Narrative:

Near magoo intersection, ZAU started aircraft on a descent to FL350, then FL330, then FL290, then FL270. Captain was PNF. First officer was PF. Aircraft speed was then slowed by ATC, then aircraft was placed on a radar vector. At FL270, captain switched to communication #2 to get ATIS and to call ground handler for gate information. First officer/PF was given communication #1 for ATC. During this time, first officer/PF started descent to FL240, stating this was given by ATC. Captain/PNF did not hear this exchange while on communication #2. Captain/PNF returned to communication #1. At FL250, ATC called saying aircraft was below assigned altitude. Captain/PNF stated we were assigned FL240 and were on assigned heading. ATC stated aircraft should be at higher altitude. ATC then gave aircraft FL250, and switched aircraft to another frequency. Next ATC frequency advised aircraft of possible pilot deviation. Aircraft continued to ord. Factors: first officer/PF new to aircraft type, but experienced. Aircraft ahead had similar flight number. Multiple altitude, heading, and speed changes in short period. 2 man flight deck. Need for PNF to leave ATC frequency to make company calls and update ATIS information. Aircraft vectored off of established arrival procedure. Speed and altitude numbers similar, ie, speed 270 KTS, FL270. Supplemental information from acn 574186: flight was on radar vectors, heading changes and descent from FL330 to FL270. The controller said that the descent was for air carrier Y. I was never told about air carrier Y as a flight number conflict. The controller also sounded uncertain of what directions she had given me.

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Original NASA ASRS Text

Title: FLT CREW OF A320 INADVERTENTLY READ BACK CLRNC FOR ACR FLT WITH SIMILAR CALL SIGN AND BEGAN DSCNT WITHOUT CLRNC.

Narrative: NEAR MAGOO INTXN, ZAU STARTED ACFT ON A DSCNT TO FL350, THEN FL330, THEN FL290, THEN FL270. CAPT WAS PNF. FO WAS PF. ACFT SPD WAS THEN SLOWED BY ATC, THEN ACFT WAS PLACED ON A RADAR VECTOR. AT FL270, CAPT SWITCHED TO COM #2 TO GET ATIS AND TO CALL GND HANDLER FOR GATE INFO. FO/PF WAS GIVEN COM #1 FOR ATC. DURING THIS TIME, FO/PF STARTED DSCNT TO FL240, STATING THIS WAS GIVEN BY ATC. CAPT/PNF DID NOT HEAR THIS EXCHANGE WHILE ON COM #2. CAPT/PNF RETURNED TO COM #1. AT FL250, ATC CALLED SAYING ACFT WAS BELOW ASSIGNED ALT. CAPT/PNF STATED WE WERE ASSIGNED FL240 AND WERE ON ASSIGNED HDG. ATC STATED ACFT SHOULD BE AT HIGHER ALT. ATC THEN GAVE ACFT FL250, AND SWITCHED ACFT TO ANOTHER FREQ. NEXT ATC FREQ ADVISED ACFT OF POSSIBLE PLTDEV. ACFT CONTINUED TO ORD. FACTORS: FO/PF NEW TO ACFT TYPE, BUT EXPERIENCED. ACFT AHEAD HAD SIMILAR FLT NUMBER. MULTIPLE ALT, HDG, AND SPD CHANGES IN SHORT PERIOD. 2 MAN FLT DECK. NEED FOR PNF TO LEAVE ATC FREQ TO MAKE COMPANY CALLS AND UPDATE ATIS INFO. ACFT VECTORED OFF OF ESTABLISHED ARR PROC. SPD AND ALT NUMBERS SIMILAR, IE, SPD 270 KTS, FL270. SUPPLEMENTAL INFO FROM ACN 574186: FLT WAS ON RADAR VECTORS, HDG CHANGES AND DSCNT FROM FL330 TO FL270. THE CTLR SAID THAT THE DSCNT WAS FOR ACR Y. I WAS NEVER TOLD ABOUT ACR Y AS A FLT NUMBER CONFLICT. THE CTLR ALSO SOUNDED UNCERTAIN OF WHAT DIRECTIONS SHE HAD GIVEN ME.

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.