Medium large transport descended to 10000 ft instead of 11000 ft.

Attributes
ACN	350210
Time
Date	199610
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : dtw
State Reference	MI
Altitude	msl bound lower : 10000 msl bound upper : 11000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : dtw
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Navigation In Use	Other Other
Flight Phase	cruise other descent other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 100 flight time total : 11000 flight time type : 3000
ASRS Report	350210
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp pilot : flight engineer
Experience	flight time last 90 days : 220 flight time total : 8000 flight time type : 1900
ASRS Report	350205
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera
Resolutory Action	other
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

In radar contact with dtw approach control, aircraft was cleared to 11000 ft from current altitude of 12000 ft. As the PNF, I read back the clearance and observed what I saw as 11000 ft in altitude window. Aircraft descended under control of first officer to assigned altitude. We were alerted by ATC that we were at 10000 ft and should climb back to 11000 ft. First officer (PF) thought he had heard 10000 ft. I missed deviation as I became busy with other duties. I believe I read back 11000 ft and observed in altitude window. Lighting from bright sun and I was wearing sunglasses. This diminishes visibility of CRT. Both communication and pilot interface with aircraft were problems. Difficult to avoid with current system.

Original NASA ASRS Text

Title: MLG DSNDED TO 10000 FT INSTEAD OF 11000 FT.

Narrative: IN RADAR CONTACT WITH DTW APCH CTL, ACFT WAS CLRED TO 11000 FT FROM CURRENT ALT OF 12000 FT. AS THE PNF, I READ BACK THE CLRNC AND OBSERVED WHAT I SAW AS 11000 FT IN ALT WINDOW. ACFT DSNDED UNDER CTL OF FO TO ASSIGNED ALT. WE WERE ALERTED BY ATC THAT WE WERE AT 10000 FT AND SHOULD CLB BACK TO 11000 FT. FO (PF) THOUGHT HE HAD HEARD 10000 FT. I MISSED DEV AS I BECAME BUSY WITH OTHER DUTIES. I BELIEVE I READ BACK 11000 FT AND OBSERVED IN ALT WINDOW. LIGHTING FROM BRIGHT SUN AND I WAS WEARING SUNGLASSES. THIS DIMINISHES VISIBILITY OF CRT. BOTH COM AND PLT INTERFACE WITH ACFT WERE PROBS. DIFFICULT TO AVOID WITH CURRENT SYS.

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.