A cl-65 flight crew misunderstood their clearance and descent below assigned altitude on an ILS.

Attributes
ACN	606025
Time
Date	200401
Day	Wed
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : zzz.airport
State Reference	US
Altitude	msl bound lower : 2500 msl bound upper : 3000
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : zzz.tracon
Operator	common carrier : air carrier
Make Model Name	Regional Jet 200 ER&LR
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 12r 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 : captain oversight : pic
Qualification	pilot : cfi pilot : instrument pilot : atp
Experience	flight time last 90 days : 180 flight time total : 8000 flight time type : 2000
ASRS Report	606025
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Events
Anomaly	altitude deviation : overshoot non adherence : far non adherence : clearance
Independent Detector	atc equipment : msaw other controllera
Resolutory Action	controller : issued advisory flight crew : returned to assigned altitude
Consequence	faa : reviewed incident with flight crew
Supplementary
Problem Areas	ATC Human Performance Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Upon approach to ILS runway 12R with copilot flying, we were instructed to descend from 3000 ft to 1800 ft MSL. I set 1800 ft on the altitude alerter to 1800 ft and copilot verified it. We were about to intercept the localizer for runway 12R when the controller said 'turn to heading 150 degrees at 2800 ft to join, cleared for the ILS runway 12R approach.' we were passing through 2800 ft when he came back and said he had a 'low altitude alert, climb immediately.' we complied immediately, leveled at 2800 ft and completed the approach. The copilot had briefed the approach and we were aware of the 2800 ft on the ILS approach. The same controller earlier had offered us a visual on the downwind. I told him we could not because we would lose sight of the field during the approach. When he gave us the 1800 ft assignment, we thought he was giving us an MVA (controller altitude) to get under the clouds for the visual. We never went to 1800 ft and never broke out of the clouds. No passenger or crew were affected by the immediate climb to 2800 ft.

Original NASA ASRS Text

Title: A CL-65 FLT CREW MISUNDERSTOOD THEIR CLRNC AND DSCNT BELOW ASSIGNED ALT ON AN ILS.

Narrative: UPON APCH TO ILS RWY 12R WITH COPLT FLYING, WE WERE INSTRUCTED TO DSND FROM 3000 FT TO 1800 FT MSL. I SET 1800 FT ON THE ALT ALERTER TO 1800 FT AND COPLT VERIFIED IT. WE WERE ABOUT TO INTERCEPT THE LOC FOR RWY 12R WHEN THE CTLR SAID 'TURN TO HDG 150 DEGS AT 2800 FT TO JOIN, CLRED FOR THE ILS RWY 12R APCH.' WE WERE PASSING THROUGH 2800 FT WHEN HE CAME BACK AND SAID HE HAD A 'LOW ALT ALERT, CLB IMMEDIATELY.' WE COMPLIED IMMEDIATELY, LEVELED AT 2800 FT AND COMPLETED THE APCH. THE COPLT HAD BRIEFED THE APCH AND WE WERE AWARE OF THE 2800 FT ON THE ILS APCH. THE SAME CTLR EARLIER HAD OFFERED US A VISUAL ON THE DOWNWIND. I TOLD HIM WE COULD NOT BECAUSE WE WOULD LOSE SIGHT OF THE FIELD DURING THE APCH. WHEN HE GAVE US THE 1800 FT ASSIGNMENT, WE THOUGHT HE WAS GIVING US AN MVA (CTLR ALT) TO GET UNDER THE CLOUDS FOR THE VISUAL. WE NEVER WENT TO 1800 FT AND NEVER BROKE OUT OF THE CLOUDS. NO PAX OR CREW WERE AFFECTED BY THE IMMEDIATE CLB TO 2800 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.