Narrative:

Small transport was handed off to me descending from 7500 ft pilot discretion to 3000 ft. On initial contact with me, small transport was instructed to 'proceed direct to TDZ for a visibility approach, no WX reporting service available for TDZ.' small aircraft had been making practice approachs to tol and was given, 'turn right heading 1-0-0 maintain 2000, 500 after completing an NDB runway 7 at tol.' this was for vector to VOR 18 final approach course at igo. Small aircraft requested holding instructions for training at vwv VOR. After issuing the holding instructions, I instructed small transport to maintain 3500 ft. This was to provide IFR sep. There was no reply. His xmissions had been intermittent, weak and his attention drifting at times. I repeated my instructions. He repeated what I thought was my instructions. Someone immediately called. The next transmission from small aircraft about the traffic off his right (small transport) alerted me to the situation. I keyed the microphone to advise small aircraft that small transport was maintaining 3500 ft. Then I noticed as small transport tag cleared another aircraft's tag that his mode C indicated 3100 ft. In the middle of the my transmission to small aircraft, I instructed small transport to maintain 3000 ft. This was an attempt to keep some sep. The aircraft didn't reply and his mode C indicated a continued descent. I then told small aircraft that I had lost communication with the aircraft. After the targets passed I informed small transport that he was stopped at 3500 ft because of the traffic he just passed. His hesitant reply was that he saw the traffic and he was level at 2000 ft. His mode C read 2400. It is my contention that the pilot knew of his restriction but was not paying attention to what he was saying or doing. This became apparent when I tried to issue traffic and issue approach clearance and he didn't reply right away. Supplemental information from acn 103129: while on IFR clearance proceeding from tol direct vwv VOR I observed another aircraft at my 2 O'clock position on a constant relative bearing and at our same altitude. After conversations with tol approach I discovered this other aircraft was assigned 3500 ft, however the small transport had descended to 2500 ft west/O clearance. Due to very bright sun I feel fortunate to have seen this other aircraft. I sincerely wish other pilots operating within the IFR/arsa system would comply with ATC instructions. Considering the purpose first officer our flight was the examination of an instrument flight instrument candidate for an additional rating, which he passed, I can assure you we were complying with ATC instructions.

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Original NASA ASRS Text

Title: SMT DESCENDED BELOW HIS ASSIGNED ALT RESULTING IN LESS THAN STANDARD SEPARATION WITH AN SMA. NMAC FILED. OPERATIONAL ERROR.

Narrative: SMT WAS HANDED OFF TO ME DSNDING FROM 7500 FT PLT DISCRETION TO 3000 FT. ON INITIAL CONTACT WITH ME, SMT WAS INSTRUCTED TO 'PROCEED DIRECT TO TDZ FOR A VIS APCH, NO WX RPTING SVC AVAILABLE FOR TDZ.' SMA HAD BEEN MAKING PRACTICE APCHS TO TOL AND WAS GIVEN, 'TURN RIGHT HDG 1-0-0 MAINTAIN 2000, 500 AFTER COMPLETING AN NDB RWY 7 AT TOL.' THIS WAS FOR VECTOR TO VOR 18 FINAL APCH COURSE AT IGO. SMA REQUESTED HOLDING INSTRUCTIONS FOR TRNING AT VWV VOR. AFTER ISSUING THE HOLDING INSTRUCTIONS, I INSTRUCTED SMT TO MAINTAIN 3500 FT. THIS WAS TO PROVIDE IFR SEP. THERE WAS NO REPLY. HIS XMISSIONS HAD BEEN INTERMITTENT, WEAK AND HIS ATTN DRIFTING AT TIMES. I REPEATED MY INSTRUCTIONS. HE REPEATED WHAT I THOUGHT WAS MY INSTRUCTIONS. SOMEONE IMMEDIATELY CALLED. THE NEXT XMISSION FROM SMA ABOUT THE TFC OFF HIS RIGHT (SMT) ALERTED ME TO THE SITUATION. I KEYED THE MIC TO ADVISE SMA THAT SMT WAS MAINTAINING 3500 FT. THEN I NOTICED AS SMT TAG CLRED ANOTHER ACFT'S TAG THAT HIS MODE C INDICATED 3100 FT. IN THE MIDDLE OF THE MY XMISSION TO SMA, I INSTRUCTED SMT TO MAINTAIN 3000 FT. THIS WAS AN ATTEMPT TO KEEP SOME SEP. THE ACFT DIDN'T REPLY AND HIS MODE C INDICATED A CONTINUED DSCNT. I THEN TOLD SMA THAT I HAD LOST COM WITH THE ACFT. AFTER THE TARGETS PASSED I INFORMED SMT THAT HE WAS STOPPED AT 3500 FT BECAUSE OF THE TFC HE JUST PASSED. HIS HESITANT REPLY WAS THAT HE SAW THE TFC AND HE WAS LEVEL AT 2000 FT. HIS MODE C READ 2400. IT IS MY CONTENTION THAT THE PLT KNEW OF HIS RESTRICTION BUT WAS NOT PAYING ATTN TO WHAT HE WAS SAYING OR DOING. THIS BECAME APPARENT WHEN I TRIED TO ISSUE TFC AND ISSUE APCH CLRNC AND HE DIDN'T REPLY RIGHT AWAY. SUPPLEMENTAL INFO FROM ACN 103129: WHILE ON IFR CLRNC PROCEEDING FROM TOL DIRECT VWV VOR I OBSERVED ANOTHER ACFT AT MY 2 O'CLOCK POS ON A CONSTANT RELATIVE BEARING AND AT OUR SAME ALT. AFTER CONVERSATIONS WITH TOL APCH I DISCOVERED THIS OTHER ACFT WAS ASSIGNED 3500 FT, HOWEVER THE SMT HAD DSNDED TO 2500 FT W/O CLRNC. DUE TO VERY BRIGHT SUN I FEEL FORTUNATE TO HAVE SEEN THIS OTHER ACFT. I SINCERELY WISH OTHER PLTS OPERATING WITHIN THE IFR/ARSA SYS WOULD COMPLY WITH ATC INSTRUCTIONS. CONSIDERING THE PURPOSE FO OUR FLT WAS THE EXAMINATION OF AN INSTRUMENT FLT INSTR CANDIDATE FOR AN ADDITIONAL RATING, WHICH HE PASSED, I CAN ASSURE YOU WE WERE COMPLYING WITH ATC INSTRUCTIONS.

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.