<< >> NASA ASRS Report 934487, February 2011

ZMA controller reported he thought an aircraft was descending based on a mode C readout and descended another aircraft based on that mode C. The first aircraft was not descending and separation was lost.

Attributes
ACN	934487
Time
Date	201102
Local Time Of Day	0001-0600
Place
Locale Reference	ZMA.ARTCC
State Reference	FL
Environment
Light	Daylight
Aircraft 1
Make Model Name	M-20 G Statesman
Flight Phase	Descent
Route In Use	None
Flight Plan	IFR
Aircraft 2
Make Model Name	PA-32 Cherokee Six/Lance/Saratoga/6X
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	None
Flight Plan	IFR
Person 1
Function	Enroute
Qualification	Air Traffic Control Fully Certified
Events
Anomaly	ATC Issue All Types Conflict Airborne Conflict Deviation - Procedural Published Material / Policy
Miss Distance	Horizontal 18000 Vertical 800

Narrative:

Aircraft 'a' had been given a crossing restriction to cross 10 miles east of xyz at 4;000 ft. Aircraft 'B' was level at 7;000 ft and needed to cross abcde at 4;000 ft. I noticed the mode C of aircraft 'a' at 059 and thought that he'd begun his descent. I then issued aircraft 'B' a descent clearance to 6;000 ft since aircraft 'a' had vacated that altitude. However; the mode C of aircraft 'a' remained at 059. The pilot had not started down after all. When conflict alert activated; I issued 20 degree turns to both aircraft; but separation had already been lost. [Do] not descend one aircraft into another.

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Original NASA ASRS Text

Title: ZMA Controller reported he thought an aircraft was descending based on a mode C readout and descended another aircraft based on that mode C. The first aircraft was not descending and separation was lost.

Narrative: Aircraft 'A' had been given a crossing restriction to cross 10 miles east of XYZ at 4;000 FT. Aircraft 'B' was level at 7;000 FT and needed to cross ABCDE at 4;000 FT. I noticed the mode C of aircraft 'A' at 059 and thought that he'd begun his descent. I then issued aircraft 'B' a descent clearance to 6;000 FT since aircraft 'A' had vacated that altitude. However; the mode C of aircraft 'A' remained at 059. The pilot had not started down after all. When conflict alert activated; I issued 20 degree turns to both aircraft; but separation had already been lost. [Do] not descend one aircraft into another.

Data retrieved from NASA's ASRS site as of April 2012 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.