Narrative:

Cleared to FL350 and climbing through FL24.3 msp ARTCC instructed our flight to descend immediately to FL200! After establishing a rapid descent, we naturally asked why? The curt response was 'what altitude were you cleared to?' not knowing which clearance the controller was referring to, we restated our last 3 altitude clrncs. After considerable discussion, it was determined that during our climb to FL350 our mode C indicated a descent which put us in conflict with an small transport that was climbing beneath us. The controller assumed that our mode C was correct, which it wasn't, and tried to descend us below the small transport. We were IMC and never saw the small transport. We asked the controller for the altitude and position of the small transport, but he would not tell us, so we don't know if a loss of sep occurred. Our mode C functioned normally until arriving in den approachs airspace, at which time it gave inaccurate altitude reports. The msp ARTCC controller assumed that our mode C was correct and that an air carrier jet had begun an unauthorized descent west/O advising ATC. The frequency was not busy and a simple radio verification of our altitude would have eliminated a potentially disastrous situation.

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Original NASA ASRS Text

Title: ACR CLIMBING TO ASSIGNED ALT TOLD TO DESCEND IMMEDIATELY. COMPLIED, THEN ASKED WHY? TOLD MODE C INDICATED A DESCENT WHICH WOULD CREATE CONFLICT.

Narrative: CLRED TO FL350 AND CLBING THROUGH FL24.3 MSP ARTCC INSTRUCTED OUR FLT TO DSND IMMEDIATELY TO FL200! AFTER ESTABLISHING A RAPID DSNT, WE NATURALLY ASKED WHY? THE CURT RESPONSE WAS 'WHAT ALT WERE YOU CLRED TO?' NOT KNOWING WHICH CLRNC THE CTLR WAS REFERRING TO, WE RESTATED OUR LAST 3 ALT CLRNCS. AFTER CONSIDERABLE DISCUSSION, IT WAS DETERMINED THAT DURING OUR CLB TO FL350 OUR MODE C INDICATED A DSNT WHICH PUT US IN CONFLICT WITH AN SMT THAT WAS CLBING BENEATH US. THE CTLR ASSUMED THAT OUR MODE C WAS CORRECT, WHICH IT WASN'T, AND TRIED TO DSND US BELOW THE SMT. WE WERE IMC AND NEVER SAW THE SMT. WE ASKED THE CTLR FOR THE ALT AND POS OF THE SMT, BUT HE WOULD NOT TELL US, SO WE DON'T KNOW IF A LOSS OF SEP OCCURRED. OUR MODE C FUNCTIONED NORMALLY UNTIL ARRIVING IN DEN APCHS AIRSPACE, AT WHICH TIME IT GAVE INACCURATE ALT RPTS. THE MSP ARTCC CTLR ASSUMED THAT OUR MODE C WAS CORRECT AND THAT AN AIR CARRIER JET HAD BEGUN AN UNAUTH DSNT W/O ADVISING ATC. THE FREQ WAS NOT BUSY AND A SIMPLE RADIO VERIFICATION OF OUR ALT WOULD HAVE ELIMINATED A POTENTIALLY DISASTROUS SITUATION.

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.