Date: 201509
Local time of day: 0001-0600
Location: ZZZ.ARTCC, US
Aircraft: Military
Operator: Military
Operating under: Part 91
Flight plan: IFR
Mission: Tactical
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Small Transport; Low Wing; 2 Turboprop Eng
Reporter organization: Government
Reporter function: Supervisor / CIC; Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Human-Machine Interface; Situational Awareness
Anomaly: ATC Issue All Types; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Contributing factors: Manuals; Procedure; Human Factors
Primary problem: Human Factors

Narrative: I was walking behind the group of controllers and observed flashing datatags on the sector radar scope.  I walked over to the 'see all' radar display and pulled up the sector traffic.  I observed [Aircraft X] due eastbound at 20;000 feet and [Aircraft Y] northbound descending out of 20;000 feet for 18;000 feet.  At this time; the aircraft were about 10 miles apart.  I observed [Aircraft Y] descending very slowly and at about 7 miles apart there was 400-500 feet vertical separation.At this point; I knew the aircraft were not going to hit and so I wanted to observe what the controller would do to ensure required minimum separation.  Would he expedite descent?  Would he turn the [Aircraft X] to avoid?  Would he do anything except watch and hope for luck?  Well; he did do something - just as [Aircraft Y] target was going to go inside the separation J Ring with 100 feet less than minimum separation; it appeared that he made the entry to turn off the mode C on that aircraft and show it level at 19;000 feet.I turned in the situation to the Operations Manager who later informed me that the minimum separation between the two targets was 4.5nm and 900 feet.  When he returned from break; I advised the controller of this finding and offered time for [a safety report].  When he returned from this task; I overheard conversations he had with other controllers - 'since he turned off the mode C; how could we tell what the separation was?'  'What's the big deal with 4.5nm and 900 feet.I believe that this situation; although well within the 80 percent rule of minimum safety risk is indeed a great safety risk.  I believe that this is a prime example of the 'drift' that is part of the system.  The controller did NOTHING to comply with prescribed requirements and when he thought he might have an issue; he turned off the mode C.  He did not proactively file [a safety report] to protect himself if he just missed the requirements and he did not notify me or anyone else in management there might have been a loss of required minimum separation.  He took no responsibility for his actions and tried to cover them up.This is a very junior controller who has just begun OJTI within the past 3 months.  I believe this is a performance issue that needs to be addressed.




Synopsis: An ARTCC Supervisor observed a Controller intentionally allow two aircraft to pass with less than prescribed separation. The aircraft were on diverging courses but passing behind with less than 1;000 feet vertical or 5 miles lateral separation. The Supervisor observed the Controller manually enter an altitude in one of the aircrafts' data block to make it appear the aircraft had the prescribed separation minima.