Narrative:

Just before intercepting localizer to runway 32 we received a TA on our TCASII immediately followed by an RA to descend. I saw the TCASII displayed an aircraft at 4200 ft (200 ft above our aircraft) and we immediately descended. I contacted approach advising what had happened. Approach cleared us for the ILS 32 and said to climb back up to 4000 ft, and we did accordingly. Approach said they showed no aircraft in the vicinity and we never had visual contact with the 'TCASII aircraft.' approximately once every hour (in my experience) the TCASII displays a 'phony' aircraft for approximately 2 seconds before disappearing. If this 'mirage' appears at the wrong place at the wrong time you get an immediate RA and must immediately act.

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Original NASA ASRS Text

Title: ALTDEV ALT EXCURSION.

Narrative: JUST BEFORE INTERCEPTING LOC TO RWY 32 WE RECEIVED A TA ON OUR TCASII IMMEDIATELY FOLLOWED BY AN RA TO DSND. I SAW THE TCASII DISPLAYED AN ACFT AT 4200 FT (200 FT ABOVE OUR ACFT) AND WE IMMEDIATELY DSNDED. I CONTACTED APCH ADVISING WHAT HAD HAPPENED. APCH CLRED US FOR THE ILS 32 AND SAID TO CLB BACK UP TO 4000 FT, AND WE DID ACCORDINGLY. APCH SAID THEY SHOWED NO ACFT IN THE VICINITY AND WE NEVER HAD VISUAL CONTACT WITH THE 'TCASII ACFT.' APPROX ONCE EVERY HR (IN MY EXPERIENCE) THE TCASII DISPLAYS A 'PHONY' ACFT FOR APPROX 2 SECONDS BEFORE DISAPPEARING. IF THIS 'MIRAGE' APPEARS AT THE WRONG PLACE AT THE WRONG TIME YOU GET AN IMMEDIATE RA AND MUST IMMEDIATELY ACT.

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.