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.

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Original NASA ASRS Text

Title: 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.

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.

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.