-xx-A1 are hard for me to understand. Others here said the same thing; when I read it to them. Maybe someone could look into that and see if it could be written better? The picture diagram of the poppet system doesn't show the valve at all. A picture of it would really help. It's absolutely my fault for this incident! What I have learned is that I have to learn the MEL's and everything about them! Again; I admit fault on my part. I did not clearly understand the two different MEL's and should have looked harder at the inboard poppet system valve. Vendor mechanic found valve locked 'closed' six days later; no event happened.

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Original NASA ASRS Text

Title: A Line Mechanic reports that his lack of experience with understanding two deferrals for an A300 aircraft contributed to his changing only the Engine Nacelle Cooling Valve on the outboard Reverser half of #1 engine; even though both Cooling valves were locked 'Open'.

Narrative: That day May 2012; I worked alone. I catch one inbound and push out the layover. There are usually work assignments on the layover. In this case it was an A300 aircraft. After pushing my outbound; I did a Service Check and four other [Task] cards that were assigned. I received a Nacelle (NAC) Cooling Valve in the [onboard aircraft parts] box for the MEL's also assigned. It was a valve for the Spray Bar Type Cooling System. I looked up the Aircraft Maintenance Manual (AMM) and looked at the MEL's. The MEL's confused me about which side was inoperative I went out to the Aircraft and opened the #1 Engine Inboard Thrust Reverser. This side has the Poppet System; and doesn't have the same type of valve I had in the box I received. I looked around at the components and did not see anything wrong with them. The valve for this type is all the way on top; and I guess I just didn't look hard enough at it. This is where I messed up! I closed this side and opened the Outboard Thrust Reverser. This side has the Spray Bar Type with the same valve I received. This one was locked 'Open' and I replaced it. I closed up the engine and ran it. The ECAM message 'Nac Cooling Valve Open' did not come on before; during or after shutdown. I thought I had fixed the problem. The same aircraft come back to ZZZ three more times and took a total of ten flights over the following six days. I do not understand how the aircraft flew that many flights and the ECAM message never came back? That message came on the ECAM that day; after the engines [were] shut down upon arrival here. How come it didn't come back for six days?I would think that my lack of experience with the MEL's had a lot to do with it. I've only been here about three months and coming from the Hangar; it's a very different world. I understand now what I did. I did not understand that (both) valves; one on each side were locked 'Open'. I did not notice the Inboard valve was locked 'Open'; but only saw the Outboard side was. Plain and simple; I just didn't understand the MEL.The actual MEL's 75-46-01-XX-B1 and 75-46-01-XX-A1 are hard for me to understand. Others here said the same thing; when I read it to them. Maybe someone could look into that and see if it could be written better? The picture diagram of the Poppet System doesn't show the valve at all. A picture of it would really help. It's absolutely my fault for this incident! What I have learned is that I have to learn the MEL's and everything about them! Again; I admit fault on my part. I did not clearly understand the two different MEL's and should have looked harder at the Inboard Poppet System valve. Vendor Mechanic found valve locked 'Closed' six days later; no event happened.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.