Narrative:

I was working the east arrival position when an mdt commuter check in on arrival at 10000 ft. He mdt was told to expect runway 23, the next arrival was an medium large transport checking in leaving 12700 ft for 11000. I instructed medium large transport to expect runway 23 and issued the mdt as traffic at 12 O'clock 2.5 mi level at 10000. The pilot questioned the altitude and I informed the pilot 'he's at 10000.' the pilot misunderstood the advisory as a descent clearance and continued past 11000 ft to 10000 ft. During this process I issued a descent clearance to the mdt to 9000 ft. I later noticed the medium large transport descending through 10500 ft as the mdt passed through 9600. I asked the medium large transport to verify his altitude. When the pilot did not respond, I instructed the medium large transport to maintain 11000 and issued the mdt traffic, the medium large transport responded with 'you gave us 10000 ft and we're already there. We have the traffic in sight.' at this point the mdt informed me that he had a TCASII RA and was climbing. I informed the mdt of the position of the trailing aircraft. The mdt climbed from below the altitude the trailing aircraft was descending to, climbed above the medium large transport then descended back below the medium large transport (see example below). It is my opinion that separation would have been maintained if TCASII RA had not occurred, ie, TCASII made a safe operation very hazardous operation for the aircraft involved.

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Original NASA ASRS Text

Title: ACR MDT ALTDEV EXCURSION FROM CLRNC ALT IN RESPONSE TO TCASII RA.

Narrative: I WAS WORKING THE E ARR POS WHEN AN MDT COMMUTER CHK IN ON ARR AT 10000 FT. HE MDT WAS TOLD TO EXPECT RWY 23, THE NEXT ARR WAS AN MLG CHKING IN LEAVING 12700 FT FOR 11000. I INSTRUCTED MLG TO EXPECT RWY 23 AND ISSUED THE MDT AS TFC AT 12 O'CLOCK 2.5 MI LEVEL AT 10000. THE PLT QUESTIONED THE ALT AND I INFORMED THE PLT 'HE'S AT 10000.' THE PLT MISUNDERSTOOD THE ADVISORY AS A DSCNT CLRNC AND CONTINUED PAST 11000 FT TO 10000 FT. DURING THIS PROCESS I ISSUED A DSCNT CLRNC TO THE MDT TO 9000 FT. I LATER NOTICED THE MLG DSNDING THROUGH 10500 FT AS THE MDT PASSED THROUGH 9600. I ASKED THE MLG TO VERIFY HIS ALT. WHEN THE PLT DID NOT RESPOND, I INSTRUCTED THE MLG TO MAINTAIN 11000 AND ISSUED THE MDT TFC, THE MLG RESPONDED WITH 'YOU GAVE US 10000 FT AND WE'RE ALREADY THERE. WE HAVE THE TFC IN SIGHT.' AT THIS POINT THE MDT INFORMED ME THAT HE HAD A TCASII RA AND WAS CLBING. I INFORMED THE MDT OF THE POS OF THE TRAILING ACFT. THE MDT CLBED FROM BELOW THE ALT THE TRAILING ACFT WAS DSNDING TO, CLBED ABOVE THE MLG THEN DSNDED BACK BELOW THE MLG (SEE EXAMPLE BELOW). IT IS MY OPINION THAT SEPARATION WOULD HAVE BEEN MAINTAINED IF TCASII RA HAD NOT OCCURRED, IE, TCASII MADE A SAFE OP VERY HAZARDOUS OP FOR THE ACFT INVOLVED.

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.