Date: 202003
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Aircraft: Large Transport
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Medium Transport
Reporter organization: Government
Reporter function: Enroute
Human factors: Communication Breakdown; Situational Awareness
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance
Detector: Automation Aircraft RA
When detected: In-flight
Result: Flight Crew Took Evasive Action; Flight Crew Returned To Clearance
Contributing factors: Human Factors
Primary problem: Human Factors

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.


Second narrative: While flying from ZZZ to ZZZ1 we received a Resolution Advisory (RA). I was serving as the First Officer and Pilot Monitoring of this flight. Between ZZZZZ and ZZZZZ1; we received a clearance from Center to climb to FL300. At approximately 29;000 feet we received an RA. The Captain (who was the Pilot on the Controls) followed the RA PFD guidance to remain within the green area. I advised ATC that we received an RA and that we were deviating to remain clear of the other aircraft. We were IMC during this entire event and could not see the other aircraft; who was above us. We then received the clear of conflict announcement. Once clear of the RA; we reengaged autopilot and continued our climb to 30;000 feet. I advised ATC that we were clear. The Captain transferred the controls to me. The altitude was set for FL300; but the autopilot captured it at I believe 30;300 feet. I input a VS descent to bring the aircraft to FL300. The flight continued without further event.We received an RA that required deviation by following the RA PFD guidance. After the RA event; the autopilot captured the altitude above the clearance level of FL300.We could have leveled off manually to prevent the automation from busting an altitude.


Synopsis: 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.