Enroute controller reported a possible loss of separation but only after the suspected conflict was ...

Narrative:

I was working a combined position with sector 10; 11; and 12 combined at 12. At approximately XA50 local I got the handoff on aircraft X a ZZZ arrival at FL390 that was in cruise; but had to be descended beneath two aircraft aircraft Y at FL360 and an aircraft Z aircraft at FL380. I issued aircraft X a crossing restriction to miss both pieces of traffic; 'cross ott at FL350'; but later corrected this to 'cross 10 miles south of ott at FL350' to over separate aircraft X from aircraft Y and aircraft Z. Aircraft X's pilot reported out of flight level 390 and was started his descent. I rogered this and continued my duties. Over the course of my next coupe of scans and saw that separation was obtained for aircraft X with aircraft Y and aircraft Z as aircraft X leveled at FL350 complying with the restriction I had given him. However; for his descent I used a interim altitude and could see his mode C altitude. Between ott and paleo; where the suspected loss occurred; aircraft X's reported mode-C altitude varied between 350 and 351. This is a common occurrence and I disregarded it was a equipment malfunction. When aircraft Y; still at FL360; got within 5 miles of aircraft X; aircraft X's reported mode-C jumped to 351 and caused a suspected loss of separation which I only became aware of after reviewing the falcon replay. I did not verify aircraft X's altitude to see if he was level at FL350 or if it was an equipment malfunction. I cannot make a recommendation for this event because I feel my procedures were correct and the restriction I gave to aircraft X ensured separation with both of the conflictions. The fluctuating mode-C altitude; in my opinion; was the cause of the separation error and it just a fault of the system between the transponder on the aircraft and the receiver on the ground.

Original NASA ASRS Text

Title: Enroute controller reported a possible loss of separation but only after the suspected conflict was identified after a reviewed on the FALCON replay system.

Narrative: I was working a combined position with sector 10; 11; and 12 combined at 12. At approximately XA50 local I got the handoff on Aircraft X a ZZZ arrival at FL390 that was in cruise; but had to be descended beneath two aircraft Aircraft Y at FL360 and an Aircraft Z aircraft at FL380. I issued Aircraft X a crossing restriction to miss both pieces of traffic; 'Cross OTT at FL350'; but later corrected this to 'Cross 10 miles south of OTT at FL350' to over separate Aircraft X from Aircraft Y and Aircraft Z. Aircraft X's pilot reported out of flight level 390 and was started his descent. I rogered this and continued my duties. Over the course of my next coupe of scans and saw that separation was obtained for Aircraft X with Aircraft Y and Aircraft Z as Aircraft X leveled at FL350 complying with the restriction I had given him. However; for his descent I used a interim altitude and could see his mode C altitude. Between OTT and PALEO; where the suspected loss occurred; Aircraft X's reported Mode-C altitude varied between 350 and 351. This is a common occurrence and I disregarded it was a equipment malfunction. When Aircraft Y; still at FL360; got within 5 miles of Aircraft X; Aircraft X's reported Mode-C jumped to 351 and caused a suspected loss of separation which I only became aware of after reviewing the FALCON replay. I did not verify Aircraft X's altitude to see if he was level at FL350 or if it was an equipment malfunction. I cannot make a recommendation for this event because I feel my procedures were correct and the restriction I gave to Aircraft X ensured separation with both of the conflictions. The fluctuating Mode-C altitude; in my opinion; was the cause of the separation error and it just a fault of the system between the transponder on the aircraft and the receiver on the ground.

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.

Attributes
ACN	1211046
Time
Date	201410
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Aircraft 1
Make Model Name	Medium Large Transport Low Wing 2 Turbojet Eng
Operating Under FAR Part	Part 121
Flight Phase	Descent
Flight Plan	IFR
Aircraft 2
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Altitude Overshoot Deviation - Procedural Clearance

Enroute controller reported a possible loss of separation but only after the suspected conflict was identified after a reviewed on the falcon replay system.