Narrative:

I was working the radar; and had numerous aircraft in and out of ZZZ. There was icing reported on the climb/descent for and on the ground (both runway and taxiways) at ZZZ; and I was trying to stay on top of conditions. The last reported conditions from the notams showed that the runway ficon was 1/1/1. Aircraft X was inbound to ZZZ from the south; and I had another aircraft that was southbound; just to aircraft X's south; that had departed from ZZZ. I had this aircraft give aircraft X flight the arrival/ground conditions; and they went back and forth for a few transmissions discussing them; as this other pilot stated that conditions were better than what the NOTAM had stated and had additional information about icing in the air that would affect the aircraft X pilot. The aircraft X pilot appeared to be concerned about the ice both on the ground and in his descent. He continued inbound; level at 9;000ft MSL; and stated that he would like to do an expedited descent into ZZZ due to the ice. I looked at my d-side; and we discussed that this meant to both of us that he would like to descend quickly to avoid the ice; and so I decided I didn't have to give him a descent until close to the last minute. I told the pilot that I would give him a pilot's discretion descent; but that my minimum IFR altitude at the IAF for the ILS runway xx that he was planning to fly; was 3;200ft; and I would not be able to descend him below that until he was on the approach. This was in plain english. He stated that he understood and wouldn't descend below 3200 until he was past [the IAF.] I assumed that since I had not stated 'descend at pilot's discretion; maintain 3;200...' and he had not read back an actual descent clearance; that he knew he was still assigned 9;000ft. The d-side subsequently cleared an aircraft; aircraft Y; out from ZZZ southbound; climbing to 6;000ft. Once he was airborne; I told him about the potential for icing at 6;000 and suggested he climb to 8;000ft. He stated that he would try out 6; but once he got there requested climb to 8;000; which I gave him. The two of them were going to cross with less than 5 miles lateral; but I believed that aircraft X understood that I had not descended him. I was then offered a break; which I took. In the briefing; I pointed out the two aircraft; and explained the situation as I understood it. I put my headset away; and returned to watch the sector for a minute from behind the new radar controller. I told him I had not yet called the traffic; as they were still approximately 10 nm apart; but that he would need to; as I had forgotten that in the recorded briefing. I forget exactly what happened next; but shortly after; we noticed that aircraft X's altitude showed him 300 feet low; and the two aircraft were quickly approaching the 5 mile separation minimum. The new r-side then told him to confirm maintain 9;000; as his altimeter showed him now at 8;600ft and descending. He then turned each of the aircraft 30 degrees to the right; and aircraft X arrested his altitude; which at the lowest was indicating either 8;400 or 8;300ft. Aircraft Y stayed level at 8;000ft. They appeared to be within 3 miles; or approximately at 3 miles lateral separation; because aircraft Y was in a 3-Mile area; so the radar controller put a 3 mile halo on him. Once they were peeled apart; the new controller re-cleared aircraft Y direct to his destination; and aircraft X to [the IAF] and for the ILS runway xx approach. He instructed the d-side to let aircraft X know when he called to cancel his IFR after landing that he had had a possible pilot deviation; as not to make him even more nervous flying through the weather on the approach. I left the area at this point; as the new controller had reestablished adequate separation. I feel bad; because looking back; I can see where I may have inserted confusion into the already complicated weather situation. I guess my biggest regret is that I did not restate that he wasstill to maintain 9;000ft after we had had our discussion about the weather and his intended descent profile. In the future I will just restate the current altitude restriction after any discussion where I may have said something that would give the pilot the idea that he or she has been cleared for a descent.

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Original NASA ASRS Text

Title: ZBW Center Controllers and a pilot involved reported that there was a miscommunication relating to the descent clearance leading to a loss of separation.

Narrative: I was working the Radar; and had numerous aircraft in and out of ZZZ. There was icing reported on the climb/descent for and on the ground (both runway and taxiways) at ZZZ; and I was trying to stay on top of conditions. The last reported conditions from the NOTAMs showed that the runway FICON was 1/1/1. Aircraft X was inbound to ZZZ from the south; and I had another aircraft that was southbound; just to Aircraft X's south; that had departed from ZZZ. I had this aircraft give Aircraft X flight the arrival/ground conditions; and they went back and forth for a few transmissions discussing them; as this other pilot stated that conditions were better than what the NOTAM had stated and had additional information about icing in the air that would affect the Aircraft X pilot. The Aircraft X pilot appeared to be concerned about the ice both on the ground and in his descent. He continued inbound; level at 9;000ft MSL; and stated that he would like to do an expedited descent into ZZZ due to the ice. I looked at my D-side; and we discussed that this meant to both of us that he would like to descend quickly to avoid the ice; and so I decided I didn't have to give him a descent until close to the last minute. I told the pilot that I would give him a pilot's discretion descent; but that my minimum IFR altitude at the IAF for the ILS RWY XX that he was planning to fly; was 3;200ft; and I would not be able to descend him below that until he was on the approach. This was in plain English. He stated that he understood and wouldn't descend below 3200 until he was past [the IAF.] I assumed that since I had not stated 'Descend at pilot's discretion; maintain 3;200...' and he had not read back an actual descent clearance; that he knew he was still assigned 9;000ft. The D-side subsequently cleared an aircraft; Aircraft Y; out from ZZZ southbound; climbing to 6;000ft. Once he was airborne; I told him about the potential for icing at 6;000 and suggested he climb to 8;000ft. He stated that he would try out 6; but once he got there requested climb to 8;000; which I gave him. The two of them were going to cross with less than 5 miles lateral; but I believed that Aircraft X understood that I had not descended him. I was then offered a break; which I took. In the briefing; I pointed out the two aircraft; and explained the situation as I understood it. I put my headset away; and returned to watch the sector for a minute from behind the new radar controller. I told him I had not yet called the traffic; as they were still approximately 10 nm apart; but that he would need to; as I had forgotten that in the recorded briefing. I forget exactly what happened next; but shortly after; we noticed that Aircraft X's altitude showed him 300 feet low; and the two aircraft were quickly approaching the 5 mile separation minimum. The new R-side then told him to confirm maintain 9;000; as his altimeter showed him now at 8;600ft and descending. He then turned each of the aircraft 30 degrees to the right; and Aircraft X arrested his altitude; which at the lowest was indicating either 8;400 or 8;300ft. Aircraft Y stayed level at 8;000ft. They appeared to be within 3 miles; or approximately at 3 miles lateral separation; because Aircraft Y was in a 3-Mile area; so the Radar Controller put a 3 mile halo on him. Once they were peeled apart; the new controller re-cleared Aircraft Y direct to his destination; and Aircraft X to [the IAF] and for the ILS RWY XX approach. He instructed the D-side to let Aircraft X know when he called to cancel his IFR after landing that he had had a possible pilot deviation; as not to make him even more nervous flying through the weather on the approach. I left the area at this point; as the new controller had reestablished adequate separation. I feel bad; because looking back; I can see where I may have inserted confusion into the already complicated weather situation. I guess my biggest regret is that I did not restate that he wasstill to maintain 9;000ft after we had had our discussion about the weather and his intended descent profile. In the future I will just restate the current altitude restriction after any discussion where I may have said something that would give the pilot the idea that he or she has been cleared for a descent.

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.