Narrative:

Passing 4000 ft during a descent from 7000 ft MSL to 3000 ft MSL while on vectors to final for runway 04R at O'hare we sighted a B-757 approaching us from right to left and below us. The B-757 was also noted on our TCASII at 1000 ft below us. About the same time the TCASII alert 'traffic, traffic' went off and I noted the same target on the TCASII at 700 ft below us. We stopped our descent at approximately 3650 ft MSL and asked approach control if we had been cleared to 3000 ft. Approach control, after a short pause, stated we were only cleared to 4000 ft. We immediately initiated a climb back to 4000 ft MSL. After climbing to 4000 ft we checked our altitude reminder set at 3000 ft and reset it to 4000 ft. At this time I do not know whether we misunderstood the descent clearance or we miss-set the altitude alert or both. Approach control then cleared us for an intercept heading for the final approach course and cleared us for the approach to runway 4R. I am not really sure how to keep these things from happening. I do know that as aircrew we must constantly be very careful when changing altitudes and when accepting clrncs. I believe all the crewmembers on my crew were performing their duties correctly, but once again, a crew can never let down their guard and must constantly use all SOP's so that things like this do not happen. The TCASII was a valuable piece of equipment during this incident. Callback conversation with reporter revealed the following information: the reporter was flying a B727-200 advanced aircraft and the other aircraft was an air carrier B757-200. The other aircraft, according to the FAA investigators, did not receive a TCASII warning. The FAA assigned the captain the penalty of a letter of warning to be active in his file for 2 yrs. The approach controller assigned the altitude of 4000 ft to this flight crew. After reviewing the tapes the reporter admits to the error, but is still unsure how it happened. He did readback 4000 ft, but set 3000 ft in the altitude alerter.

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Original NASA ASRS Text

Title: ALTDEV ALT OVERSHOT -- ACR DSNDS BELOW ITS ASSIGNED ALT, GETS A TCASII WARNING AND, AFTER A DISCUSSION WITH THE APCH CTLR, LEVELS AT THE ASSIGNED ALT.

Narrative: PASSING 4000 FT DURING A DSCNT FROM 7000 FT MSL TO 3000 FT MSL WHILE ON VECTORS TO FINAL FOR RWY 04R AT O'HARE WE SIGHTED A B-757 APCHING US FROM R TO L AND BELOW US. THE B-757 WAS ALSO NOTED ON OUR TCASII AT 1000 FT BELOW US. ABOUT THE SAME TIME THE TCASII ALERT 'TFC, TFC' WENT OFF AND I NOTED THE SAME TARGET ON THE TCASII AT 700 FT BELOW US. WE STOPPED OUR DSCNT AT APPROX 3650 FT MSL AND ASKED APCH CTL IF WE HAD BEEN CLRED TO 3000 FT. APCH CTL, AFTER A SHORT PAUSE, STATED WE WERE ONLY CLRED TO 4000 FT. WE IMMEDIATELY INITIATED A CLB BACK TO 4000 FT MSL. AFTER CLBING TO 4000 FT WE CHKED OUR ALT REMINDER SET AT 3000 FT AND RESET IT TO 4000 FT. AT THIS TIME I DO NOT KNOW WHETHER WE MISUNDERSTOOD THE DSCNT CLRNC OR WE MISS-SET THE ALT ALERT OR BOTH. APCH CTL THEN CLRED US FOR AN INTERCEPT HDG FOR THE FINAL APCH COURSE AND CLRED US FOR THE APCH TO RWY 4R. I AM NOT REALLY SURE HOW TO KEEP THESE THINGS FROM HAPPENING. I DO KNOW THAT AS AIRCREW WE MUST CONSTANTLY BE VERY CAREFUL WHEN CHANGING ALTS AND WHEN ACCEPTING CLRNCS. I BELIEVE ALL THE CREWMEMBERS ON MY CREW WERE PERFORMING THEIR DUTIES CORRECTLY, BUT ONCE AGAIN, A CREW CAN NEVER LET DOWN THEIR GUARD AND MUST CONSTANTLY USE ALL SOP'S SO THAT THINGS LIKE THIS DO NOT HAPPEN. THE TCASII WAS A VALUABLE PIECE OF EQUIP DURING THIS INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR WAS FLYING A B727-200 ADVANCED ACFT AND THE OTHER ACFT WAS AN ACR B757-200. THE OTHER ACFT, ACCORDING TO THE FAA INVESTIGATORS, DID NOT RECEIVE A TCASII WARNING. THE FAA ASSIGNED THE CAPT THE PENALTY OF A LETTER OF WARNING TO BE ACTIVE IN HIS FILE FOR 2 YRS. THE APCH CTLR ASSIGNED THE ALT OF 4000 FT TO THIS FLC. AFTER REVIEWING THE TAPES THE RPTR ADMITS TO THE ERROR, BUT IS STILL UNSURE HOW IT HAPPENED. HE DID READBACK 4000 FT, BUT SET 3000 FT IN THE ALT ALERTER.

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.