Narrative:

Aircraft X had checking in level at FL360 with a mode C readout of FL360. Approximately 20 miles inside of our airspace; he began a distorted conversation with the radar that we both had difficulty making out between the pilots poor english and broken sentences. He more than once referred to himself with the wrong call sign and was trying to communicate something about his altimeters and FL330 and FL300. A the end of this cumbersome exchange; I understood that he had some part of his mode C that wasn't working correctly but that he was using the other side. Due to the fact that the pilot was so hard to understand; I had a concern that the pilots may be hypoxic and while the radar caught up with the traffic I entered new pref routing to ZZZ in order to listen to pilot's readback. The pilot read back the clearance clearly. He came back and verified that he was cleared direct zzzzz at that time; which the radar confirmed. I resigned to just advise the next sector to monitor the pilot's transmissions for anything out of the ordinary. Within another 10 miles of flying; the pilot asked what altitude the aircraft that just passed near to him was at. The radar informed him that the aircraft was at FL330. The pilot then said something that informed us that he thought he was at FL330; although it was still unclear which mode C in his cockpit was correct. I turned around and asked the radar controller behind me who was working said traffic at FL330; aircraft Y if he had seen any traffic. The radar reported to me that the pilot only saw traffic below him. I do not know if there was any other traffic in the vicinity below aircraft Y. Hearing this response; however; and being trained to trust the equipment; I was doubtful that aircraft X was in fact at FL330. I did point aircraft X out to the two sectors below our stratum and across the control room from me and told them that the aircraft may be at FL330; as he seemed to be saying; and that he seemed to have lost his mode C; which I thought would make him negative rvsm; requiring additional separation and that we were treating him as an emergency. I also told radar to separate a ZZZZ inbound laterally from aircraft X. It still seemed possible from listening to the pilot talk that he may be hypoxic; so I voiced my concern that the aircraft was negative rvsm and should be descended regardless. There was confusion at this point as the radar did not know what altitude aircraft X was actually at; and how to go about descending him. The supervisor was saying that the pilot needed to report that he was negative rvsm; which made no sense under the circumstances. As the aircraft approached out lateral boundary as it headed north west bound; I did hear him audibly request to descend 1;000 feet to FL320. I included that request as I coordinated the situation with area 2 sectors so that they could block all of the altitudes between FL330-360. I also passed along my assessment that the aircraft was likely negative rvsm. It became clearer as the aircraft approached the area 2 boundary that they did in fact report to be at FL330; which I also forwarded along with the pilot's request for FL320 and that we were treating it as a [priority]. If aircraft X was in fact at FL330; he passed very close to aircraft Y; I would guess less than 2 miles laterally. It seems likely that if the aircraft was in fact level at FL330 that the pilot's did not follow some procedure to correct their mode C readout. If not; then there obviously needs to be a procedure for this circumstance; whatever it was. If aircraft X was actually at FL330 the danger presented here cannot be overstated given that no TCAS alerts would ever go off; and that no traffic or safety alerts would ever be issued; there was a very serious possibility of a midair collision between aircraft X and aircraft Y.

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Original NASA ASRS Text

Title: ZOB Controllers reported a near mid-air collision when a CL60 with incorrect altitude information disabled conflict alerts and TCAS.

Narrative: Aircraft X had checking in level at FL360 with a MODE C readout of FL360. Approximately 20 miles inside of our airspace; he began a distorted conversation with the radar that we both had difficulty making out between the pilots poor English and broken sentences. He more than once referred to himself with the wrong call sign and was trying to communicate something about his altimeters and FL330 and FL300. A the end of this cumbersome exchange; I understood that he had some part of his Mode C that wasn't working correctly but that he was using the other side. Due to the fact that the pilot was so hard to understand; I had a concern that the pilots may be hypoxic and while the radar caught up with the traffic I entered new pref routing to ZZZ in order to listen to pilot's readback. The pilot read back the clearance clearly. He came back and verified that he was cleared direct ZZZZZ at that time; which the RADAR confirmed. I resigned to just advise the next sector to monitor the pilot's transmissions for anything out of the ordinary. Within another 10 miles of flying; the pilot asked what altitude the aircraft that just passed near to him was at. The RADAR informed him that the aircraft was at FL330. The pilot then said something that informed us that he thought he was at FL330; although it was still unclear which MODE C in his cockpit was correct. I turned around and asked the RADAR controller behind me who was working said traffic at FL330; Aircraft Y if he had seen any traffic. The radar reported to me that the pilot only saw traffic below him. I do not know if there was any other traffic in the vicinity below Aircraft Y. Hearing this response; however; and being trained to trust the equipment; I was doubtful that Aircraft X was in fact at FL330. I did point Aircraft X out to the two sectors below our stratum and across the control room from me and told them that the aircraft may be at FL330; as he seemed to be saying; and that he seemed to have lost his mode C; which I thought would make him NEGATIVE RVSM; requiring additional separation and that we were treating him as an emergency. I also told Radar to separate a ZZZZ inbound laterally from Aircraft X. It still seemed possible from listening to the pilot talk that he may be hypoxic; so I voiced my concern that the aircraft was negative RVSM and should be descended regardless. There was confusion at this point as the RADAR did not know what altitude Aircraft X was actually at; and how to go about descending him. The Supervisor was saying that the pilot needed to report that he was negative RVSM; which made no sense under the circumstances. As the aircraft approached out lateral boundary as it headed north west bound; I did hear him audibly request to descend 1;000 feet to FL320. I included that request as I coordinated the situation with AREA 2 sectors so that they could block all of the altitudes between FL330-360. I also passed along my assessment that the aircraft was likely negative RVSM. It became clearer as the aircraft approached the AREA 2 boundary that they did in fact report to be at FL330; which I also forwarded along with the pilot's request for FL320 and that we were treating it as a [priority]. If Aircraft X was in fact at FL330; he passed very close to Aircraft Y; I would guess less than 2 miles laterally. It seems likely that if the aircraft was in fact level at FL330 that the pilot's did not follow some procedure to correct their MODE C readout. If not; then there obviously needs to be a procedure for this circumstance; whatever it was. If Aircraft X was actually at FL330 the danger presented here cannot be overstated given that no TCAS alerts would ever go off; and that no traffic or safety alerts would ever be issued; there was a very serious possibility of a midair collision between Aircraft X and Aircraft Y.

Data retrieved from NASA's ASRS site 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.