Narrative:

While on a vector turn from downwind to left base we were instructed to descend from 3000 to 2000' MSL. We were VMC above WX conducting a CAT III-a to 800' RVR. Controller was busy, traffic moderate. I heard first officer read back 2000' and thought, 'that's below the OM crossing altitude; he must be trying to avoid another aircraft.' after the first officer read back 2000' and the controller didn't say anything, I did what I was told. As I passed 2400' descending the controller asked what we were doing. Controller wanted 2800'. We later worked it out over the phone. Next time I'm not going below the OM crossing altitude prior to the outer unless he gives me a good reason. Callback conversation with reporter revealed the following: it was learned that the approach controller supervisor admitted a possible controller error. An aircraft accident earlier at a nearby airport had upset the ATC routine so the readback was not monitored. Reporter thinks this is another case where controller workload was a factor.

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Original NASA ASRS Text

Title: ACR MLG ALT DEVIATION EXCURSION FROM CLRNC ALT ON RADAR VECTOR TO IAP ILS APCH AT IND. FLT CREW HEARD AND READ BACK 2000'. APCH CTLR SAYS HE CLEARED THE ACFT TO 3000'.

Narrative: WHILE ON A VECTOR TURN FROM DOWNWIND TO LEFT BASE WE WERE INSTRUCTED TO DSND FROM 3000 TO 2000' MSL. WE WERE VMC ABOVE WX CONDUCTING A CAT III-A TO 800' RVR. CTLR WAS BUSY, TFC MODERATE. I HEARD F/O READ BACK 2000' AND THOUGHT, 'THAT'S BELOW THE OM XING ALT; HE MUST BE TRYING TO AVOID ANOTHER ACFT.' AFTER THE F/O READ BACK 2000' AND THE CTLR DIDN'T SAY ANYTHING, I DID WHAT I WAS TOLD. AS I PASSED 2400' DSNDING THE CTLR ASKED WHAT WE WERE DOING. CTLR WANTED 2800'. WE LATER WORKED IT OUT OVER THE PHONE. NEXT TIME I'M NOT GOING BELOW THE OM XING ALT PRIOR TO THE OUTER UNLESS HE GIVES ME A GOOD REASON. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: IT WAS LEARNED THAT THE APCH CTLR SUPVR ADMITTED A POSSIBLE CTLR ERROR. AN ACFT ACCIDENT EARLIER AT A NEARBY ARPT HAD UPSET THE ATC ROUTINE SO THE READBACK WAS NOT MONITORED. RPTR THINKS THIS IS ANOTHER CASE WHERE CTLR WORKLOAD WAS A FACTOR.

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.