Air Traffic Control Fully Certified
Communication Breakdown; Situational Awareness
ATC Issue All Types; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance
Air Traffic Control Issued New Clearance
Human Factors; Procedure
I was on D9. R9 was working about 6 inbounds to ATL on the HONIE arrival with minimal in trail requirement. The first aircraft in line; Air Carrier X descended early; and that plus his loss of the higher altitude tailwind caused the second aircraft in line to begin to overtake. The R-Side slowed the second aircraft (Air Carrier Y) to 250 KTS and turned them 15 [degrees left] to counteract the overtake. This then caused the third aircraft (Air Carrier Z) to begin to overtake by a 130 KTS difference. Air Carrier Z was told 'begin your speed reduction' referring to the 250 KTS assignment at HONIE. Air Carrier Z's speed did not change much and when he began to get close to Air Carrier Y; R9 issued 'Air Carrier Z just maintain one six thousand.' His Mode C showed 16;800 FT at that time and he did not respond. R9 again said; 'Air Carrier Z; just maintain one six thousand.' He responded; 'Unable.' I said to R9; 'Unable? What does that mean?' R9 transmitted; 'Air Carrier Z; unable?' He responded; 'Unable; we are through 15;600 FT;' and he continued descending; intending to cross HONIE at 14;000 FT. R9 issued; 'Air Carrier Z stop your descent and turn 20 degrees right.' About that time the aircraft lost separation. 'Begin your speed reduction' is an ambiguous clearance; should use 'reduce to 250 KTS.'
ZTL Controller described a loss of in trail separation with ATL arrival aircraft; claiming the phraseology used by the RADAR Controller was a bit ambiguous.