Narrative:

Aircraft X was enroute; level at 13000. Aircraft Y was enroute level at 12000 with pilot's discretion descent to 6000. The aircraft were on opposite direction routes. I quoted traffic to both aircraft when they were approximately 20 nm apart. Aircraft Y acknowledged the traffic quote but aircraft X did not. As the aircraft were nearing 10 nm apart I went to quote traffic again to aircraft X and at that moment I noticed aircraft X's mode C readout was 12600. I feared aircraft X had taken the previous traffic quote as a descent clearance and instructed aircraft X to maintain 13000. He informed me that he was unable to maintain altitude due to 'moderate turbulence and severe downdrafts'. I issued a traffic alert to aircraft X and a thirty degree left turn then issued a thirty degree left turn to aircraft Y. These actions did not maintain standard IFR separation. After separation was re-established I cleared each aircraft back on course. I queried aircraft X as to whether or not he had heard my first traffic quote. He replied that he had but was unable to hold altitude due to the turbulence and downdrafts. He stated that his autopilot had disengaged and he was struggling to get back to 13000. He also stated that it was impossible to hold altitude for a few seconds. The pilot of aircraft Y concurred with the flight conditions that aircraft X reported. I entered the PIREPS into erids (enroute information display system) and asked both pilots if they had each other in sight and if they had any TCAS resolutions or advisories from their onboard equipment. Both pilots replied negative on both accounts. I requested flight strips on both aircraft and reported the incident to my supervisor.this event occurred on a midnight shift. I was completely unaware of the flight conditions that the aircraft encountered. Most aircraft I had worked up to that point had been enroute jet aircraft at high altitude and jet aircraft climbing out of sfo; oak; sjc and smf airports and none had reported anything worse than light turbulence. I worked one light twin in the area where the event occurred at 14000 and lower approximately 4 hours prior to the event and the pilot had no issues with turbulence. The cwsu (center weather service unit) was closed at the time of the event. I believe that had the cwsu been open; they may have been able to warn of the potential for the flight conditions that these two aircraft encountered; and I would have warned the aircraft and used more than 1000 ft separation because the aircraft were on opposite direction courses. I do not recall any sigmets or airmets in effect for my geographic area at the time of the event.neither pilot informed me of any damage to crew nor aircraft; however I did not specifically ask them. Due to the nature of the turbulence reported; I should have made a better effort to ascertain that all crew and passengers were not injured; aircraft were not damaged and that no further assistance was needed from either pilot.

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Original NASA ASRS Text

Title: ZOA Center Controller reported a loss of separation when an aircraft was unable to maintain altitude due to moderate turbulence and severe downdrafts.

Narrative: Aircraft X was enroute; level at 13000. Aircraft Y was enroute level at 12000 with pilot's discretion descent to 6000. The aircraft were on opposite direction routes. I quoted traffic to both aircraft when they were approximately 20 nm apart. Aircraft Y acknowledged the traffic quote but Aircraft X did not. As the aircraft were nearing 10 nm apart I went to quote traffic again to Aircraft X and at that moment I noticed Aircraft X's mode C readout was 12600. I feared Aircraft X had taken the previous traffic quote as a descent clearance and instructed Aircraft X to maintain 13000. He informed me that he was unable to maintain altitude due to 'moderate turbulence and severe downdrafts'. I issued a traffic alert to Aircraft X and a thirty degree left turn then issued a thirty degree left turn to Aircraft Y. These actions did not maintain standard IFR separation. After separation was re-established I cleared each aircraft back on course. I queried Aircraft X as to whether or not he had heard my first traffic quote. He replied that he had but was unable to hold altitude due to the turbulence and downdrafts. He stated that his autopilot had disengaged and he was struggling to get back to 13000. He also stated that it was impossible to hold altitude for a few seconds. The pilot of Aircraft Y concurred with the flight conditions that Aircraft X reported. I entered the PIREPS into ERIDS (Enroute Information Display System) and asked both pilots if they had each other in sight and if they had any TCAS resolutions or advisories from their onboard equipment. Both pilots replied negative on both accounts. I requested flight strips on both aircraft and reported the incident to my supervisor.This event occurred on a midnight shift. I was completely unaware of the flight conditions that the aircraft encountered. Most aircraft I had worked up to that point had been enroute jet aircraft at high altitude and jet aircraft climbing out of SFO; OAK; SJC and SMF airports and none had reported anything worse than light turbulence. I worked one light twin in the area where the event occurred at 14000 and lower approximately 4 hours prior to the event and the pilot had no issues with turbulence. The CWSU (Center Weather Service Unit) was closed at the time of the event. I believe that had the CWSU been open; they may have been able to warn of the potential for the flight conditions that these two aircraft encountered; and I would have warned the aircraft and used more than 1000 ft separation because the aircraft were on opposite direction courses. I do not recall any SIGMETS or AIRMETS in effect for my geographic area at the time of the event.Neither pilot informed me of any damage to crew nor aircraft; however I did not specifically ask them. Due to the nature of the turbulence reported; I should have made a better effort to ascertain that all crew and passengers were not injured; aircraft were not damaged and that no further assistance was needed from either pilot.

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.