Narrative:

On this flight we had a MEL'ed FMS; and had been flying the entire flight in 'raw data.' as the PNF; I had spent a good portion of the flight explaining to my first officer about the limitations of what we can and cannot do with the FMS inoperative. (Unable to do RNAV arrs; direct an intersection; etc.) when we were cleared for the ILS runway 28 approach into chicago; we were told to maintain 5000 ft MSL until crossing wavie intersection. My first officer; thinking that the approach mode on our flight director would not work because of the inoperative FMS; disconnected the autoplt as we were cleared for the ILS approach. This was a misunderstanding; as the flight director would work; and was required to be used by our operations specifications for this approach. I armed the approach mode for her; and continued monitoring her flying as the autoplt was off. I did not notice that when the GS became alive; and intercepted; that we were not quite to wavie intersection yet...as the flight director started to command a descent; we followed it down; causing us to be almost 300 ft low as we crossed wavie intersection. I think some of the factors to this event happening were the inoperative FMS and my first officer's lack of knowledge on abnormal operations without the FMS. Also; when the approach clearance was given; the altitude restr was not spoken very clearly. When later queried by ATC I did remember the instruction; but I feel it could have been spoken more clearly (not mumbled by the controller).

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Original NASA ASRS Text

Title: A CL-600 FLT CREW ON AN ILS APPROACH TO ORD WERE 300 FT LOW AT WAVIE; DUE IN PART TO THE PROCEDURE BEING FLOWN IN RAW DATA BECAUSE OF AN INOP FMS.

Narrative: ON THIS FLT WE HAD A MEL'ED FMS; AND HAD BEEN FLYING THE ENTIRE FLT IN 'RAW DATA.' AS THE PNF; I HAD SPENT A GOOD PORTION OF THE FLT EXPLAINING TO MY FO ABOUT THE LIMITATIONS OF WHAT WE CAN AND CANNOT DO WITH THE FMS INOP. (UNABLE TO DO RNAV ARRS; DIRECT AN INTXN; ETC.) WHEN WE WERE CLRED FOR THE ILS RWY 28 APCH INTO CHICAGO; WE WERE TOLD TO MAINTAIN 5000 FT MSL UNTIL XING WAVIE INTXN. MY FO; THINKING THAT THE APCH MODE ON OUR FLT DIRECTOR WOULD NOT WORK BECAUSE OF THE INOP FMS; DISCONNECTED THE AUTOPLT AS WE WERE CLRED FOR THE ILS APCH. THIS WAS A MISUNDERSTANDING; AS THE FLT DIRECTOR WOULD WORK; AND WAS REQUIRED TO BE USED BY OUR OPS SPECS FOR THIS APCH. I ARMED THE APCH MODE FOR HER; AND CONTINUED MONITORING HER FLYING AS THE AUTOPLT WAS OFF. I DID NOT NOTICE THAT WHEN THE GS BECAME ALIVE; AND INTERCEPTED; THAT WE WERE NOT QUITE TO WAVIE INTXN YET...AS THE FLT DIRECTOR STARTED TO COMMAND A DSCNT; WE FOLLOWED IT DOWN; CAUSING US TO BE ALMOST 300 FT LOW AS WE CROSSED WAVIE INTXN. I THINK SOME OF THE FACTORS TO THIS EVENT HAPPENING WERE THE INOP FMS AND MY FO'S LACK OF KNOWLEDGE ON ABNORMAL OPS WITHOUT THE FMS. ALSO; WHEN THE APCH CLRNC WAS GIVEN; THE ALT RESTR WAS NOT SPOKEN VERY CLRLY. WHEN LATER QUERIED BY ATC I DID REMEMBER THE INSTRUCTION; BUT I FEEL IT COULD HAVE BEEN SPOKEN MORE CLRLY (NOT MUMBLED BY THE CTLR).

Data retrieved from NASA's ASRS site as of January 2009 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.