Narrative:

On flying the walet 5 arrival into mem, mem approach cleared us to descend to 6000'. On the descent, we noticed an mlt on a northbound heading that could be a possible conflict, although I assumed him to be at a lower altitude--probably 5000'. As I continued the descent toward 6000', it appeared that the mlt might be at or near 6000'. We questioned mem approach and he responded that the mlt had been cleared to 2000' and showed 4000' on mode C. The controller then asked the mlt pilot his altitude and he responded that it was 6000'. At this point I began a turn toward the north to provide sep. I estimate that we passed within 1/2 mi of the mlt. Since we had the mlt in sight, there was no danger of collision. However, had it been dark of had there been WX, the danger of a collision would have been very real. On arrival at mem, I called mem approach on the phone and spoke about the incident. He told me that he was getting the information together and that he was waiting to talk to the mlt pilot. Since I did not hear the controller assign a lower altitude to the mlt, I do not know whether the mlt pilot or the controller was at fault. The situation was made especially dangerous by the apparent failure of the mlt mode C since it was indicating an altitude of 4000' when the airplane was at 6000'.

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Original NASA ASRS Text

Title: ACR DESCENDED TO ALT OCCUPIED BY ANOTHER ACFT.

Narrative: ON FLYING THE WALET 5 ARR INTO MEM, MEM APCH CLRED US TO DSND TO 6000'. ON THE DSCNT, WE NOTICED AN MLT ON A NBOUND HDG THAT COULD BE A POSSIBLE CONFLICT, ALTHOUGH I ASSUMED HIM TO BE AT A LOWER ALT--PROBABLY 5000'. AS I CONTINUED THE DSCNT TOWARD 6000', IT APPEARED THAT THE MLT MIGHT BE AT OR NEAR 6000'. WE QUESTIONED MEM APCH AND HE RESPONDED THAT THE MLT HAD BEEN CLRED TO 2000' AND SHOWED 4000' ON MODE C. THE CTLR THEN ASKED THE MLT PLT HIS ALT AND HE RESPONDED THAT IT WAS 6000'. AT THIS POINT I BEGAN A TURN TOWARD THE N TO PROVIDE SEP. I ESTIMATE THAT WE PASSED WITHIN 1/2 MI OF THE MLT. SINCE WE HAD THE MLT IN SIGHT, THERE WAS NO DANGER OF COLLISION. HOWEVER, HAD IT BEEN DARK OF HAD THERE BEEN WX, THE DANGER OF A COLLISION WOULD HAVE BEEN VERY REAL. ON ARR AT MEM, I CALLED MEM APCH ON THE PHONE AND SPOKE ABOUT THE INCIDENT. HE TOLD ME THAT HE WAS GETTING THE INFO TOGETHER AND THAT HE WAS WAITING TO TALK TO THE MLT PLT. SINCE I DID NOT HEAR THE CTLR ASSIGN A LOWER ALT TO THE MLT, I DO NOT KNOW WHETHER THE MLT PLT OR THE CTLR WAS AT FAULT. THE SITUATION WAS MADE ESPECIALLY DANGEROUS BY THE APPARENT FAILURE OF THE MLT MODE C SINCE IT WAS INDICATING AN ALT OF 4000' WHEN THE AIRPLANE WAS AT 6000'.

Data retrieved from NASA's ASRS site as of August 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.