Air carrier descended below assigned altitude while on approach.

Attributes
ACN	138162
Time
Date	199002
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : crl airport : dtw
State Reference	MI
Altitude	msl bound lower : 10500 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : dtw
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	descent : approach
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 228 flight time total : 7850 flight time type : 210
ASRS Report	138162
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 220 flight time total : 15000
ASRS Report	138338
Events
Anomaly	altitude deviation : overshoot non adherence : clearance other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance other
Consequence	faa : reviewed incident with flight crew
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were inbound to dtw, landing runway 21R, level 12000. The frequency was extremely congested. I was flying and started a descent to 10000'. At 10500' the controller told us to climb to 11000'. I was surprised at the tone in his voice (as if we had done something wrong) and climbed to 11000. The captain said he thought that was what we were cleared to. I said I thought we had been cleared to 10000. Captain said we had been told to slow to 210. All I had heard was to ten. Factor -- fatigue: I had been 1 day off in last 8 days with a lot of commutes. Congested frequency: lot of rapid fire instructions in last 10 mins of flight. Also one person talking and other flying. Person, ie, captain read back one clearance, but I heard another. Supplemental information from acn 138338: dtw approach control cleared us from 12000 to 11000 MSL. 11000 was set in alerter. There was much conversation between controllers and aircraft. We had had many step down dscnts in our flight that evening. Somehow the first officer descended to 10000. It was not caught by myself. There was no near miss and approach control advised us that there was no traffic conflict.

Original NASA ASRS Text

Title: ACR DESCENDED BELOW ASSIGNED ALT WHILE ON APCH.

Narrative: WE WERE INBND TO DTW, LNDG RWY 21R, LEVEL 12000. THE FREQ WAS EXTREMELY CONGESTED. I WAS FLYING AND STARTED A DSCNT TO 10000'. AT 10500' THE CTLR TOLD US TO CLIMB TO 11000'. I WAS SURPRISED AT THE TONE IN HIS VOICE (AS IF WE HAD DONE SOMETHING WRONG) AND CLIMBED TO 11000. THE CAPT SAID HE THOUGHT THAT WAS WHAT WE WERE CLRED TO. I SAID I THOUGHT WE HAD BEEN CLRED TO 10000. CAPT SAID WE HAD BEEN TOLD TO SLOW TO 210. ALL I HAD HEARD WAS TO TEN. FACTOR -- FATIGUE: I HAD BEEN 1 DAY OFF IN LAST 8 DAYS WITH A LOT OF COMMUTES. CONGESTED FREQ: LOT OF RAPID FIRE INSTRUCTIONS IN LAST 10 MINS OF FLT. ALSO ONE PERSON TALKING AND OTHER FLYING. PERSON, IE, CAPT READ BACK ONE CLRNC, BUT I HEARD ANOTHER. SUPPLEMENTAL INFORMATION FROM ACN 138338: DTW APCH CTL CLRED US FROM 12000 TO 11000 MSL. 11000 WAS SET IN ALERTER. THERE WAS MUCH CONVERSATION BETWEEN CTLRS AND ACFT. WE HAD HAD MANY STEP DOWN DSCNTS IN OUR FLT THAT EVENING. SOMEHOW THE F/O DESCENDED TO 10000. IT WAS NOT CAUGHT BY MYSELF. THERE WAS NO NEAR MISS AND APCH CTL ADVISED US THAT THERE WAS NO TFC CONFLICT.

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.