Crew misinterpreted clearance and descended below GS intercept altitude.

Attributes
ACN	492234
Time
Date	200011
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : lex.airport
State Reference	KY
Altitude	msl bound lower : 2500 msl bound upper : 3100
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : lex.tracon
Operator	common carrier : air carrier
Make Model Name	Beech 1900
Operating Under FAR Part	Part 135
Navigation In Use	ils localizer & glide slope : 22 Other
Flight Phase	descent : approach
Route In Use	approach : instrument precision arrival : on vectors
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : multi engine pilot : instrument pilot : commercial
Experience	flight time last 90 days : 250 flight time total : 1600 flight time type : 300
ASRS Report	492234
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp pilot : commercial pilot : cfi
Experience	flight time last 90 days : 100 flight time total : 7800 flight time type : 1800
ASRS Report	491859
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : clearance non adherence : company policies non adherence : published procedure other anomaly other
Independent Detector	other controllera
Resolutory Action	controller : issued new clearance
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

On approach to lex ILS runway 22 VMC conditions, control gave heading and descent to maintain until established cleared for ILS runway 22 approach. After verifying with captain what I thought was descend and maintain 2500 ft, I read back the entire clearance with 2500 ft as altitude to descend and maintain. No response was heard after my readback. Approximately 2 mins later control said maintain 3100 ft until bridle cleared for the approach, contact tower. I questioned control as to the altitude we were given and he said contact tower. I had heard the altitude given in the clearance wrong and had read it back, but was never corrected. By the time we knew the correct altitude we were over the fix and cleared to land. After landing we contacted tower to verify the situation and they said communication could have been better on both ends, but we were still within vectoring altitudes.

Original NASA ASRS Text

Title: CREW MISINTERPRETED CLRNC AND DSNDED BELOW GS INTERCEPT ALT.

Narrative: ON APCH TO LEX ILS RWY 22 VMC CONDITIONS, CTL GAVE HEADING AND DSCNT TO MAINTAIN UNTIL ESTABLISHED CLRED FOR ILS RWY 22 APCH. AFTER VERIFYING WITH CAPT WHAT I THOUGHT WAS DSND AND MAINTAIN 2500 FT, I READ BACK THE ENTIRE CLRNC WITH 2500 FT AS ALT TO DSND AND MAINTAIN. NO RESPONSE WAS HEARD AFTER MY READBACK. APPROX 2 MINS LATER CTL SAID MAINTAIN 3100 FT UNTIL BRIDLE CLRED FOR THE APCH, CONTACT TWR. I QUESTIONED CTL AS TO THE ALT WE WERE GIVEN AND HE SAID CONTACT TWR. I HAD HEARD THE ALT GIVEN IN THE CLRNC WRONG AND HAD READ IT BACK, BUT WAS NEVER CORRECTED. BY THE TIME WE KNEW THE CORRECT ALT WE WERE OVER THE FIX AND CLRED TO LAND. AFTER LNDG WE CONTACTED TWR TO VERIFY THE SIT AND THEY SAID COM COULD HAVE BEEN BETTER ON BOTH ENDS, BUT WE WERE STILL WITHIN VECTORING ALTS.

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.