ZTL controller described a loss of in trail separation with atl arrival aircraft; claiming the phras...

Narrative:

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.'

Original NASA ASRS Text

Title: 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.

Narrative: 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.'

Data retrieved from NASA's ASRS site as of July 2013 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.

Attributes
ACN	1014457
Time
Date	201206
Local Time Of Day	1801-2400
Place
Locale Reference	ZTL.ARTCC
State Reference	GA
Aircraft 1
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	STAR HONIE
Flight Plan	IFR
Aircraft 2
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	Descent
Route In Use	Vectors
Flight Plan	IFR
Person 1
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Deviation - Altitude Overshoot Deviation - Procedural Clearance

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.