ATC clears an aircraft to descend through an occupied altitude. Separation is lost.

Attributes
ACN	230349
Time
Date	199301
Day	Wed
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : rdu
State Reference	NC
Altitude	msl bound lower : 9600 msl bound upper : 10000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : rdu
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	descent other
Route In Use	enroute airway : rdu enroute airway : zdc
Flight Plan	IFR
Aircraft 2
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
ASRS Report	230349
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Experience	flight time last 90 days : 280 flight time total : 4400 flight time type : 280
ASRS Report	230344
Events
Anomaly	non adherence : far non adherence : required legal separation
Independent Detector	other controllera other flight crewa
Resolutory Action	controller : issued new clearance other
Consequence	faa : investigated
Miss Distance	horizontal : 6000 vertical : 300
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Took a handoff on northbound aircraft X over aldan at 9000 ft and I was already working an aircraft level at 10000 ft. Rdu tower and ZDC have an LOA that states propellers cross aldan at 8000 ft unless coordinated. The air carrier B was a propeller and had descended to 10000 ft to stop him above the 9000 ft traffic (aircraft X). After working out some other business in the sector I saw that rdu had accepted the handoff and air carrier B needed to be switched. I gave him a frequency change to rdu and at the last min noticed he was a propeller and cleared him to 8000 ft with the 9000 ft traffic ahead on about 5-8 mi. I realized the mistake and called rdu to tell them the situation and to stop the air carrier B at 10000 ft or turn him. With the lack of immediate concern or action rdu could not keep separation from being lost.

Original NASA ASRS Text

Title: ATC CLRS AN ACFT TO DSND THROUGH AN OCCUPIED ALT. SEPARATION IS LOST.

Narrative: TOOK A HDOF ON NBOUND ACFT X OVER ALDAN AT 9000 FT AND I WAS ALREADY WORKING AN ACFT LEVEL AT 10000 FT. RDU TWR AND ZDC HAVE AN LOA THAT STATES PROPS CROSS ALDAN AT 8000 FT UNLESS COORDINATED. THE ACR B WAS A PROP AND HAD DSNDED TO 10000 FT TO STOP HIM ABOVE THE 9000 FT TFC (ACFT X). AFTER WORKING OUT SOME OTHER BUSINESS IN THE SECTOR I SAW THAT RDU HAD ACCEPTED THE HDOF AND ACR B NEEDED TO BE SWITCHED. I GAVE HIM A FREQ CHANGE TO RDU AND AT THE LAST MIN NOTICED HE WAS A PROP AND CLRED HIM TO 8000 FT WITH THE 9000 FT TFC AHEAD ON ABOUT 5-8 MI. I REALIZED THE MISTAKE AND CALLED RDU TO TELL THEM THE SIT AND TO STOP THE ACR B AT 10000 FT OR TURN HIM. WITH THE LACK OF IMMEDIATE CONCERN OR ACTION RDU COULD NOT KEEP SEPARATION FROM BEING LOST.

Data retrieved from NASA's ASRS site as of July 2007 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.