Narrative:

On a march 2013 morning; mechanic Y and I; as mechanic X; took a turnover from graveshift mechanics on the replacement of the #2 engine oil cooler shroud. The installation was done; only remaining steps were to apply firewall sealant and close engine cowling and do operational and leak checks. The cooler was only removed to facilitate maintenance; same serial number cooler re-installed. We read the task for oil cooler installation; and printed it off along with the task for engine leak check. (Paragraph-5D); not realizing we had failed to notice the task for oil pressure check and adjustment (paragraph-5E). I thought we had put the task copies on the aircraft before we pulled out of the hangar; and didn't realize we didn't have them onboard until we were running the engines. In the interest of time; we decided that it was a standard leak check and proceeded with the runs; warming the oil. We had several other checks to do. From reading the task earlier; I incorrectly remembered the oil temperature (temp) called for in the task as being 60 degrees. As I recall; we achieved an oil temp in the low to mid 60's. We missed the part about the 80% nh. We monitored oil pressure being in the normal range as part of standard run procedures. After shutdown; mechanic Y performed a leak check of the cooler; with no leaks noted; then he signed-off the non-routine [write-up/ task card] while I cleaned our mess in the hangar. In-flight enroute; the pilot reported oil pressure fluctuations; then a steady decline in oil pressure along with oil pressure caution light. The engine was not shutdown; and the flight diverted. On-call maintenance reported large quantities of oil on cowling and no oil showing in quantity sight gauge or dipstick. The event occurred because an o-ring was cut on installation of the inboard oil cooler line at the oil cooler. The cooler shroud has sharp edges. The leak did not happen until the aircraft was in-flight; approximately 25-minutes from [my divert airport] is my understanding. I and a fellow mechanic were sent to [my divert airport] to investigate what happened. We began engine removal and when the suspect line was removed; it was noted that the oil o-ring had a slice out of the outer edge. Me and mechanic Y; (who was also in [my divert airport] for the engine replacement); were notified by a supervisor that the flight data recorder (fdr) and or engine monitoring unit (emu) download showed that we had missed a step. We then reviewed the associated task involved and realized the mistakes that had been made. We should have taken the time to shutdown the engines and get the printed task; complacency.

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Original NASA ASRS Text

Title: A Line Mechanic reported a damaged O-Ring seal for an Air/Oil Cooler line on #2 Engine had caused an inflight diversion of a company Q-400 aircraft; due to a steady loss of oil pressure and oil quantity.

Narrative: On a March 2013 morning; Mechanic Y and I; as Mechanic X; took a turnover from graveshift mechanics on the replacement of the #2 Engine Oil Cooler shroud. The installation was done; only remaining steps were to apply firewall sealant and close engine cowling and do Operational and Leak Checks. The cooler was only removed to Facilitate Maintenance; same Serial Number cooler re-installed. We read the task for Oil Cooler Installation; and printed it off along with the task for Engine Leak Check. (Paragraph-5D); not realizing we had failed to notice the Task for Oil Pressure Check and adjustment (Paragraph-5E). I thought we had put the task copies on the aircraft before we pulled out of the hangar; and didn't realize we didn't have them onboard until we were running the engines. In the interest of time; we decided that it was a standard leak check and proceeded with the runs; warming the oil. We had several other checks to do. From reading the Task earlier; I incorrectly remembered the oil temperature (temp) called for in the task as being 60 degrees. As I recall; we achieved an oil temp in the low to mid 60's. We missed the part about the 80% NH. We monitored oil pressure being in the normal range as part of standard run procedures. After shutdown; Mechanic Y performed a Leak Check of the cooler; with no leaks noted; then he signed-off the Non-Routine [Write-up/ Task Card] while I cleaned our mess in the hangar. In-flight enroute; the pilot reported oil pressure fluctuations; then a steady decline in oil pressure along with oil pressure caution light. The engine was not shutdown; and the flight diverted. On-call Maintenance reported large quantities of oil on cowling and no oil showing in quantity sight gauge or dipstick. The event occurred because an O-ring was cut on installation of the inboard oil cooler line at the oil cooler. The cooler shroud has sharp edges. The leak did not happen until the aircraft was in-flight; approximately 25-minutes from [my divert airport] is my understanding. I and a fellow Mechanic were sent to [my divert airport] to investigate what happened. We began engine removal and when the suspect line was removed; it was noted that the oil O-ring had a slice out of the outer edge. Me and Mechanic Y; (who was also in [my divert airport] for the engine replacement); were notified by a Supervisor that the Flight Data Recorder (FDR) and or Engine Monitoring Unit (EMU) download showed that we had missed a step. We then reviewed the associated task involved and realized the mistakes that had been made. We should have taken the time to shutdown the engines and get the printed task; complacency.

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.