Narrative:

Small aircraft X under control of ZMP who was directing approachs into tvc. I had to overfly the area of the approachs which meant I had to stay high. My en route altitude was 6000' and I was told to descend to 4500' when approaching the tvc area because my destination airport was about 20 mi beyond. Upon reaching 4500' I was asked for my altitude. I replied '4.5,' which was my altitude, and the controller told me to turn off my altitude squawk because he was showing 4200'. I turned it off. I was then cleared to descend to 3000' (I thought) and I read back, 'out of 4.5 for 3000,' and started a descent. I checked the charts of the NDB approach to acb and found that 3000' was a reasonable altitude for vectors to the approach. (In fact, I had been cleared to descend to 3700'.) shortly after reading back the descent to 3000', I was asked my altitude, and I replied, '3.5.' the controller responded (from my memory), '3.5, uh, ok.' I continued the descent and leveled at 3000'. Shortly the controller again asked my altitude and I replied, '3000.' at this point the controller told me, 'you are supposed to be at 3700, sir,' and I replied, 'I don't think so, but I'm climbing back up.' then the controller asked me a question about what I read back, and I wasn't sure what he wanted, so I asked him, 'which readback?'. Nothing much further was said until the controller cleared me for the approach and said, 'it appears that there has been a system error between you and aircraft Y??? Will you please call the center when you get on the ground?' after talking with the center people twice, they said I had been cleared to 3700', but I did read back 3000' and the controller didn't catch the readback. Then, when later I reported '3.5,' he again failed to catch the problem. The center people said that it appeared to have been a controller error (a supervisor). I agree--most of the problem may lie with the controller--but there are 2 things I intend to do to help eliminate this in the future: 1) listen more carefully. 2) be more clear in my readbacks and responses than I was. If I had said, 'descending through 3.5,' or '3.5 down to 3000,' instead of just '3.5,' the controller would most likely have caught the error with little harm done. As it was, it was my impression that he was confused by my report of '3.5,' thinking that maybe I was just a little low and would correct. 3) I doubt if using the more correct '30-500' instead of just '3.5' would make any different in this or most situations, but I intend to try to stick to more standard phraseology.

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Original NASA ASRS Text

Title: SMA X HAD LESS THAN STANDARD SEPARATION FROM ACFT Y. SYSTEM ERROR. SEPARATION ESTIMATE VERTICAL: 300', HORIZONTAL: 1 MILE.

Narrative: SMA X UNDER CTL OF ZMP WHO WAS DIRECTING APCHS INTO TVC. I HAD TO OVERFLY THE AREA OF THE APCHS WHICH MEANT I HAD TO STAY HIGH. MY ENRTE ALT WAS 6000' AND I WAS TOLD TO DSND TO 4500' WHEN APCHING THE TVC AREA BECAUSE MY DEST ARPT WAS ABOUT 20 MI BEYOND. UPON REACHING 4500' I WAS ASKED FOR MY ALT. I REPLIED '4.5,' WHICH WAS MY ALT, AND THE CTLR TOLD ME TO TURN OFF MY ALT SQUAWK BECAUSE HE WAS SHOWING 4200'. I TURNED IT OFF. I WAS THEN CLRED TO DSND TO 3000' (I THOUGHT) AND I READ BACK, 'OUT OF 4.5 FOR 3000,' AND STARTED A DSNT. I CHKED THE CHARTS OF THE NDB APCH TO ACB AND FOUND THAT 3000' WAS A REASONABLE ALT FOR VECTORS TO THE APCH. (IN FACT, I HAD BEEN CLRED TO DSND TO 3700'.) SHORTLY AFTER READING BACK THE DSNT TO 3000', I WAS ASKED MY ALT, AND I REPLIED, '3.5.' THE CTLR RESPONDED (FROM MY MEMORY), '3.5, UH, OK.' I CONTINUED THE DSNT AND LEVELED AT 3000'. SHORTLY THE CTLR AGAIN ASKED MY ALT AND I REPLIED, '3000.' AT THIS POINT THE CTLR TOLD ME, 'YOU ARE SUPPOSED TO BE AT 3700, SIR,' AND I REPLIED, 'I DON'T THINK SO, BUT I'M CLBING BACK UP.' THEN THE CTLR ASKED ME A QUESTION ABOUT WHAT I READ BACK, AND I WASN'T SURE WHAT HE WANTED, SO I ASKED HIM, 'WHICH READBACK?'. NOTHING MUCH FURTHER WAS SAID UNTIL THE CTLR CLRED ME FOR THE APCH AND SAID, 'IT APPEARS THAT THERE HAS BEEN A SYS ERROR BTWN YOU AND ACFT Y??? WILL YOU PLEASE CALL THE CENTER WHEN YOU GET ON THE GND?' AFTER TALKING WITH THE CENTER PEOPLE TWICE, THEY SAID I HAD BEEN CLRED TO 3700', BUT I DID READ BACK 3000' AND THE CTLR DIDN'T CATCH THE READBACK. THEN, WHEN LATER I RPTED '3.5,' HE AGAIN FAILED TO CATCH THE PROB. THE CENTER PEOPLE SAID THAT IT APPEARED TO HAVE BEEN A CTLR ERROR (A SUPVR). I AGREE--MOST OF THE PROB MAY LIE WITH THE CTLR--BUT THERE ARE 2 THINGS I INTEND TO DO TO HELP ELIMINATE THIS IN THE FUTURE: 1) LISTEN MORE CAREFULLY. 2) BE MORE CLR IN MY READBACKS AND RESPONSES THAN I WAS. IF I HAD SAID, 'DSNDING THROUGH 3.5,' OR '3.5 DOWN TO 3000,' INSTEAD OF JUST '3.5,' THE CTLR WOULD MOST LIKELY HAVE CAUGHT THE ERROR WITH LITTLE HARM DONE. AS IT WAS, IT WAS MY IMPRESSION THAT HE WAS CONFUSED BY MY RPT OF '3.5,' THINKING THAT MAYBE I WAS JUST A LITTLE LOW AND WOULD CORRECT. 3) I DOUBT IF USING THE MORE CORRECT '30-500' INSTEAD OF JUST '3.5' WOULD MAKE ANY DIFFERENT IN THIS OR MOST SITUATIONS, BUT I INTEND TO TRY TO STICK TO MORE STANDARD PHRASEOLOGY.

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.