ARTCC controller error when controller instructed flight crew of an air carrier to descend and TCASII RA showed, and crew observed, another aircraft at the altitude to which they were assigned.

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Attributes
ACN	485907
Time
Date	200008
Day	Mon
Local Time Of Day	1201 To 1800
Place
State Reference	MI
Altitude	msl bound lower : 24000 msl bound upper : 25000
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zob.artcc
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	descent : vacating altitude
Flight Plan	IFR
Aircraft 2
Controlling Facilities	artcc : zob.artcc
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 120 flight time total : 2800 flight time type : 460
ASRS Report	485907
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude conflict : airborne less severe non adherence : clearance non adherence : required legal separation other anomaly other
Independent Detector	atc equipment other atc equipment : radar aircraft equipment : tcas other flight crewa
Resolutory Action	controller : issued new clearance controller : separated traffic flight crew : took precautionary avoidance action
Miss Distance	horizontal : 12000 vertical : 500
Supplementary
Problem Areas	Flight Crew Human Performance ATC Human Performance
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Lga-ord. Received a clearance to descend from FL250 to FL240. Midway through descent, we noticed a target on TCASII and then acquired the aircraft visually. The descent was stopped and the captain queried the controller. The controller issued a new clearance back to FL250. We complied. There was no further instruction or comment. The controller was very busy. We read back the initial clearance and were not corrected.

Original NASA ASRS Text

Title: ARTCC CTLR ERROR WHEN CTLR INSTRUCTED FLC OF AN ACR TO DSND AND TCASII RA SHOWED, AND CREW OBSERVED, ANOTHER ACFT AT THE ALT TO WHICH THEY WERE ASSIGNED.

Narrative: LGA-ORD. RECEIVED A CLRNC TO DSND FROM FL250 TO FL240. MIDWAY THROUGH DSCNT, WE NOTICED A TARGET ON TCASII AND THEN ACQUIRED THE ACFT VISUALLY. THE DSCNT WAS STOPPED AND THE CAPT QUERIED THE CTLR. THE CTLR ISSUED A NEW CLRNC BACK TO FL250. WE COMPLIED. THERE WAS NO FURTHER INSTRUCTION OR COMMENT. THE CTLR WAS VERY BUSY. WE READ BACK THE INITIAL CLRNC AND WERE NOT CORRECTED.

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.