Narrative:

We were crusing at fl 350 about 15 minutes prior to zzzzz slightly north of airway after being cleared direct to zzzzz from a position prior to ZZZZZ1; talking to center and the controller advised us of traffic 1:30 and 4 miles and 500 ft. Above us. About 10 seconds later the first officer advised me that he had the traffic in sight. The approaching aircraft was behind the center fuselage window frame piece; so I didn't see the aircraft for a few more seconds. I then saw a twin engine military fighter out the first officer's window in a steep left bank turn passing very close to our 1:30 position; and slightly above our altitude. TCAS indicated traffic 400 ft. Above our position. A few seconds later we received a TCAS RA 'descend' command; but the RA voice and vertical speed guidance ended before the first officer could turn off the autopilot. We know that RA's should always be honored; but in this case the advisory ended within a few seconds of when it started. That aircraft was already flying away from us off our right side when the RA sounded. It was impossible to estimate his speed; but in this case the TCAS advisory sounded well after a collision would have occurred had we been a collision course. The first officer indicated the aircraft looked like an F-18. We queried the controller who stated that the aircraft passed within a mile horizontally and 500 ft. Vertically; and he thought it was american (I don't know how he would know its nationality); and that these types of incidents have been occurring for over a month. My guess is that the aircraft was significantly closer than a mile since I could see the seams in the metal panels on the bottom of the aircraft; and it looked as large in the windscreen as B747's we follow on oceanic crossings 1;000 ft. Above when they are almost directly above us.military aircraft flying close to very heavily traveled civilian airways and airspace not talking to controllers. I have no idea if the military aircraft had his transponder on for the entire encounter.more coordination between military aircraft and civilian controllers. Improved TCAS software (if military aircraft had transponder on during entire flight).

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Original NASA ASRS Text

Title: MD-11 Captain reported an NMAC event with a military aircraft flying close to heavily traveled oceanic routes.

Narrative: We were crusing at FL 350 about 15 minutes prior to ZZZZZ slightly north of airway after being cleared direct to ZZZZZ from a position prior to ZZZZZ1; talking to Center and the controller advised us of traffic 1:30 and 4 miles and 500 ft. above us. About 10 seconds later the First Officer advised me that he had the traffic in sight. The approaching aircraft was behind the center fuselage window frame piece; so I didn't see the aircraft for a few more seconds. I then saw a twin engine military fighter out the First Officer's window in a steep left bank turn passing very close to our 1:30 position; and slightly above our altitude. TCAS indicated traffic 400 ft. above our position. A few seconds later we received a TCAS RA 'Descend' command; but the RA voice and vertical speed guidance ended before the First Officer could turn off the autopilot. We know that RA's should always be honored; but in this case the advisory ended within a few seconds of when it started. That aircraft was already flying away from us off our right side when the RA sounded. It was impossible to estimate his speed; but in this case the TCAS advisory sounded well after a collision would have occurred had we been a collision course. The First Officer indicated the aircraft looked like an F-18. We queried the controller who stated that the aircraft passed within a mile horizontally and 500 ft. vertically; and he thought it was American (I don't know how he would know its nationality); and that these types of incidents have been occurring for over a month. My guess is that the aircraft was significantly closer than a mile since I could see the seams in the metal panels on the bottom of the aircraft; and it looked as large in the windscreen as B747's we follow on oceanic crossings 1;000 ft. above when they are almost directly above us.Military aircraft flying close to very heavily traveled civilian airways and airspace not talking to controllers. I have no idea if the military aircraft had his transponder on for the entire encounter.More coordination between military aircraft and civilian controllers. Improved TCAS software (if military aircraft had transponder on during entire flight).

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.