Potential conflict averted as alert ARTCC radar controller intervenes with TA given and an amended clearance altitude change. Previous controller had allegedly issued clearance for descent to occupied altitude and failed to catch operational deviation in clearance readback hearback.

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Attributes
ACN	217123
Time
Date	199206
Day	Sat
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : gve
State Reference	MD
Altitude	msl bound lower : 25000 msl bound upper : 29000
Environment
Flight Conditions	VMC
Light	Dusk
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent other
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Light Transport, Low Wing, 2 Turboprop Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : flight engineer pilot : instrument
Experience	flight time last 90 days : 220 flight time total : 2300 flight time type : 800
ASRS Report	217123
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : overshoot other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	Other
Miss Distance	horizontal : 5000 vertical : 1000
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	other

Narrative:

En route to phl, we were given the clearance to cross 15 NM northeast of gve at FL240. I read back the clearance and set 240 in the altitude window. Passing FL250, approximately 12 DME northeast of gve, center queried our passing altitude. We told him FL250. He told us to maintain FL250 for traffic at 11 O'clock, 1 mi, FL240. We complied and got a visual sighting of the traffic, a light twin. Noting the closeness of the spacing, we asked the controller what happened. He stated the previous controller had issued the descent to 290, not 240, but not to worry, there's no problem. We discussed it in the cockpit, and all 3 of us (we had a jumpseat rider from another company), heard 240, which was what I read back. I believe it was an honest mistake, the controller may have thought he said 290, he may have been thinking 290, but what came out was 240. No harm was done, we also got a TA on TCASII shortly after center told us to maintain FL250.

Original NASA ASRS Text

Title: POTENTIAL CONFLICT AVERTED AS ALERT ARTCC RADAR CTLR INTERVENES WITH TA GIVEN AND AN AMENDED CLRNC ALT CHANGE. PREVIOUS CTLR HAD ALLEGEDLY ISSUED CLRNC FOR DSCNT TO OCCUPIED ALT AND FAILED TO CATCH OPDEV IN CLRNC READBACK HEARBACK.

Narrative: ENRTE TO PHL, WE WERE GIVEN THE CLRNC TO CROSS 15 NM NE OF GVE AT FL240. I READ BACK THE CLRNC AND SET 240 IN THE ALT WINDOW. PASSING FL250, APPROX 12 DME NE OF GVE, CTR QUERIED OUR PASSING ALT. WE TOLD HIM FL250. HE TOLD US TO MAINTAIN FL250 FOR TFC AT 11 O'CLOCK, 1 MI, FL240. WE COMPLIED AND GOT A VISUAL SIGHTING OF THE TFC, A LIGHT TWIN. NOTING THE CLOSENESS OF THE SPACING, WE ASKED THE CTLR WHAT HAPPENED. HE STATED THE PREVIOUS CTLR HAD ISSUED THE DSCNT TO 290, NOT 240, BUT NOT TO WORRY, THERE'S NO PROBLEM. WE DISCUSSED IT IN THE COCKPIT, AND ALL 3 OF US (WE HAD A JUMPSEAT RIDER FROM ANOTHER COMPANY), HEARD 240, WHICH WAS WHAT I READ BACK. I BELIEVE IT WAS AN HONEST MISTAKE, THE CTLR MAY HAVE THOUGHT HE SAID 290, HE MAY HAVE BEEN THINKING 290, BUT WHAT CAME OUT WAS 240. NO HARM WAS DONE, WE ALSO GOT A TA ON TCASII SHORTLY AFTER CTR TOLD US TO MAINTAIN FL250.

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.