Narrative:

While on descent into midway on the gsh 3 arrival we were given a vectored heading of 240 degrees off of the STAR. ATC then gave us an expedite descent and cleared us to intercept runway 31 localizer. The PIC (PNF) dialed in 4000 ft and we both confirmed 4000 ft. While in the descent, I asked the PNF if he was sure about 4000 ft and he stated that we were cleared to 4000 ft. While passing through 4500 ft ATC advised us that our clearance was to 6000 ft. At that point the PNF stated negative we were cleared to 4000 ft. ATC then told us to maintain 4000 ft. Upon landing at midway at XX08Z we were told to contact ZAU. The PIC (PNF) spoke to an ATC controller who advised the PIC that separation was lost between ourselves and another aircraft (type unknown). Controller stated that he listened to the tapes and that we were cleared to 6000 ft and that the PIC read back 6000 ft. The controller said he would make the required report to flight standards with the recommendation that no action be taken since 80% separation was maintained which was 4 mi +500 ft. What could have been done to prevent this situation is the flight crew (ourselves) should have reconfirmed our altitude with ATC. Supplemental information from acn 384293: I read back the clearance as given and reset the altitude select. The PF was involved in reducing the power and extending the air brakes to slow to 250 KTS and comply with the 'expedite requirement.' just prior to intercepting the localizer, ATC stated that we were assigned 6000 ft and that they showed us descending through 5000 ft. I looked at the altitude select and saw that it was selected to 4000 ft, not 6000 ft. Not believing that I had selected the wrong altitude I stated to ATC that we had been cleared to 4000 ft and expedite, and did he want us to climb to 6000 ft, to which he replied 'negative.' our procedure for altitude changes is for the pilot receiving the clearance to read back the altitude, reset the altitude select, restate the altitude to the other pilot, observe that the pilot looks at the altitude select and confirms the selection by repeating the altitude assigned. Should a discrepancy exist, a request to ATC for confirmation is made. Part, or all, of this procedure was obviously not followed or the improper altitude selection would have been detected and corrected.

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Original NASA ASRS Text

Title: THE FO OF A BRITISH HAWKER BAE125, SERIES 800, OVERSHOT DSCNT ALT DUE TO THE CAPT PLACING THE WRONG ALT IN THE ALT ALERTER. ATC ADVISED THAT THERE WAS A LOSS OF SEPARATION WITH ANOTHER ACFT AND SUBSEQUENTLY CLRED THEM TO THE LOWER ALT TO WHICH THEY WERE HEADED. ATC TAPES REVEALED THAT THE FLC HAD ACKNOWLEDGED THE HIGHER ALT AND READ IT BACK.

Narrative: WHILE ON DSCNT INTO MIDWAY ON THE GSH 3 ARR WE WERE GIVEN A VECTORED HDG OF 240 DEGS OFF OF THE STAR. ATC THEN GAVE US AN EXPEDITE DSCNT AND CLRED US TO INTERCEPT RWY 31 LOC. THE PIC (PNF) DIALED IN 4000 FT AND WE BOTH CONFIRMED 4000 FT. WHILE IN THE DSCNT, I ASKED THE PNF IF HE WAS SURE ABOUT 4000 FT AND HE STATED THAT WE WERE CLRED TO 4000 FT. WHILE PASSING THROUGH 4500 FT ATC ADVISED US THAT OUR CLRNC WAS TO 6000 FT. AT THAT POINT THE PNF STATED NEGATIVE WE WERE CLRED TO 4000 FT. ATC THEN TOLD US TO MAINTAIN 4000 FT. UPON LNDG AT MIDWAY AT XX08Z WE WERE TOLD TO CONTACT ZAU. THE PIC (PNF) SPOKE TO AN ATC CTLR WHO ADVISED THE PIC THAT SEPARATION WAS LOST BTWN OURSELVES AND ANOTHER ACFT (TYPE UNKNOWN). CTLR STATED THAT HE LISTENED TO THE TAPES AND THAT WE WERE CLRED TO 6000 FT AND THAT THE PIC READ BACK 6000 FT. THE CTLR SAID HE WOULD MAKE THE REQUIRED RPT TO FLT STANDARDS WITH THE RECOMMENDATION THAT NO ACTION BE TAKEN SINCE 80% SEPARATION WAS MAINTAINED WHICH WAS 4 MI +500 FT. WHAT COULD HAVE BEEN DONE TO PREVENT THIS SIT IS THE FLC (OURSELVES) SHOULD HAVE RECONFIRMED OUR ALT WITH ATC. SUPPLEMENTAL INFO FROM ACN 384293: I READ BACK THE CLRNC AS GIVEN AND RESET THE ALT SELECT. THE PF WAS INVOLVED IN REDUCING THE PWR AND EXTENDING THE AIR BRAKES TO SLOW TO 250 KTS AND COMPLY WITH THE 'EXPEDITE REQUIREMENT.' JUST PRIOR TO INTERCEPTING THE LOC, ATC STATED THAT WE WERE ASSIGNED 6000 FT AND THAT THEY SHOWED US DSNDING THROUGH 5000 FT. I LOOKED AT THE ALT SELECT AND SAW THAT IT WAS SELECTED TO 4000 FT, NOT 6000 FT. NOT BELIEVING THAT I HAD SELECTED THE WRONG ALT I STATED TO ATC THAT WE HAD BEEN CLRED TO 4000 FT AND EXPEDITE, AND DID HE WANT US TO CLB TO 6000 FT, TO WHICH HE REPLIED 'NEGATIVE.' OUR PROC FOR ALT CHANGES IS FOR THE PLT RECEIVING THE CLRNC TO READ BACK THE ALT, RESET THE ALT SELECT, RESTATE THE ALT TO THE OTHER PLT, OBSERVE THAT THE PLT LOOKS AT THE ALT SELECT AND CONFIRMS THE SELECTION BY REPEATING THE ALT ASSIGNED. SHOULD A DISCREPANCY EXIST, A REQUEST TO ATC FOR CONFIRMATION IS MADE. PART, OR ALL, OF THIS PROC WAS OBVIOUSLY NOT FOLLOWED OR THE IMPROPER ALT SELECTION WOULD HAVE BEEN DETECTED AND CORRECTED.

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.