Air carrier descended to altitude occupied by another aircraft.

Attributes
ACN	121321
Time
Date	198908
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : emi
State Reference	MD
Altitude	msl bound lower : 10800 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zdc
Operator	general aviation : corporate
Make Model Name	Light Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent : approach descent other
Route In Use	arrival star : star enroute airway : zdc
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 4 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent : approach descent other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 11500 flight time type : 3000
ASRS Report	121321
Person 2
Affiliation	Other
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	conflict : airborne less severe non adherence : published procedure non adherence : required legal separation
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Miss Distance	horizontal : 12000 vertical : 800
Supplementary
Primary Problem	ATC Human Performance
Air Traffic Incident	Operational Error

Narrative:

Descending on the westminster one arrival to bwi, we were given several descent altitudes. Last clearance was to cross ruane intersection at 10,000' and we read back 10,000'. As we were descending we saw an air carrier widebody transport 90 degree to our flight path and about 2 mi ahead and below. We remarked what a sight and did not perceive any conflict. Several seconds later (10-15) as we were about to level at 10,000, the controller said to climb back to 11,000'. We did immediately. We didn't know if the controller gave us the wrong altitude, or we heard it wrong and read back 10,000' and he didn't catch it. In our operation we have a policy that the pilot who is communicating reads back the altitude and sets the altitude alerter, and the pilot flying calls out the altitude climbing or descending to.

Original NASA ASRS Text

Title: ACR DESCENDED TO ALT OCCUPIED BY ANOTHER ACFT.

Narrative: DESCENDING ON THE WESTMINSTER ONE ARR TO BWI, WE WERE GIVEN SEVERAL DSCNT ALTS. LAST CLRNC WAS TO CROSS RUANE INTXN AT 10,000' AND WE READ BACK 10,000'. AS WE WERE DESCENDING WE SAW AN ACR WDB 90 DEG TO OUR FLT PATH AND ABOUT 2 MI AHEAD AND BELOW. WE REMARKED WHAT A SIGHT AND DID NOT PERCEIVE ANY CONFLICT. SEVERAL SECONDS LATER (10-15) AS WE WERE ABOUT TO LEVEL AT 10,000, THE CTLR SAID TO CLIMB BACK TO 11,000'. WE DID IMMEDIATELY. WE DIDN'T KNOW IF THE CTLR GAVE US THE WRONG ALT, OR WE HEARD IT WRONG AND READ BACK 10,000' AND HE DIDN'T CATCH IT. IN OUR OPERATION WE HAVE A POLICY THAT THE PLT WHO IS COMMUNICATING READS BACK THE ALT AND SETS THE ALT ALERTER, AND THE PLT FLYING CALLS OUT THE ALT CLIMBING OR DESCENDING TO.

Data retrieved from NASA's ASRS site as of August 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.