Narrative:

I was assigned to aircraft X; a 787 coming in from ZZZZ going back to ZZZ. Upon arrival it required an ETOPS pre-departure check; engine oils serviced; and an MEL for cabin interphone inoperative to be cleared and [a] restoration [of] the aircraft's [maintenance release]. As soon as the aircraft arrived at gate xx; maintenance received a call from ramp notifying us that there was something sticking out of from the left-hand wing just above #1 engine pylon. I knew they were talking about the speed tape that tech support and I installed for a write-up we cleared earlier that day.routinely; I go directly to the flight deck to do my ETOPS inspections and take note of engine oil quantity. Per the task card; everything was in order and I radioed my lead asking for a lift and some speed tape for the wing. While waiting for him; I ran some tests and cycled various circuit breakers in an attempt to clear [the] cabin interphone message on the center aisle tuning control panel (tcp). The message did not go away and I didn't have enough ground time to troubleshoot further. I would have to 're-defer' this MEL later.once I finished the inside; I went downstairs when my lead called me over. He was on the lift and told me he will return with the wider roll of speed tape. I said okay; I will join after I have accomplished servicing the engine oils and doing my walk-around. I grabbed the 3 engine oil cans I needed and continued to service the #1 engine which only needed 1 qt. After filling up the #1 engine I moved on to engine #2 which needed 2 qts. Once finished; I stored the used cans and did my walk-around. Finally; I went back upstairs to the flight deck to ensure the engine oil levels were sufficient and they were.while in the flight deck; I planned on waiting for the flight crew so I could brief them about the inoperative cabin interphone message on the tcp that I was going to 're-defer.' in the meantime; I decided to keep attempting to troubleshoot. It was my first experience with the 787 dealing with a system fault which failed to show up on the EICAS status page. Usually any little discrepancy would be displayed on the status page. My theory is perhaps it was because the interphone was still functional. I kept trying to think of anything I could do to just get rid of the inoperative message on the tcp.my time was up when the flight crew arrived. After briefing them about the issue; I joined my lead who just finished up with the wing. He had me go up on the lift to verify the speed tape installation and it was satisfactory. We finished up at the gate and returned back to the office. I signed off all of the assigned work for the aircraft. [The] ZZZ planner assisted us with the 're-defer' on the MEL and I issued the aircraft's [maintenance release]. Later on that day; I received a phone call from my manager informing me that aircraft X had to divert to ZZZ1 due to excessive loss of oil from the #2 engine. It was [in] that moment [that] I realized I may have been too distracted from the other workload [to] properly secure the filler cap on the oil tank.

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Original NASA ASRS Text

Title: Technicians reported a flight that lost an excessive amount of oil in-flight due to a missing oil cap.

Narrative: I was assigned to Aircraft X; a 787 coming in from ZZZZ going back to ZZZ. Upon arrival it required an ETOPS pre-departure check; engine oils serviced; and an MEL for CABIN INTERPHONE INOP to be cleared and [a] restoration [of] the aircraft's [Maintenance Release]. As soon as the aircraft arrived at Gate XX; Maintenance received a call from Ramp notifying us that there was something sticking out of from the left-hand wing just above #1 Engine Pylon. I knew they were talking about the speed tape that Tech Support and I installed for a write-up we cleared earlier that day.Routinely; I go directly to the Flight Deck to do my ETOPS inspections and take note of engine oil quantity. Per the Task Card; everything was in order and I radioed my Lead asking for a lift and some speed tape for the wing. While waiting for him; I ran some tests and cycled various circuit breakers in an attempt to clear [the] CABIN INTERPHONE message on the center aisle Tuning Control Panel (TCP). The message did not go away and I didn't have enough ground time to troubleshoot further. I would have to 're-defer' this MEL later.Once I finished the inside; I went downstairs when my Lead called me over. He was on the lift and told me he will return with the wider roll of speed tape. I said okay; I will join after I have accomplished servicing the engine oils and doing my walk-around. I grabbed the 3 engine oil cans I needed and continued to service the #1 Engine which only needed 1 QT. After filling up the #1 Engine I moved on to Engine #2 which needed 2 QTS. Once finished; I stored the used cans and did my walk-around. Finally; I went back upstairs to the Flight Deck to ensure the engine oil levels were sufficient and they were.While in the Flight Deck; I planned on waiting for the flight crew so I could brief them about the INOP CABIN INTERPHONE message on the TCP that I was going to 're-defer.' In the meantime; I decided to keep attempting to troubleshoot. It was my first experience with the 787 dealing with a system fault which failed to show up on the EICAS Status Page. Usually any little discrepancy would be displayed on the status page. My theory is perhaps it was because the interphone was still functional. I kept trying to think of anything I could do to just get rid of the INOP message on the TCP.My time was up when the flight crew arrived. After briefing them about the issue; I joined my Lead who just finished up with the wing. He had me go up on the lift to verify the speed tape installation and it was satisfactory. We finished up at the gate and returned back to the office. I signed off all of the assigned work for the aircraft. [The] ZZZ Planner assisted us with the 're-defer' on the MEL and I issued the aircraft's [Maintenance Release]. Later on that day; I received a phone call from my Manager informing me that Aircraft X had to divert to ZZZ1 due to excessive loss of oil from the #2 Engine. It was [in] that moment [that] I realized I may have been too distracted from the other workload [to] properly secure the filler cap on the oil tank.

Data retrieved from NASA's ASRS site 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.