Air Traffic Control Fully Certified
Communication Breakdown; Situational Awareness
Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude
Air Traffic Control Issued New Clearance
Aircraft; Human Factors
Was taking a relief briefing on the sector when a Global Express at FL450 requested lower. The working Controller answered; 'We have your request; it will be 9 minutes.' The Global Express answered with 'roger.' After the briefing; the Global Express said they would like an off course vector for lower. I answered; 'Roger turn 15 degrees left vectors for your descent; expect lower in 4 minutes.' The Global Express answered; 'Roger 15 left.' Two minutes later; the Global Express was 7 miles from a Citation at FL430 and said; 'Unable to maintain altitude; need to descend immediately.' I responded; 'Roger fly heading 330.' No answer. 'Global Express fly heading 170.' At this moment the Global Express started descending losing separation with the Citation. [I] told the Citation to 'descend and maintain FL400 immediately.' No response. Went back to the Global Express and said 'descend and maintain FL430.' 'Unable' was the response. The Global Express descended through FL420 and I told the Global Express to descend and maintain FL400. The Global Express answered; 'Roger descending to FL400.' Went back to the Citation and said; 'Climb and maintain FL430.' The Citation response; 'Roger never left FL430.' I put the Global Express back on course after separation was insured. If a pilot has an imminent need; they need to vocalize those needs before it becomes an emergency.
Enroute Controller described a loss of separation event when an aircraft needed an immediate descent; but failed to verbalize the need to ATC.