Air carrier breaks out of IMC, follows wrong aircraft due off course.

Attributes
ACN	225874
Time
Date	199211
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	airport : stl
State Reference	MO
Altitude	msl bound lower : 2400 msl bound upper : 3000
Environment
Flight Conditions	IMC
Light	Dusk
Aircraft 1
Controlling Facilities	tower : stl
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Navigation In Use	Other
Flight Phase	descent : approach
Route In Use	enroute : on vectors
Flight Plan	IFR
Aircraft 2
Operator	common carrier : air carrier
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Flight Phase	descent : approach
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 240 flight time total : 17500 flight time type : 15000
ASRS Report	225874
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : atp
Events
Anomaly	non adherence : clearance other anomaly other other spatial deviation
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : became reoriented
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

We were being vectored to the lda 12L and captured the localizer. Approximately 3 mi ahead of us was another aircraft at our altitude (3600 ft) and at 12 O'clock, as shown on our TCASII. We were IMC with winds at our altitude at 180/40 KTS. We were told by approach that we were following another aircraft to 12L at 5 mi ahead of us. The controller made no mention of the aircraft at now 2 1/2 mi, and then switched us to tower. We were now inside jaame and descending to 2400 ft and looking for the aircraft at 12 O'clock and 2 mi. As we broke out of the overcast at about 3000 ft MSL, we picked up the aircraft visually and also saw the airport. What I didn't realize was that the strong crosswind had lessened as we went below the overcast and that caused us to drift off the lda to the south, slightly. Then we asked the tower what the other aircraft was doing and tower said that he was on final for 12R and we were off course. Distraction of now knowing what that other aircraft was doing along with numerous TCASII warnings caused this deviation.

Original NASA ASRS Text

Title: ACR BREAKS OUT OF IMC, FOLLOWS WRONG ACFT DUE OFF COURSE.

Narrative: WE WERE BEING VECTORED TO THE LDA 12L AND CAPTURED THE LOC. APPROX 3 MI AHEAD OF US WAS ANOTHER ACFT AT OUR ALT (3600 FT) AND AT 12 O'CLOCK, AS SHOWN ON OUR TCASII. WE WERE IMC WITH WINDS AT OUR ALT AT 180/40 KTS. WE WERE TOLD BY APCH THAT WE WERE FOLLOWING ANOTHER ACFT TO 12L AT 5 MI AHEAD OF US. THE CTLR MADE NO MENTION OF THE ACFT AT NOW 2 1/2 MI, AND THEN SWITCHED US TO TWR. WE WERE NOW INSIDE JAAME AND DSNDING TO 2400 FT AND LOOKING FOR THE ACFT AT 12 O'CLOCK AND 2 MI. AS WE BROKE OUT OF THE OVCST AT ABOUT 3000 FT MSL, WE PICKED UP THE ACFT VISUALLY AND ALSO SAW THE ARPT. WHAT I DIDN'T REALIZE WAS THAT THE STRONG XWIND HAD LESSENED AS WE WENT BELOW THE OVCST AND THAT CAUSED US TO DRIFT OFF THE LDA TO THE S, SLIGHTLY. THEN WE ASKED THE TWR WHAT THE OTHER ACFT WAS DOING AND TWR SAID THAT HE WAS ON FINAL FOR 12R AND WE WERE OFF COURSE. DISTR OF NOW KNOWING WHAT THAT OTHER ACFT WAS DOING ALONG WITH NUMEROUS TCASII WARNINGS CAUSED THIS DEV.

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.