B737 crew descended prematurely, crossing the initial approach fix 500 ft low.

Attributes
ACN	502890
Time
Date	200103
Day	Fri
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : ord.airport
State Reference	IL
Altitude	msl bound lower : 3500 msl bound upper : 4000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	tracon : c90.tracon
Operator	common carrier : air carrier
Make Model Name	B737 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Navigation In Use	ils localizer & glide slope : 27r 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
Experience	flight time last 90 days : 220 flight time total : 4000 flight time type : 550
ASRS Report	502890
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Events
Anomaly	altitude deviation : excursion from assigned altitude altitude deviation : crossing restriction not met non adherence : company policies non adherence : published procedure non adherence : clearance
Independent Detector	other flight crewa other flight crewb
Resolutory Action	none taken : detected after the fact
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Cleared ILS runway 27R at ord after extensive vectors and 3 runway changes. We started our descent to the final approach altitude (2200 ft) 1 or 2 mi prematurely. We crossed ebens at approximately 3500 ft, when we noticed the published altitude is 4000 ft or 2500 ft -- whichever authority/authorized by ATC. After ebens, the published final approach altitude is 2200 ft, at which we picked up the GS almost immediately after ebens and made an uneventful approach and landing. ATC never said a word during the event. The slight deviation most likely occurred due to focusing too much attention on the close proximity of traffic -- both on the parallel and on the approach in front of us -- coupled with ATC congestion and heavy workload at the time.

Original NASA ASRS Text

Title: B737 CREW DSNDED PREMATURELY, XING THE INITIAL APCH FIX 500 FT LOW.

Narrative: CLRED ILS RWY 27R AT ORD AFTER EXTENSIVE VECTORS AND 3 RWY CHANGES. WE STARTED OUR DSCNT TO THE FINAL APCH ALT (2200 FT) 1 OR 2 MI PREMATURELY. WE CROSSED EBENS AT APPROX 3500 FT, WHEN WE NOTICED THE PUBLISHED ALT IS 4000 FT OR 2500 FT -- WHICHEVER AUTH BY ATC. AFTER EBENS, THE PUBLISHED FINAL APCH ALT IS 2200 FT, AT WHICH WE PICKED UP THE GS ALMOST IMMEDIATELY AFTER EBENS AND MADE AN UNEVENTFUL APCH AND LNDG. ATC NEVER SAID A WORD DURING THE EVENT. THE SLIGHT DEV MOST LIKELY OCCURRED DUE TO FOCUSING TOO MUCH ATTN ON THE CLOSE PROX OF TFC -- BOTH ON THE PARALLEL AND ON THE APCH IN FRONT OF US -- COUPLED WITH ATC CONGESTION AND HVY WORKLOAD AT THE TIME.

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.