Air carrier aircraft descends below what ATC thinks he was cleared to.

Attributes
ACN	118583
Time
Date	198907
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ord
State Reference	IL
Altitude	msl bound lower : 6500 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	Widebody, Low Wing, 3 Turbojet Eng
Navigation In Use	Other
Flight Phase	cruise other cruise other
Route In Use	enroute : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : atp pilot : instrument
Experience	flight time last 90 days : 130 flight time total : 10000 flight time type : 50
ASRS Report	118583
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : instrument pilot : commercial pilot : atp
Experience	flight time last 90 days : 150 flight time total : 22000 flight time type : 150
ASRS Report	117448
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	controller : issued new clearance
Consequence	Other
Supplementary
Primary Problem	Ambiguous
Air Traffic Incident	Pilot Deviation Operational Deviation other

Narrative:

We were cleared to 8000' on vector for runway 4R. All 3 pilots heard our clearance to 4000' and heading change to (I believe) 110 degrees. Captain dialed 4000' on altitude preselect and I started down. At 6500' approach control asked, 'how low are you going?' we replied, '4000'.' ATC said, 'you were only cleared to 8, maintain 6000 for now.' west leveled and maintained 6000'. We had acknowledged the 4000' clearance. Supplemental information from acn 117448: all three of us in the cockpit felt we had been cleared to 4000'--that 4000' had been read back and 4000' had been set in the altitude alert window and armed.

Original NASA ASRS Text

Title: ACR ACFT DESCENDS BELOW WHAT ATC THINKS HE WAS CLEARED TO.

Narrative: WE WERE CLRED TO 8000' ON VECTOR FOR RWY 4R. ALL 3 PLTS HEARD OUR CLRNC TO 4000' AND HDG CHANGE TO (I BELIEVE) 110 DEGS. CAPT DIALED 4000' ON ALT PRESELECT AND I STARTED DOWN. AT 6500' APCH CTL ASKED, 'HOW LOW ARE YOU GOING?' WE REPLIED, '4000'.' ATC SAID, 'YOU WERE ONLY CLRED TO 8, MAINTAIN 6000 FOR NOW.' W LEVELED AND MAINTAINED 6000'. WE HAD ACKNOWLEDGED THE 4000' CLRNC. SUPPLEMENTAL INFO FROM ACN 117448: ALL THREE OF US IN THE COCKPIT FELT WE HAD BEEN CLRED TO 4000'--THAT 4000' HAD BEEN READ BACK AND 4000' HAD BEEN SET IN THE ALT ALERT WINDOW AND ARMED.

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.