A flight crew failed to maintain assigned altitude on descent into yyz.

Attributes
ACN	437540
Time
Date	199905
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	navaid : yyz.vor
State Reference	ON
Altitude	msl bound lower : 7000 msl bound upper : 9000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : czyz.tracon tower : stl.tower
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Operating Under FAR Part	Part 121
Navigation In Use	other vortac
Flight Phase	cruise : enroute altitude change
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 : 180 flight time total : 11000 flight time type : 5600
ASRS Report	437530
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : cfi pilot : flight engineer
Experience	flight time last 90 days : 150 flight time total : 9000 flight time type : 150
ASRS Report	437551
Events
Anomaly	altitude deviation : overshoot non adherence : clearance
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

At 10000 ft approaching toronto, was given descent to 9000 ft. Thought controller said 5000 ft so read back 5000 ft and set 5000 ft in altitude alerter. Descending through 7000 ft, controller asked what altitude we were descending to and we were now cleared to 4000 ft. I said we were originally descending to 5000 ft and were now continuing to 4000 ft.

Original NASA ASRS Text

Title: A FLC FAILED TO MAINTAIN ASSIGNED ALT ON DSCNT INTO YYZ.

Narrative: AT 10000 FT APCHING TORONTO, WAS GIVEN DSCNT TO 9000 FT. THOUGHT CTLR SAID 5000 FT SO READ BACK 5000 FT AND SET 5000 FT IN ALT ALERTER. DSNDING THROUGH 7000 FT, CTLR ASKED WHAT ALT WE WERE DSNDING TO AND WE WERE NOW CLRED TO 4000 FT. I SAID WE WERE ORIGINALLY DSNDING TO 5000 FT AND WERE NOW CONTINUING TO 4000 FT.

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.