Narrative:

I was working the radar position; not excessively busy but a little more complex due to the amount of rerouted traffic from the covid-19 closures. Aircraft X; a large transport heading to ZZZ; was already descended to 310; perhaps unnecessarily but to avoid faster traffic climbing behind. Aircraft Y; a medium transport bound for ZZZ1; had recently departed and was being step-climbed beneath traffic in level flight; requesting FL330 for the final. Initially climbed to FL270; I climbed the aircraft to FL300 to level beneath aircraft X and received the correct readback of 300. In the meantime; the d-side replaced the interim altitude with a local interim; showing FL330 beneath; so the aircraft could be placed in handoff status to the adjacent center and the climb could be issued later. Traffic was called when the two aircraft were side-by-side; to allow that aircraft Y understood why he would have to level and in case aircraft X saw him on the 'fish finder.' within moments; aircraft Y reported an RA; and stopped his climb at approximately FL296. I called traffic once again; and the pilot rogered back. I called once again to reiterate whether he was technically responding to the RA. Aircraft Y only reported 'clear;' and I returned with a roger. Shortly thereafter aircraft Y climbed from FL296 to approximately FL303 and I asked him to verify level at FL300; to which he responded in the affirmative. The aircraft; who had reported level; appeared to be continuing [to] climb. I asked him to verify his altimeter and he returned 29.92; and stated something about adjusting vertical rates. Still not clear as to whether he was continuing his response; or whether there was a mode-C aberration (being near the radar site); I was taking the pilot at his word that he reported level and with the proper altimeter. At their closest the aircraft appeared to be ~2 NM apart with 400 feet of separation. When the aircraft were finished separating and I was able to safely issue the climb to FL330; I shipped him to the next center.the conditions of the RA were not clear; to be sure. As to whether the pilot was still responding or was through with the resolution was never properly elaborated; which led to some confusion. The pilot also indicated a vertical rate issue; which has been happening more frequently of late with empty planes climbing quickly. The radar site also being fairly close to the event also leads me to doubt the mode-C. Was that a bad computer extrapolation; or was that the pilot climbing through his assigned altitude which he confirmed multiple times? And with the lack of clarity really; was he still responding to the RA? In short; the RA should probably have not happened. With empty planes and instead non-expectation of correct performance; the aircraft should possibly be separated by 2;000 feet for climbers. Until this coronavirus outbreak is over and we resume normal operations; I know I won't be trying to climb or descend aircraft the way I would under normal circumstances.

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Original NASA ASRS Text

Title: Center Controller and an air carrier First Officer reported an airborne conflict that resulted in an RA and a minimal altitude deviation. The Controller advised caution when climbing and descending aircraft during the COVID-19 pandemic because aircraft performance is not always as expected due to lighter-than-normal loads.

Narrative: I was working the radar position; not excessively busy but a little more complex due to the amount of rerouted traffic from the COVID-19 closures. Aircraft X; a Large Transport heading to ZZZ; was already descended to 310; perhaps unnecessarily but to avoid faster traffic climbing behind. Aircraft Y; a Medium Transport bound for ZZZ1; had recently departed and was being step-climbed beneath traffic in level flight; requesting FL330 for the final. Initially climbed to FL270; I climbed the aircraft to FL300 to level beneath Aircraft X and received the correct readback of 300. In the meantime; the D-side replaced the interim altitude with a local interim; showing FL330 beneath; so the aircraft could be placed in handoff status to the adjacent Center and the climb could be issued later. Traffic was called when the two aircraft were side-by-side; to allow that Aircraft Y understood why he would have to level and in case Aircraft X saw him on the 'fish finder.' Within moments; Aircraft Y reported an RA; and stopped his climb at approximately FL296. I called traffic once again; and the pilot rogered back. I called once again to reiterate whether he was technically responding to the RA. Aircraft Y only reported 'clear;' and I returned with a roger. Shortly thereafter Aircraft Y climbed from FL296 to approximately FL303 and I asked him to verify level at FL300; to which he responded in the affirmative. The aircraft; who had reported level; appeared to be continuing [to] climb. I asked him to verify his altimeter and he returned 29.92; and stated something about adjusting vertical rates. Still not clear as to whether he was continuing his response; or whether there was a Mode-C aberration (being near the radar site); I was taking the pilot at his word that he reported level and with the proper altimeter. At their closest the aircraft appeared to be ~2 NM apart with 400 feet of separation. When the aircraft were finished separating and I was able to safely issue the climb to FL330; I shipped him to the next Center.The conditions of the RA were not clear; to be sure. As to whether the pilot was still responding or was through with the resolution was never properly elaborated; which led to some confusion. The pilot also indicated a vertical rate issue; which has been happening more frequently of late with empty planes climbing quickly. The radar site also being fairly close to the event also leads me to doubt the Mode-C. Was that a bad computer extrapolation; or was that the pilot climbing through his assigned altitude which he confirmed multiple times? And with the lack of clarity really; was he still responding to the RA? In short; the RA should probably have not happened. With empty planes and instead non-expectation of correct performance; the aircraft should possibly be separated by 2;000 feet for climbers. Until this Coronavirus outbreak is over and we resume normal operations; I know I won't be trying to climb or descend aircraft the way I would under normal circumstances.

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.