Flight crew of DC9-30 descended prematurely prior to OM at mdw, il.

Attributes
ACN	424060
Time
Date	199812
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : mdw
State Reference	IL
Altitude	msl bound lower : 2500 msl bound upper : 2500
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Operator	common carrier : air carrier
Flight Phase	landing other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Experience	flight time last 90 days : 150 flight time total : 8100 flight time type : 700
ASRS Report	424060
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Experience	flight time last 90 days : 90 flight time total : 17000 flight time type : 6600
ASRS Report	424064
Events
Anomaly	altitude deviation : excursion from assigned altitude
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Air Traffic Incident	Pilot Deviation

Narrative:

Approach control cleared us from 3000 ft to 2500 ft MSL. Upon entering a left base, we were given a heading to intercept the mdw runway 31C localizer. Subsequently, we cleared for the approach and assigned an airspeed of 170 KTS until runts. There was some confusion with the approach plate (mdw ILS runway 31C). Note: ATC can clear you to 2100 ft MSL. I started to descend to 2100 ft MSL. At around 2200 ft MSL, I noticed the discrepancy between the altitude alerter set at 2500 ft. We were approximately 2.0 DME from runts. I climbed back to 2500 ft MSL until GS intercept. We continued the approach to a successful landing. There was no conflict with traffic.

Original NASA ASRS Text

Title: FLC OF DC9-30 DSNDED PREMATURELY PRIOR TO OM AT MDW, IL.

Narrative: APCH CTL CLRED US FROM 3000 FT TO 2500 FT MSL. UPON ENTERING A L BASE, WE WERE GIVEN A HDG TO INTERCEPT THE MDW RWY 31C LOC. SUBSEQUENTLY, WE CLRED FOR THE APCH AND ASSIGNED AN AIRSPD OF 170 KTS UNTIL RUNTS. THERE WAS SOME CONFUSION WITH THE APCH PLATE (MDW ILS RWY 31C). NOTE: ATC CAN CLR YOU TO 2100 FT MSL. I STARTED TO DSND TO 2100 FT MSL. AT AROUND 2200 FT MSL, I NOTICED THE DISCREPANCY BTWN THE ALT ALERTER SET AT 2500 FT. WE WERE APPROX 2.0 DME FROM RUNTS. I CLBED BACK TO 2500 FT MSL UNTIL GS INTERCEPT. WE CONTINUED THE APCH TO A SUCCESSFUL LNDG. THERE WAS NO CONFLICT WITH TFC.

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.