Date: 201510
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Second aircraft: PC-12
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Situational Awareness
Second reporter function: Enroute
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: I was working Sectors combined with a Radar Associate. Both sectors cover the east and west sides of the mountains so both sectors have transceivers on both sides of the mountains. We often split the sector off just to handle the overlapping transmissions from 4 zones even before traffic levels become a factor; but we hadn't yet at this time. I was stepping down the Aircraft X over Aircraft Y with a plan to have them at 12000 feet and 10000 feet. I told Aircraft Y to descend from 19000 feet to 11000 feet and was stepping down Aircraft X as I was able. Other situations were happening in the other zones of the two sectors between these transmissions. I could see it was taking too long so I told the Aircraft Y 'no delay through 12000 feet.' He really did pick up the descent rate quite well. As the Aircraft X passed Aircraft Y I called traffic and the Aircraft Y had Aircraft X in sight. As Aircraft Y got to 12000 feet and Aircraft X was through 14000 feet (3 miles in front of Aircraft Y) I gave Aircraft X 'descend and maintain 12000 feet' betting by the time I ended my transmission Aircraft Y would be through 11500 feet. Aircraft Y leveled at 12000 feet. Other traffic called; blocking the frequency. By the time I could tell Aircraft Y to maintain visual separation Aircraft X was descending through 13000 feet.I should expect blocked transmissions at the worst possible moment. I should have restated the assigned altitude when I went back to hurry Aircraft Y through an intermediate altitude and I should not have assumed the descent rate of Aircraft Y would continue.


Second narrative: I was asked to work the Radar Associate position for the sectors as another controller was working the Radar side. Shortly after sitting down on position I saw Aircraft Y was about 15 miles west of [fix] VORTAC descending on the [STAR] arrival. Since he was a [nearby airport] Propeller lander; his crossing restriction should have been [fix] at 10;000. Aircraft Y was at approximately 16;000 feet when the radar controller issued 'Descend and maintain 11;000 feet. I asked him if he will be getting him down to 10;000 feet since he is a propeller; to which the radar controller said yes I will. About 10 miles east of Aircraft Y's position was Aircraft X who was about 100 knots faster and out of approximately 24;000 feet descending.I became occupied for sometime with a pointout and coordinating with another sector as they were closing and there was another flight who checked on with us when he was in another sector's airspace. After my briefing with the other sector I looked back at [fix] and saw that Aircraft X had an interim altitude of 14000 feet; which was above the altitude of Aircraft Y. A few seconds later I noticed that Aircraft X was leaving 14000 feet and descending to 12000 feet while an interim altitude of 14000 feet was STILL in the data block. Aircraft Y was still descending and at approximately 12000 feet. At this point; due to speeds; Aircraft X was maybe 3 miles in front of Aircraft Y and both aircraft were at 12000 feet. The radar controller attempted to use visual separation; but by this time it appeared to me that there was 400 foot vertical and 3.5 miles of separation. The radar controller told me that he told Aircraft Y to expedite through 12000 feet; and the pilot must have stopped at 12000 feet instead. I did not hear this instruction as I was coordinating with a different sector at that time. The improper use of interim altitudes made it appear to me that Aircraft X was vertically separated. This radar controller frequently uses interim altitudes in a sporadic manner. Sometimes he will use them and other times he will not; which makes the radar screen not accurate to what the aircraft are actually doing. Also; the use of 'Expedite' does not provide any positive control to the situation. When I asked the radar controller if he was going to descend Aircraft Y down to 10000 feet 10 miles west of [fix]; there was no reason he descended him to 11000 feet.


Synopsis: An ARTCC Controller was descending one aircraft above another descending aircraft. Both aircraft were on the same route and the higher one was overtaking the lower one. The Controller descended the higher and faster aircraft to 12000 feet. The aircraft reached 12000 feet and the lower and slower aircraft was still at 12000 feet less than 3 miles behind. Some of the Controller's radio transmissions were blocked as they tried to issue instructions at the last minute to prevent the occurrence.